The study of orthopteran signaling systems has become highly interdisciplinary, with research on communication being carried out from a number of points of view and by persons of varied backgrounds and training. Research on neuromuscular, developmental, and genetic aspects of signaling is being carried out in a number of laboratories around the world, sometimes concurrently at one place. Systematists continue to use signaling as a tool in distinguishing between species, to determine phylogenetic relationships among related species, and to provide basic information on the diversity of signaling systems. Orthoptera are also increasingly being used to study evolutionary and ecological principles, circadian rhythms, density dynamics, and population spacing patterns. Much information on this group is already available in several excellent reviews (Alexander, 1960a, 1962a, 1967, 1968; Walker, 1957, 1962). The present chapter reconsiders topics discussed previously, although from a slightly different viewpoint, as well as others not previously discussed. My own background in grasshopper and cricket biology unavoidably has resulted in a more comprehensive treatment of behavior in these groups. The review by Barth and Lester (1973) should be consulted for an introduction to the literature on blattid behavior.
Communication vs. Sensory Perception
On the basis of fitness changes that result from transfer of information between two organisms, three types of transmission can be recognized: (1) An act of information transfer that increases the fitness of the emitter and the receiver alike, causing selection to improve the mechanisms of emission and reception. (2) Emission as an incidental effect of other behavior or structures in which the fitness of the emitter remains unaltered. Under such conditions no evolution of emission occurs. (3) Emission incidental to other attributes, but which decreases the fitness of the emitter, causing selection to diminish the amount of information transmitted. Information emission by prey that informs predators of the prey's location is under negative selection, but the ability to receive information is under positive selection. A kind of chase between the emitters and the exploiting receivers ensues (Otte, 1975a, 1975b). Some highly interesting results in the evolution of cryptic coloration and posturing are possible (Robinson, 1969a, 1969b). This chapter focuses principally on mutualistic interactions between emitters and receivers, but also considers cases of intraspecific signal exploitation.
Mechanisms of Signaling: Some Aspects of Evolution
Orthoptera have exploited most available modes of signaling potentially open to them. By most reasonable guesses, olfaction and touch probably preceded vision and hearing. As expected, nocturnally active groups tend to rely more on olfaction and hearing, while diurnal groups rely more on vision. Among nocturnal groups, cockroaches depend heavily on olfactory and tactile signals (Barth, 1965, 1970; Roth and Barth, 1964). Such a condition may be representative of the ancestral Orthopteroid stock. Gryllids and tettigoniids represent lineages that have entered the adaptive zone of acoustics. Certain nocturnal cricket genera (Apterogryllus, Apteronemobius, Amusurgus, and others) have secondarily lost their acoustic ability and may now rely mainly on olfactory cues (Alexander and Otte, in prep.). Among diurnal taxa, Acridoidea have evolved relatively acute vision and rely greatly on visual signaling, but subgroups of primarily nocturnal families have undergone secondary reversions from one signaling system to another. Certain diurnal crickets (for example, Metioche, Homoeoxipha; Fig. 1 ) have large eyes and have lost or partially lost acoustic signaling as a result of adopting a more diurnal habit (Alexander and Otte, in prep.). Implied here is the idea that the evolution of one system of signaling changes the relative importance of another— there occurs a tradeoff where one system increases in importance or acuity at the expense of another. No species appears to excel in all modes of signaling.
Fig. 2 is a partially hypothetical and partially factual representation of what the relative acuities of different groups may be like. The clustering of taxa is expected if a tradeoff between signaling modes is the rule. Orthoptera cannot (or at least do not) simultaneously have high visual, acoustic, and olfactory acuity. It also shows that each of the three adaptive zones is dominated by a major group. Diurnal and nocturnal zones can also be recognized, the latter having acoustic and olfactory subzones. Anatomical investigations into changes in neural networks that accompany changes in signaling would be of major interest.
From the standpoint of signaling methods, the Acridoidea are clearly the most diverse group. At least eleven methods of acoustic signaling are known in this group alone (Table 1). Crickets and tettigoniids, the other acