The class Crustacea contains a very large number of species, which exhibit great diversity. Small forms, such as Branchiopoda, Ostracoda, and Copepoda, have simple, perhaps basic, patterns of behavior. Large forms, as exemplified by crayfish, crabs, and hermit crabs (Malacostraca), show complex social behavior and communication systems. Even truly social crustaceans have recently been discovered, forming closed societies, comparable to some of the more primitive societies of social insects like wasps. A number of crustaceans, belonging to different subclasses, have become parasites with reduced but highly specialized behavior patterns, and some of these have developed communicatory mechanisms not only affecting behavior but also development. It is thus difficult to draw a clear-cut picture of crustacean communication. However, since most studies have involved the Malacostraca, this chapter will mainly deal with this subclass.
METHODS OF COMMUNICATION
Communication signals may be transmitted by chemical, mechanical, acoustical, and optical means, or by combinations of these.
The use of chemical signals is probably the basic method of communication in crustaceans. Combined with tactile stimuli, it may govern the behavior in sexual recognition and pair formation in most of the Entomostraca and even in most Malacostraca. It has been substantiated, however, only in a few cases.
In barnacles, a chemicagl has been shown to induce settling and crowding in cypris larvae. This substance is contained in the shells of settled barnacles and acts quite specifically, for cyprids of Balanus balanoides react to the "settling factor" of conspecific adults to a greater extent than to that of other species. This factor is a protein probably similar to the arthropodin in the cuticle of other arthropods. Cypris larvae react only to surfaces coated with this substance, not to solutions of it (for references, see Frings and Frings, 1968). The function of this substance is to induce crowding, which may be necessary in sedentary animals to ensure cross fertilization.
Another substance is released by well-nourished barnacles, and this induces the newly hatched nauplii to become active and to leave the mantle cavity. It thus ensures that nauplii emerge when food is available.
Chemical stimuli are also responsible for aggregation in terrestrial isopods such as Oniscus, Porcellio, and Armadillidium. These animals react to the odor of conspecifics, especially under dessicated conditions (Kuenen and Nooteboom, 1963).
Pair formation and mating are in many crustaceans initiated by chemical signals. The substances by which males recognize receptive or precopulatory-molt females may be located on or in the surface of the female's cuticle or they may be released. The sense employed by the male is contact chemoreception in the former, but distance chemoreception in the latter. The factors involved have not been identified.
In the first case a male will recognize a receptive or potentially receptive female only after physical contact with her exoskeleton. This has been reported for the crabs Callinectis sapidus and Carcinus maenas and for the shrimps Leander squilla, Palaemonetes vulgaris, and Pandalus borealis (for references, see Frings and Frings, 1968), and proved for the hermit crab Pagurus bernhardus (Hazlett, 1970) and the wood louse Porcellio dilatatus (Legrand, 1958). In Pagurus the male immediately responds by grasping when his chelipeds or legs have touched a receptive female. This response is even elicited when the female has been wrapped in a piece of cloth, preventing adequate visual stimuli. In the wood louse Porcellio the male's ability to detect a receptive female is lost when the last segment of the antenna is removed. Contact chemoreception is also responsible for the recognition of family members in the social isopod Hemilepistus (see below).
Distance chemoreception in pair formation has been reported for the copepod Labidocera aestiva, some talitrids (Amphipoda), some mysid shrimps (Clutter 1969), the lobster Homarus americanus, the snapping shrimp Synalpheus hemphilli, and the crab Hemigrapsus oregonensis (for references, see Frings and Frings, 1968). It has been proved only by Ryan (1966) for the crab Portunus sanguilentus. In this crab, a chemical is present in the urine of receptive females. Males placed in water previously occupied by such a female exhibit searching and courting behavior.
Sex determination in a few crustaceans is phenotypic and strongly influenced by the presence or absence of conspecific animals. In some species of the parasitic isopod family Bopyridae the first larva to arrive at its host will develop into a female. Larvae that arrive later will settle on the female and transform into males (Reinhard, 1949; Reverberi and Pitotti, 1942). A complex sex determination has been found in the tanaid Heterotanais oerstedi (Bückle-Ramirez, 1965). Females may transform into males, and larvae kept together with an adult male or female will develop into females or males, respectively. The mechanisms involved in sex determination in these cases are unknown but it seems most probable that pheromones play an essential role.
Communication by tactile stimuli is probably very important in many crustaceans; during courting, mating, or fighting, male and female tap or touch each other with their antennae, chelipeds, or legs. Mating and agonistic behavior involving such movements, in most cases combined with chemical or visual signals, have been described for numerous crustaceans of different orders. However, careful analysis of the communicative value of such movements are scarce. Usually it is difficult, if not impossible, to separate such stimuli from other communicative actions. Hazlett (1970) studied the responses of hermit crabs to tactile stimuli. These crabs can often be observed to move one or both cheliped mani out and back through an arc of about 30°. This "flicking" act is most common when a crab, which has withdrawn into its gastropod shell, is grasped by another crab. It thus probably represents a rather unspecific defense movement.
Hazlett was able to induce flicking in Pagurus bernhardus by touching various parts of the chelipeds or legs with a glass rod. He also showed that flicking is the response to tactile stimuli when the crab is withdrawn in its shell. It is inhibited by visual inputs and cannot be elicited when the crab is in the walking position with eyes uncovered. Flicking is further influenced by previous experience with an opponent. Crabs that have withdrawn in response to the aggressive display of another crab of equal size are much more likely to execute flicking than animals that have had an encounter with a much larger opponent. Another behavior elicited by tactile input is the "dislodging shaking." This is the movement performed by a hermit crab on whose shell a conspecific crab is crawling. It causes the offending crab to crawl off the shaking crab's shell or it dislodges the offender. Dislodging shaking can be elicited by placing a weight on the crab's shell, and this weight has to be larger in larger crabs or in crabs with larger shells.
The role of tactile stimuli in the communication system of fiddler crabs (genus Uca) was studied by Altevogt (1957), von Hagen (1970c), and Salmon (1965). Salmon, studying U. pugilator, showed that sexual discrimination depends on tactile stimuli during the night. A male gently touched with a grass leaf responds with courtship behavior, but more intense contact triggers aggressive display. Males of U. vocator, like those of other species, respond to dummies prepared of cardboard, but the response is greatly increased after the model has touched the male.
Communication by tactile stimuli, combined perhaps with chemical signals, is probably most important in many nocturnal and cavernicolous crayfish, isopods, and amphipods, but none of these have been investigated. The deep-sea gala-theid Munidopsis polymorpha probably uses another, perhaps more common method of mechanical communication. When two animals approach each other they perform waving or trembling movements with their chelipeds that may cause water disturbances detectable over short distances. It seems that this behavior is a courtship display (Jakob Parzefall, pers. comm.).
The use of acoustic signals is another method of mechanical communication. Many crustaceans are able to produce sounds. The methods by which these are emitted vary in different species, and one species may employ two or three different methods. For a review see Guinot-Dumortier and Dumortier (1960).
Hissing or rattling sounds may be produced by rubbing the second article of the antennae against the edge of the rostrum in spiny lobsters (Panulirus argas, P. guttatus: Moulton, 1957; Hazlett and Winn, 1962), by friction of the walking legs against the carapace in several species of freshwater crabs (Potamon), or by rubbing the maxillipeds against each other in the freshwater crab Pseudothelphusa garmani and in terrestrial hermit crabs (Birgus, Coenobita). These are stridulatory sounds. Similar sounds are produced in other crabs (several Ocypode species, Sesarma angustipes, S. ricordi) by forcing respiratory water through the exhalation opening beneath the epistome (burbling) (von Hagen, 1968). Spiny lobsters have been observed to respond to the stridulatory sounds of conspecific cage mates. In the crabs, however, these types of sounds are not used in intraspecific encounters, and their function may be to deter predators.
The most conspicuous sounds produced by crustaceans are those made by the snapping shrimps Alpheus and Synalpheus. In these shrimps, one of the chelipeds is enlarged and bears a huge chela. When widelyopened a pair of smooth disks are held togetherby the cohesive forces of water.This allows thecloser muscle of the claw togenerate a large amount of tension before theseforces are overcome. A rapid closing is thus facilitated (Ritzmann, 1973).The claw with its specialized tubercleand depression on opposite parts (Volz, 1938) thereby produces a loud crack and a jet of water. The function of this is still uncertain, but Hazlett and Winn (1962) and Nolan and Salmon (1970) have shown that snapping plays an important role in agonistic encounters of homosexual individuals. Since the snapping frequency is increased at dawn and during the night, one of its functions may be to produce agonistic signals when the visual signal of the opened claw is not effective. A similar crack is emitted by the mantis shrimp Gonodactylus oerstedii. It is produced by striking the raptatorial appendage against something hard.
Whereas in the cases cited above, the communicatory role of sound production is still uncertain, recent studies by Altevogt (1966), von Hagen (1962, 1970), Salmon (1965, 1967, 1971), and others have indicated that acoustic communication is very important in terrestrial and semiterrestrial crabs of the families Ocypodidae and Grapsidae. Indeed, it has been shown that species of fiddler crabs and ghost crabs are very sensitive to substrate-borne vibrations, and Ocypode quadrata seems even able to detect airborne sounds of about 3 kHz (Horch and Salmon, 1969). The sense organs employed are the vibration receptors in the joints of the walking legs (Salmon and Horch, 1972, 1973). These crabs are able to use as sources of information a multitude of vibrations originating from conspecifics and perhaps from predators, too.
Thus, the substrate-borne vibrations produced by a rapidly fleeing Uca tangeri act as an alarm signal, inducing other crabs to retreat into their burrows or to stay there if already hidden (Altevogt, 1966). Another acoustic by-product is the sound produced by a male waving its large cheliped. At the end of each waving act the claw is moved downward and applies a vibration impulse to the substratum. This is perceived by hidden crabs and probably recognized by its rhythm, which, of course, is identical to the waving rhythm. It induces the crabs to leave their burrows and to start waving also. It may also be responsible for synchronous waving in fiddler crabs out of direct sight of each other (Gordon, 1958).
Besides these vibrations, which are epiphenomena of other (e.g., locomotory) activities, direct production of vibratory signals occurs in ghost crabs (genus Ocypode), fiddler crabs (genus Uca), and some other Ocypodidae (e.g., Dotilla), and in some Grapsidae. Though these are probably perceived as substrate-borne vibrations, they cause airborne sounds detectable by the human ear. The methods by which these sounds are emitted are different. A number of species drum on the substratum. "Honking" sounds are produced by several Uca species (e.g., U. mordax and U. burgersi) by convulsions of the large cheliped. Striking the walking legs against the ground also aids the sound production. Other species (e.g., U. tangeri, U. pugilator) push the base of the enlarged claw against the substrate or against one leg (percussion), thus emitting the well-known "rapping" sounds. U. thayeri is able to use both methods of sound production (von Hagen, 1973b). Drumming by striking the claws against the substratum has also been observed in Dotilla blanfordi and in Ocypode quadrata. Ocypode ceratophthalmus is able to produce three different types of vibrations (Hughes, 1966): rapping sounds by drumming on the ground, rasping sounds by stridulation, and hissing sounds by burbling (see p. 000). Stridulation is achieved by rubbing a ridge of tubercles on the inner side of one claw against a tubercle on the second article of the same appendage. The grapsids Sesarma rectum and S. curacaoense beat one claw against the other, which is placed on the ground (von Hagen, 1967).
The best-known cases of acoustic communication in crustaceans are those of Uca tangeri and some other Uca species. In Uca tangeri two different "rapping" signals are emitted by the males (Fig. 1). Short drumrolls, consisting of one to three pulses and repeated at intervals of about one second, represent the spontaneous activity, corresponding to the low intensity or spontaneous waving. Longer drumrolls, consisting of seven to twelve pulses, are produced by males sensing the substratum-borne vibrations of an approaching conspecific animal. Altevogt (1966) was able to simulate these vibration signals by drumming with his fingers on the substratum. This elicited typical behavior providing the vibrations were of the correct intensity and rhythm. When drumming near a male's burrow, the male answered, then appeared, took up an aggressive posture, but quickly disappeared at the sight of the author. When drumming near a female's burrow, the female appeared and, as long as her eyes were concealed, even allowed pulling on her leg and simulated carapace feeding, both components of the courtship behavior of this species.
Tropical mud flats and mangrove areas are usually inhabited by more than one species of Uca and Ocypode, all of which may produce sounds. These vibration signals are of special importance in species that are also active during the night and in those that inhabit the uppermost intertidal areas, with grassy vegetation reducing visibility. Salmon and Atsaides (1968) found that most Uca species, including U. pugilator, produce only one type of drumroll. The sounds of different species vary in the number of pulses, the time intervals between pulses, and the time intervals between consecutive drumrolls (Figs. 1, 2). All of these probably contribute to species recognition and may thus serve as isolating mechanisms. Even the frequencies of the sounds may be different. The rapping sounds of Uca pugilator contain maximal energies between 600 and 2,-400 Hz, whereas those of U. rapax have maximal energies between 300 and 600 Hz. Whether these differences are detectable seems questionable. Salmon (1971) showed that U. rapax is more sensitive to vibrations between 480 and 1,000 Hz than is U. pugilator.
Many crustaceans are able to produce light (Harvey, 1961). Indeed, the chemical basis of light production of the ostracod Cypridina hilgendorfii is one of the best-investigated examples of bioluminescence (Johnson et al., 1961; McElroy and Seeliger, 1962). In these and other species that emit luminescent clouds, bioluminescence may have no communicatory function but is used to blind predators temporarily and to conceal the animal. The most highly evolved light-emitting organs are found in the Euphausiacea. These organs resemble eyes in having, besides the luminescent layers, lenses, reflecting layers, and mechanisms analogous to eyelids to turn off the light. But even in these, the function of bioluminescence is unknown. Countershading is one of the possibilities suggested (Nicol, 1962; Clarke, 1963). Reports that shoals or swarms of euphausids often flash simultaneously only suggest that light production is seen by other members of the swarm and may, at least, be used to synchronize the animals. It may also serve to prevent dispersion of the animals and to facilitate species recognition.
Many crustaceans with well-developed eyes use conspicuous postures or movements, often combined with the display of brightly colored body parts for communication, and some even change the structure of their environment in the same context.
Interpretation of body postures or movements as displays with communicatory function must, however, be made with caution. Species that are active during the night also adopt certain postures during intraspecific encounters. Heckenlively (1970), for example, conducted a statistical study of body postures during agonistic interaction in the crayfish Orconectes virilis andcame to the conclusion that "antennal position seems to be important in crayfish aggression, both as an indicator of the aggressive state of the individual and as a threat display to the opponent." It is not known, however, whether a crayfish is able to recognize the antennal position of its opponent and what features indicateits aggressive state. It was shown by Bovbjerg (1956) that blinded crayfish also engage in aggressive interactions.
To settle this question, dummies can be used. Von Hagen (1962, 1970c), studying Uca tangeri and U. vocator, conducted experiments with different models, crabs, and pieces of cork or cardboard of different forms and colors to study the features that elicit certain agonistic or courtship displays. Similarly, Hazlett (1969c) studied the communicatory role of cheliped presentation and leg postures in agonistic interactions of hermit crabs, using dummies prepared from dried specimens.
Visual displays have been most extensively studied in several hermit crabs (Hazlett, 1966a, 1966b, 1968a, 1968c, 1969a, 1969c, 1972a) and in fiddler crabs and a few other Ocypodidae (Altevogt, 1955, 1957, 1959, 1969; Crane, 1943, 1957, 1958, 1966, 1967; Griffin, 1968; von Hagen, 1962, 1970a, 1970b, 1972a, 1972b, 1973a, 1973b, 1973c; Salmon, 1965, 1967; Salmon and Atsaides, 1968; Warner, 1970; Yamaguchi, 1971).
In hermit crabs, visual displays are employed in agonistic encounters. Though these may lead to precopulatory behavior, this is triggered by contact chemoreception and consists mainly of tapping and stroking movements with the chelipeds or legs and by rocking or rotating the female's shell. The agonistic displays involve raising one or more ambulatory legs and presentation or extension of one or both chelipeds (Fig. 3).
The well-known conspicuous claw waving of fiddler crabs can lead to either aggression or mating, depending on whether a male or a female approaches the performing male. Here, sexual recognition depends on visual cues; the female has two small chelipeds, and in the male one of them is extremely enlarged and sometimes brightly colored.
Usually there are two, sometimes three, different forms of claw waving in one species, corresponding to differences in courting intensity. Spontaneous activity is performed whether other conspecifics are absent or present. Itis changed into the courting activity at the sight of an approaching female. In this mating signal, the waving movements are performed faster and seem to bemore exaggerated.
The waving movements vary in different species and thus they probably contribute to species recognition and isolation (Fig. 4). The waving act may be a smooth, continuous movement or it may be interrupted once or several times. It may be performed either while stationary, in front of the male's burrow, or during short, rhythmic bursts of locomotion. Coloration and color change contribute to the conspicuousness of the display. Many species are cryptically colored at the onset of each low-tide activity period, when the animals are feeding or constructing burrows. Later, when social activities become more frequent, the large claw and sometimes even the carapace and other appendages brighten up in some species.
These activities of different Uca species have been well described by the authors mentioned, and even excellent films on several species are now available (Altevogt, 1964a, 1964b; Altevogt and Altevogt, 1968a, 1968b, 1968c, 1968d, 1968e, 1968f; von Hagen, 1972a, 1972b, 1973a, 1973b, 1973c).
Claw waving has also been described for several other subsocial ocypodid crabs, e.g., Dotilla blanfordi (Altevogt, 1966) and Heloecius cordifrons and Hemiplax latifrons (Griffin, 1968). Simpler display patterns, usually cheliped presentation, occur in other Ocypodidae, Grapsidae, and among other crab families (Schöne, 1968; Warner, 1970; Wright, 1968).
Another method of courtship display and territory demarcation has evolved in Ocypode saratan (Linsenmair,1967) and perhaps also in O. quadrata (Horch and Salmon,1969). Like other ocypodid crabs, ghost crabs construct burrows. The sand, however, is not evenly distributed around the burrow by O. saratan, but used to build a pyramid. These pyramids attract both females ready to mate and males searching for a territory. This was clearly shown by Linsenmair (1967), who was able, by building a few pyramids, to trigger the development of a new mating colony (see below).
INFORMATION TRANSFER BY DISPLAYING CRABS
Communication between two animals implies transfer of information. That communication occurs is often quite obvious because the animal to which the display is directed responds in a predictable way. The information content per display and the amount of information transferred per display is obviously a function of the number of possible displays and the number of possible responses. It is low if both are low. Hazlett and Bossert (1965, 1966) conducted statistical investigations on the communications system in aggressive encounters of hermit crabs. They found thirteen to fifteen possible behavioral acts, most of them visual displays. The average amount of information transferred ranged from 0.35 bits per display in Clibanarius triclor to 0.52 bits per display in Pagurus marshi. This latter species tends to be camouflaged by accumulated debris. The rate of information transmission ranged from 0.4 to 4.4 bits per sec in different species. Dingle (1969), studying the aggressive behavior of the mantis shrimp Gonodactylus bredini, arrived at similar values. He observed ten behavioral acts, visual displays as well as tactile stimuli, and calculated mean values of information transmission of 0.78 bits per display and 1.82 bits per interaction. During sixty-minute-observation periods, information transmission increased during the first ten minutes, then slowly decreased with the establishment of dominance-subordinate relationships. Similar values of 1.00 to 1.57 bits per interaction have also been found in the spider crab Microphrys bicornutus and in hermit crabs by character analysis (Hazlett and Estabrook, 1974a, 1974b). It was further shown that the uncertainty about a given act is reduced by the knowledge of the previous act.
THE MEANING OF DISPLAY
The meaning of spontaneous claw waving or sound production of ocypodid crabs has been discussed. These activities are sometimes performed in the absence of other conspecific animals. Some authors (Hediger, 1933, Peters, 1955; Vervey, 1930) have concluded that this behavior has a territorial function. Others (Altevogt, 1966; von Hagen, 1962) believe that its function is to attract females and that this behavior is therefore a low-intensity mating signal roughly comparable to the appetence behavior of other animals. They reject the term "territorial display" because it would characterize activities of negative social value. However, in many animals of different phyla the same display may serve to mark a territory and to attract females, and a clear-cut distinction and definition of both activities may be impossible. It may therefore be more interesting to observe and describe the responses to the displays by different animals of the community.
The assumption that the spontaneous claw waving and sound production in Uca species is primarily a mating signal, indicating the male's maturity and physiological state, is correct. The display is comparable to an advertisement, and the message may be circumscribed by the para-phase: "Here is a mature male." Females ready to mate will be attracted by this signal. The sight of the female then triggers the higher-intensity waving, which is a typical courting activity. This, in turn, may induce the female to enter the male's burrow or to permit the exchange of tactile stimuli that later lead to underground or surface mating. Males in search of a burrow or territory may also be attracted, and ritualized fighting may result. Further, crabs previously hidden may leave their burrows and, if they are males, start waving too.
The pyramid of Ocypode saratan is a similar advertisement. The male's mating territory consists of a spiral burrow, the pyramid, a path from the burrow entrance tothe pyramid, and a small area surrounding these structures. The male usually waits inside the burrow, and the pyramid alone is sufficient to attract females. The pyramid also attracts males in search of a mating place. The males either try to take over the territory by stridulating in front of the burrow entrance until the owner appears and a ritualized fighting starts, or they may start buildinganew burrow and pyramid nearby. Thus, at the onset of the breeding season the first male that has a mating burrow and pyramid determines where a mating colony will develop. Each mating territory is occupied for four to eight days, during which time the male does not feed. Thereafter, he migrates to a feeding place, which may be a mile off. After some days of feeding the male searches for a mating place again. The population therefore always contains wandering males, and these may be attracted by the pyramids, throughout the breeding season. Clearly, the pyramid is at the same time a mating signal to receptive females, a territory signal to some males, and a social attractant to other males.
The most conspicuous displays shown by many crabs are cheliped presentations. During agonistic encounters, some crabs and other decapods stretch their chelipeds toward the opponent. Other species maximally unfold their chelipeds (Fig. 5). At the end of the encounter the chelipeds may be folded again, with the claws pointing medially. It is most likely that unfolding and folding of the chelipeds was originally a defense movement, not a display. Defense and attack are most effective if executed with maximally spread chelipeds and the body held high on extended legs, whether this posture is seen by the opponent or not (see, for comparison, the similar defense postures in scorpions and whip scorpions, in which visual communication can be neglected). This behavior is also effective during defense against predators and will probably function in species with poor eyesight and in those that are active at night. In a number of crabs ritualized fighting has developed from this posture; the crabs push against each other with maximally spread chelipeds (for details see Schöne, 1961, 1968).
In many crabs with well-developed eyes this defense posture or the complete movement, or one or the other part of it, has become a signal, a threat display shown in agonistic interactions. In these, the chelae are not stretched toward the opponent but held in a posture that ensures maximal conspicuousness.
A complex courtship behavior is missing in many aquatic crabs; the male rapes the female, to which it has been attracted by contact or distance chemoreception. In some terrestrial crabs, especially fiddler crabs, claw waving is performed as a courtship signal. It is more likely that this movement has evolved from the greatly exaggerated and ritualized threat display, which is still performed in agonistic interactions (Schöne 1968; Wright 1968), than from an exaggerated locomotion (parade) (Altevogt, 1957). It has then triggered the development of the extreme cheliped asymmetry in fiddler crabs.
This type of cheliped presentation has been called "lateral merus" display by Wright (1968). Another type of display called "chela forward" has evolved in several unrelated ocypodid and grapsid crabs. Here, the merus of the cheliped is stretched forward and the chelae point downward (Fig. 5). In higher-intensity displays the chelae may move up and down or even rotate. In Eriocheir and Pachygrapsus the "chela forward" is used as a courtship display; in other grapsids and ocypodids it is an agonistic display. Wright assumes that this type of display has evolved independently in different groups, first as a mating signal that has subsequently changed its function to an agonistic display. This, however, does not explain the origin of the "chela forward" display. Perhaps the "chela forward" is a cheliped presentation, like the "lateral merus," which started from the feeding posture of those terrestrial crabs in which the chelae point downward in front of the body and are slowly moved alternately up and down to the mouth parts.
INDIVIDUAL RECOGNITION AND MONOGAMY
Individual recognition has been reported for a number of decapods: the crayfish Orconectes and Cambarellus, the cleaning shrimp Stenopus hispidus, and the hermit crab Pagurus bernhardus. It has been observed in the harlequin shrimp (Hymenocera picta: for references, see Wickler and Seibt, 1970; Seibt and Wickler, 1972) and in the social isopods Hemilepistus reaumuri (Linsenmair, 1971, 1972) and H. aphghanicus (Schneider, 1971).
Monogamy has also been reported for a number of decapods (Seibt and Wickler, 1972; Wickler and Seibt, 1970). Some of them live in sponges or burrows (Alpheus, Synalpheus: Alpheidae; Spongicola: Stenopidae), others in molluscs or tunicates (some Pontoniinae). Periclimenes affinis occurs in pairs on the anthozoan Discosoma. However, it is not known how long these pairings exist and whether they are facilitated by individual recognition or by the fact that both animals inhabit the same host or tube and each mate independently drives away other conspecifics of its own sex.
In the harlequin shrimp (Hymenocera picta) monogamy is due to individual recognition. The pair stays together for months, and if separated the male will select his former mate out of several other females. It is believed that chemical cues are most important in individual recognition.
In the social isopod Hemilepistus reaumuri monogamy lasts until the death of one member of the pair.These isopods inhabit some of the deserts and semideserts of northern Africa and Asia. They construct long, deep burrows in the hard desert soil. Each burrowis guarded by one member of the pair, and only the other member is allowed to enter. It is recognized by contact chemoreception and investigated with the antennae. These isopods form family groups that are closed societies, consisting of male, female, and their offspring. The young animals are first fed by food collected by the foraging parent. Later they forage also but freely enter their parents' burrow before noon and at night, when temperatures become unfavorable. The young animals also recognize their parents, but the parents do not recognize each individual young. Inside the burrow the offspring establish a family-specific chemical character that is recognized by the parents; young animals of other parents are devoured. The family odor is a mixture of the individual pheromones of the family. Under experimental conditions young animals of other parents can accept the family character. When kept together with the offspring of one particular pair for some days, during which time the pair has no access to the offspring, this pair will later tolerate the strangers also. However, if some young animals are removed from their parents for a period of weeks they lose the family odor and are subsequently treated as strangers. The source of the pheromone is not known. The individual and family-specific odors are learned, and the adults are able to learn characters throughout the breeding period. At the beginning of the next breeding season (spring), the families disperse and new societies are formed. The development of these behavioral and communications systems enable this isopod genus to live in arid areas that cannot be inhabited by any other crustacean.
The courtship behavior of spiders has aroused interest since antiquity. Many spiders are especially ferocious predators, not hesitating to devour members of their own species. Males attempting to mate, therefore, have to use precise signals in order to be accepted as a mate instead of as a meal. The same problem seems to exist in some of the other arachnid orders, though to a much lesser extent. Mating has been observed in at least some members of all arachnid orders except the Palpigradi, the habits of which are literally unknown. Agonistic behavior, sometimes highly ritualized fighting, has been reported in some of the orders. However, spiders are the only arachnids in which attempts to analyze the communications systems have been made.
Scorpions perform complex mating dances. In Euscorpius the male grasps the female's chelae and steps back and forth, shows jerking or trembling movements, taps the female's genital area with his forelegs, and stings the female in the articular membrane at the base of her palpal chela (Angermann, 1957; Weygoldt, 1973a). Similar behavior patterns have been observed in other genera (Alexander, 1957, 1958a; Abushama, 1968; Gamier and Stockmann, 1972; Rosin and Shulov, 1963; Matthiesen, 1968). Finally, a spermatophore is deposited and the female is pulled over it. The mating dance seems to stimulate the female to accept the spermatophore. It is not known, however, which of the special movements of the dance (e.g., the sting or the tapping of the female gonopore) are essential signals. Nor is it known whether there is a mutual exchange of signals. To the observer it seems as if in many species the female does not respond to any of these movements and, at the end of the mating dance, is as reluctant as at the beginning. Mating usually takes hours, and the amount of information transmitted per action is probably very low.
How a female is recognized by a male is not known; the observations suggest that contact chemoreception is involved.
Some scorpions, when cornered, produce sounds. The methods of sound production vary in different species (Alexander, 1958b; Rosin and Shulov, 1961). It is not known whether any of these species use sound production as a method of communication.
Though not closely related, many pseudo-pproach it from the correct side. Even in some of the scorpions perform mating dances similar to those of scorpions (Kew, 1912; Vachon, 1938; Weygoldt, 1969, 1970). But these have evolved within the order. Members of the more primitive families have no mating behavior at all. The males produce spermatophores in the absence of females (most families) or after short physical contact with a female, whether receptive or not (some Olpiidae). Females are most likely attracted to the spermatophores by chemical sex attractants. The male Serianus carolinensis (Olpiidae) surrounds its spermatophores with signal threads, which make it easier for the female to find the spermatophore and to approach it from the correct side. Even in some of these nonmating species, communication signals between individuals have been observed. A number of species are gregarious (Chthoniidae, Garypidae, Sternophoridae, Cheiridiidae). When one animal is closely approached by another, it may respond by vibrating or shaking movements of the pedipalps and rocking movements of the body. This behavior often spreads through the whole group and results in the usual spacing of the animals. A similar behavior has been observed in some Chernetidae. In Lasiochernes pilosus chemical communication is probably also involved. Excited animals produce a strong odor, which seems to stimulate other individuals.
The mating dances of most Cheliferoidea pose the same problems as those of the scorpions. They probably produce special sets of mechanical stimuli, combined perhaps with chemical signals. A courting male of Withius sub-ruber (Withiidae), for example, rapidly vibrates his third pair of legs and thus perhaps stimulates the trichobothria on the female's palpal chelae. The Cheliferidae perform mating dances without physical contact of the mates. The males court by jumping or vibrating movements in front of the female, thus probably stimulating her trichobothria, and display their ram's-horn organs (Fig. 6). These carry apical glands (Heurtault, 1972) which most likely emit airborne chemical substances. A female thus courted approaches the male and keeps her palpal chelae close to the tips of his ram's-horn organs.
Mating dances have also been observed in whip scorpions (Klingel, 1963; Sturm, 1958, 1973; Weygoldt, 1971, 1973b). The male grasps the antenniform legs of the female and rubs their tips with his chelicerae. The female is also stroked and tapped with the male's pedipalps and antenniform legs. Again, it is not known which of these actions are essential signals. The female is finally stimulated and embraces the male's opisthosoma (Thelyphonidae) or flagellum (Schizomidae). The spermatophore is deposited in this position. In Mastigoproctus, spermatophore formation takes two to three hours, during which time the female remains motionless.
Agonistic behavior between males has been observed in Mastigoproctus. With pedipalps widely opened the animals push against each other, at the same time tapping each other with the antenniform legs. Soon one animal gives up and performs rocking movements, thereby slowly retreating.
Whip spiders or tailless whip scorpions possess a complex arrangement of trichobothria on their walking legs, which provide a fine sense of distance mechanoreception, and extremely elongated antenniform first legs covered with mechanoreceptors and chemoreceptors (Foelix et al., 1975). During courting, a male sitting close to a female rapidly vibrates one or both antenniform legs (Charinus, Tarantula) or taps the female in a characteristic rhythm (Heterophrynus = Admetus) (Klingel, 1963; Weygoldt, 1972a, 1972b, 1972d, 1972f, 1974). Occasionally the male leaps or steps forward, his raptatorial pedipalps extended. The receptive female thereupon retreats but approaches again immediately afterward and stretches one antenniform leg toward the male. Thus, there is probably a mutual exchange of signals. After deposition of the spermatophore the male alters the movements of his antenniform legs or performs a regular dance, and the female approaches and steps over the spermatophore. However, the exact communicatory role of these movements is not known.
Male and sometimes even female whip spiders perform highly ritualized fighting when encounteringanother animal of their own sex and species (Weygoldt, 1972c, 1972e). In some species these are probably used to defend territories. After some irregular mutual tapping the combatants moveapart until each animal can just reach its opponent with one antenniform leg (Fig. 7). Performing slow tapping or stroking movements, the animals coordinate with each other. Suddenly they stop and, with pedipalps widely opened, each steps forward. A short but vehement pushing-and-pulling struggle follows, after which the animals separate. Later a characteristic movement of one antenniform leg of the dominant animal is sufficient to induce retreat of the subordinate.
Some whip spiders are able to stridulate (Millot, 1949; Shear, 1970a), but the communicatory role of sound production is unknown.
This is the largest order of the arachnids (except the mites) and the one with the highest degree of diversity. Numerous authors have studied the mating habits of different spiders (for a summary and a bibliography, see Platnick, 1971). Spiders have evolved a variety of communication systems in different families and species. Communication is achieved by chemical, mechanical, and visual signals.
The use of chemical signals is the most primitive means of communication in spiders. It has probably been retained in some form in all families. In many spiders courtship starts after the male has accidentally touched a female. This has been observed in theraphosids, in numerous haplogyne, and in some entelegyne spiders. Though conclusive evidence is scarce, it seems most likely that contact chemoreception is involved. For example, autotomized female appendages no longer elicit courtship responses from males when washed in ether and dried (Kaston, 1936). The tarsi of the pedipalps and the first pair of legs carry many chemoreceptive hairs (Foelix, 1970a, 1970b). Contact chemoreception also aids in the finding of a female by a male. Bristowe and Locket (1926) and many others have noticed that male lycosid spiders start courtship behavior when placed into a container previously occupied by mature females. In this case it is some factor of the drag line of the female that elicits the male behavior. The factor is species-specific. Males of the European species of Trochosa react only to the drag lines of con-specific mature females (Engelhardt, 1964). The substratum over which a female Pardosa lapidicina has walked does not trigger any courtship behavior if the female was prevented from producing silk (Dondale and Hegdekar, 1973). The presumed sex pheromone is quickly inactivated by water. This ensures that only fresh drag lines are attractive to males. The factor is not a necessary stimulus in the courtship behavior of lycosid spiders; it ensures that the sexes meet, but it can be bypassed by direct contact of a female by a male.
Similarly, the webs of mature females elicit courtship in conspecific males. In the theridiid spiders Steatoda bipunctata and Teutana grossa, a web washed in water and ether is no longer attractive to males (Gwinner-Hanke, 1970). In the agelenid Tegenaria atrica (K.-G. Collatz, pers. comm.) the factor is soluble in petroleum benzine and can thus be transferred to another, previously unattractive web under experimental conditions. Contact chemoreception is also the most important means of mutual recognition in social spiders (see below).
An airborne sex attractant is emitted by mature females of Crytophora cicatrosa (Blanke, 1973b). Its production starts a number of days after the female's final molt, reaches a peak at about the twentieth day, and thereafter slowly decreases. It also stops two days after copulation of the female. Crane (1949b) also found some evidence for the action of airborne pheromones in Salticidae.
When a male spider has found the web of a mature female he starts emitting courtship signals. Agelenidae and Amaurobiidae drum on the web, and the female, if receptive, waits motionless or responds by shaking her web. Male orb weaver spiders (Araneidae, Tetragnathidae) at first remain at the periphery of the female's web and start plucking certain threads with their legs. In many species the males produce mating threads connected to the female's web and pluck or shake these in a certain rhythm, often combined with vibrating movements. These signals cause movements and vibrations of the web that can be perceived by the female and that are quite different from the more irregular struggling movements of a prey organism entangled in the web. The females respond by shaking or tapping on the web. There is clearly a mutual exchange of signals, which finally results in the female approaching and accepting the copulatory position. The male Cyrtophora citricola, for example, plucks the mating thread with his third legs, and the female, if receptive, grasps the mating thread with her third legs. Thus the male, when approaching, is guided directly toward the female's epigyne (Blanke, 1972). The plucking or vibrating movements applied to the mating threads or webs are probably species-specific. Males of other species may be chased away or devoured by the female. In fact, they seldom enter the web of another species because of different chemical sex attractants. A very interesting observation, however, is that of Czajka (1963) on Ero furcata. The female of this spider-eating spider is capable of imitating the courtship signals of the male of Meta segmentata and uses this to approach and capture the females of this species.
Similar plucking and shaking signals, sometimes combined with stridulatory vibrations, have been observed in Theridiidae, Linyphiidae, and Dictynidae. Many linyphiid males, instead of attaching mating threads, bite away threads or whole parts of the female's web (van Helsdingen, 1965). Males of Linyphia triangularis remain in the female's web for some days. During this time they are the dominant member of the pair. If a second male enters the web, threat display and fighting occurs. Several levels of aggressive behavior can be distinguished: approach with abdominal jerking, threat display with forelegs and chelicerae spread and abdominal whirring, and three phases of fighting (Rovner, 1968b).
Tactile stimuli are probably involved in the mating of all spiders. In many species the male, after approaching the female, taps or strokes her body, and in lycosid and salticid spiders the female responds by twisting her opisthosoma to facilitate insertion of the male copulatory organ. Tactile stimuli are also important in the brood care of wolf spiders. Newly hatched spiders settle on their mother's opisthosoma for a number of days. They fail to do so if the female abdomen has been shaved or covered with cloth. Hairy surfaces elicit attachment behavior, and the young spiders are more likely to settle on the back of a freshly killed spider abdomen of another species than on their own, shaved mother (Engelhard, 1964; Rovner et al., 1973).
Many spiders are able to produce sounds, either by drumming on the substratum or by stridulation. Stridulatory structures have evolved on different parts of the body (for details, see Chrysanthus, 1953 and Legendre, 1963). In most cases the function of sound production is unknown. Some theraphosids, when cornered, produce a loud hissing sound resembling that of snakes, and it seems likely that this repels possible predators. Legendre, following Berland (1932), rejects this possibility and assumes that sound production is a by-product of other activities. His arguments, however, are not convincing.
Clearly, sound production was originally a by-product of movements of the palps, legs, or the opisthosoma.Such movements are performed by many spidersduring courtship, and some of these use the vibrations caused by such movements as communicatory signals. This, in turn, probably triggered the evolution of special sound-producing structures and behavior mechanisms. Receptors may be the slit sense organs on the legs.In Cupiennius salei a single large slit sense organ on the tarsus of the legs is sensitive to surface vibrations and even to airborne sounds (Barth, 1967), and the lyriform organs of Achaearanea tepidariorum are sensitive to vibrations of the substratum (Walcott and van der Kloot, 1959). (For a discussion of sound perception in spiders see also Chryanthus, 1953; Legendre 1963; Frings and Frings, 1966; Liesenfeld, 1961; Walcott, 1963). The auditory function of the trichobothria, which are sensitive to minute air movements (Görner, 1965; Görner and Andrews 1969), is still a matter of speculation.
Acoustical communication has been studied in the theridiid spiders Steatoda bipunctata and Teutana grossa (Gwinner-Hanke, 1970) and in some Lycosidae (Buckle, 1972; Harrison, 1969; Rovner, 1967). It probably occurs in other species also. Although there is some evidence to suggest that the sounds may be perceived directly in the species mentioned above, it is more likely that the vibrations of the substratum or the webs are usually detected. Both theridiids, Steatoda and Teutana, possess stridulatory organs between prosoma and opisthosoma. The prosoma carries rows of parallel ridges, and the opisthosoma has a row of cuticular tubercles with hairs that can be moved over the ridges. Both species stridulate during courtship. In Steatoda, the female responds to the stridulation even when the male is not sitting in the female's web. Sound production is a necessary part of courtship behavior. Stridulation is also used in agonistic behavior between males. Males that are made unable to stridulate do not start courtship. In Teutana the communicatory role of sound production is more obscure. Females do not seem to respond to stridulation, and sometimes courtship may take place without any sound production. Fighting males never stridulate.
Sound production in wolf spiders has long been noticed by many naturalists (Rovner, 1967). Courtship in lycosids involves up-and-down movements of the pedipalps and first legs and rapid vibrations of the opisthosoma. From these movements different methods of sound production have evolved. The males of many species use their palps to drum on the substratum, usually on dry dead leaves in the natural habitat. This has been reported for Cupiennius salei (Melchers, 1963), Lycosa rabida (Rovner, 1967), L. gulosa (Lahee, 1904; Harrison, 1969), Schizocosa avida (Buckle, 1972), S. crassipes (Kaston, 1936), and several species of Pardosa (e.g., Hallander, 1967; Dumais et al., 1973). Other species, e.g., Alopecosa aculeata, scrape the surface of the ground with their pedipalps (Buckle, 1972). Vibrations of the opisthosoma may produce sounds when the abdomen hits the ground. This has been observed in Alopecosa pulverulenta (Bristowe and Locket, 1926), Lycosa gulosa (Harrison, 1969), and Hygrolycosa rubrofasciata (O. von Helversen, unpublished results) (Fig. 8). Males of this latter species possess an interesting sound-producing apparatus. The cuticle of the ventral side of the opisthosoma is thickened and hardened and covered with specialized knobbed hairs. In Pardosa fulvipes, on the other hand, a stridulatory apparatus is present, consisting of cuticular ridges on the surface of the book lung covers and specialized hairs on the coxae of the fourth legs (Kronestedt 1973). Whether this is used in courtship behavior is not known.
Oscillograms or sonagrams of the sounds of Lycosa rabida, L. gulosa, Schizocosa avida, and Alopecosa aculeata have been published by Rovner (1967), Harrison (1969), and Buckle (1972). Lycosa rabida is the species that has been studied most intensively. Sounds are produced during courtship and agonistic behavior (threat display). Courtship signals consist of several brief bursts of pulses followed by a long, continuous train of pulses with a mean of 29 pulses per sec. Each pulse has a duration of 3 to 6 msec. The courtship sound ends abruptly and is followed by a period of silence. Females respond to the courtship sounds by leg-waving display and approach behavior, even when recorded sounds are played back through a loud speaker. Lycosa rabida is active not only under daylight conditions. When a male that has sensed the contact sex pheromone of a female starts drumming, it can be found by a receptive female during the night or in the dense undergrowth of the field. During threat display short bursts of pulses are emitted by the males.
Experiments with palpless males indicate that, under daylight conditions, the acoustic signals are not essential. Females readily respond by leg waving and will approach a male performing only the visual courtship displays.
The use of visual signals in courtship and agonistic behavior has been studied in some lycosid spiders and in Salticidae, including Lyssomaninae, and in Oxyopidae.
Many wolf spiders are active during the day and, when sensing the contact pheromone of a female or when seeing a femalelike spider, will perform elaborate courtship movements involving waving of the first legs and pedipalps. In Pardosa and Lycosa species, especially, these appendages are conspicuously colored, often black. In Lycosa rabida, the courting male assumes a distinct posture with forelegs flexed. Then the palps, in alternation, are raised and waved in a circular path (palpal rotation: Kaston, 1936). After performing a number of palpal rotations, one of the forelegs is lifted and extended in a tapping movement, simultaneously with opisthosomal vibrations and palpal drumming. These courtship activities are alternated with periods of inactivity during which the female responds by waving her forelegs and approaching. Rovner (1968a) showed that palpal rotations are not essential signals, for receptive females also respond to performing palpless males. There is a mutual exchange of signals; in response to the leg waving of the female the frequency of the male's courtship activities increase.
Similar visual signals are employed in Pardosa. A male of Pardosa amentata stops at the sight of a female and raises one pedipalp. He then makes a step, simultaneously lowers both palps, stops again and raises the other palp. The palps, when lowered, vibrate, and the first legs and the abdomen vibrate at the same time. These are very distinctive movements, and the male walks around the female, step by step, approaches and mounts. In Pardosa amentata the approach behavior of the male is thus part of the courtship sequence (Vlijm and Dijkstra, 1966). Similar though different displays have been observed in P. hortensis and P. nigriceps. In P. lugubris, however, the male remains stationary and courts by raising the palps and the forelegs. Both are lowered stepwise; at the same time they vibrate and the opisthosoma also vibrates. After a number of such courtship sequences the male runs toward the female and mounts. There is often more than one species of Pardosa in the same environment, and the females are similar in appearance. The differences in male courtship behavior therefore probably contribute to species recognition and function as pre-mating isolating mechanisms.
Agonistic behavior among male wolf spiders can often be observed. Threat displays in Lycosa rabida (Rovner, 1968a) and Schizocosa crassipes (Aspey, 1974)involve a number of different movements of stepping, extension, and raising of the first pair of legs. It appears that these spiders have evolved a communication system similar to that described for some crabs but, until more evidence is available, this conclusion may be premature. Under laboratory conditions a dominance-subordinate relationship is soon established, and the subordinate male assumes a submission posture when attacked by a dominant male.
Males of the European species of Trochosa also perform courtship movements that include lifting and extending of their forelegs, vibrations of the palps, and quick up-and-down movements of the opisthosoma. However, these species are usually not active during the day, and copulation takes place even when the females are blinded. Engelhardt (1964), therefore, assumes that the courtship displays in these species do not have a communicatory function but are a by-product indicating the physiological state of the male or have the function of self-stimulation. One might conclude, then, that the courtship movements of the Trochosa species are vestigial behavioral patterns inherited from wolf spiders that were more active during the day. However, the displays are different in the four species, and it is not known whether they can be sensed by the female by means of her trichobothria or other mechanoreceptors.
Salticid spiders rely entirely on vision in their courtship and agonistic behavior. The males of most species are brightly colored and display the colored parts of the body in a species-specific manner (Bristowe, 1929; Drees, 1962; Legendre and Llinares, 1970; Plett, 1962). Usually different courtship and threat display movements are shown. Waving movements may be performed with the pedipalps, the first, or even the third legs (Figs. 9 and 10). The males may rock or jump sidewise in front of the females or perform other conspicuous movements. Receptive females usually wait nearly motionless. Crane (1948, 1949) has published a wonderful synthesis of salticid behavior, including the generalized scheme of salticid courtship display (1949b) shown in Table 1.
THE EVOLUTION OF DISPLAY
Mechanical, acoustical, and visual displays probably serve for species recognition and mutual stimulation (for discussions on this subject see Peckham and Peckham, 1889, 1890; Montgomery, 1910; Berland, 1912; Bristowe, 1929; and Crane 1949b). The display movements have probably evolved from activities with other functions. It is likely that the plucking or shaking signals of a male web spider arose from the orientation movements with which a spider investigates whether there is another spider or animal in the web. Similarly, visual displays most likely evolved from tactile, chemical, and tactochemical orientation movements and, perhaps also, from defense postures. Many spiders, especially theraphosids, when walking in a strange environment, perform waving movements with their first legs, and defense posture with the first or first and second legs raised is easily assumed. These very conspicuous postures, together with orientation leg waving, do not play a communicatory role since theryphosids are not visually oriented. However, with the acquisition of better vision these behavior patterns could have become communicatory signals that subsequently become more conspicuous through exaggeration of the whole movement or parts of it.
Social phenomena exist in a number of spiders belonging to different families (Kullmann, 1968, 1969c, 1972;Shear, 1970b). The social phenomena involve mutual tolerance, interattraction (that is, the fact that individuals are attracted and remain close to conspecifics), and cooperation. Temporary societies are established in some spiders in which the offspring of a female remain with their mother and are nourished by her until they are able to take their own prey. This has been studied in the agelenid Coelotes terrestris (Tretzel, 1961a, 1961b), the theridiids Theridion notatum and T. impressum, and in a number of Eresidae (Kullmann, 1969a, 1969b; Kullmann et al., 1972). In Coelotes terrestris the young spiders remain in the female's web until they are half grown. They gather around their mother when she has prey. They are attracted by the movements of prey wrapping and feeding, and by stroking her first legs they "beg" for food. A warning signal is used by a female that has been disturbed. A few hard tapping movements on the web induce the young spiders to retreat into the web funnel.
In the subsocial Theridiidae and Eresidae young spiders are nourished by a fluid regurgitated from the female's gut (Kullmann and Kloft, 1969; Kullmann et al., 1972). No begging signals have been observed in these species. In the recognition of the young spiders by their mother, contact chemoreception is involved. The characteristic slow movements of the young spiders usually do not elicit aggressive behavior in the female, but when a young animal moves quickly it is touched and investigated with the forelegs. Conspecific young spiders of other females are accepted; other spiders are eaten or chased away. Tolerance and interattraction disappear later; the young spiders finally devour their dying mother, separate, and become mutually aggressive.
Permanent spider societies have evolved in a number of species of different families (Kullmann, 1968, 1972; Shear, 1970b). The best-investigated species are Agelena consociata (Agelenidae: Krafft, 1966, 1969, 1970a, 1970b, 1971, 1972) and some Stegodyphus species (Eresidae: Kullmann, 1969a, 1970b; Wickler, 1973; Jacson and Joseph, 1973). Unlike insect and isopod societies, these spider societies are open. New conspecifics and even spiders of related species of the same genus are tolerated and sometimes integrated into the society. Experiments by Krafft ( 1970b, 1972) and Kullmann and Zimmermann (1972) indicate that mutual recognition is explained by contact pheromones and perhaps also by the surface structure of the spider's cuticle. Conspecific animals placed on a vibrator and thus behaving like prey are touched and investigated with the forelegs but not bitten. The source of the pheromone that inhibits killing of conspecifics is not known. Even pieces of cuticle turned upside down are less likely to be bitten than are prey organisms or pieces of elder marrow. This pheromone is not only responsible for mutual tolerance but probably also for the interattraction, that is, the tendency of the spiders to sit close to each other, especially when separated from the society and in a strange environment. In Agelena consociata a certain hierarchy has been demonstrated. Some spiders are less easily attracted to their conspecifics. One such animal alone carries a freshly killed prey animal into the interior of the web. Cooperation is achieved by more indirect interactions. The web is constructed cooperatively, but the performance of each spider is determined not by special signals of other individuals but by those parts of the web already present. Similarly, cooperation in prey capture is a function of the size, intensity, and duration of the struggling movements of the prey organism. The larger and more vigorous the prey the more spiders participate in its capture. When the prey is finally killed, however, only the dominant animal transports it to the young spiders.
Little is known about communication in other arachnids. Mating preliminaries usually involve tapping or stroking movements or other tactile stimuli. In sopugids the female is raped by the male, embraced by pedipalps and pinched with the chelicerae. If the assault is vigorous enough the female immediately assumes a certain posture and becomes completely immovable (Junqua, 1966; Muma, 1966). If not, the male is chased away or devoured. Pheromones, though probably generally present, have been demonstrated only in a few cases. Males of the harvestman Ischyropsalis hellwigi and some other species of the same genus present cheliceral glands to the females during courtship, and the secretion probably stimulates chemoreceptors on the tips of the female chelicerae (Martens 1969a). In the tick Argas persicus an assembly pheromone has been demonstrated by Leahy et al. (1973). An interesting case of chemical communication has been observed in the mite Myrmonyssus phaelenodectes by Treat (1958, 1969). This mite is a parasite of the noctuid moth Pseudaletia and destroys one of its host's tympanic organs. The first mite to arrive at its host invades one of the tympanic organs and marks a trail to this organ by walking back and forth and depositing a trail pheromone on the host's thorax. Mites that arrive later follow this trail and invade the same organ. It is thus ensured that the parasitized moth is not completely deafened, for hearing, in these moths, is used to escape bats.
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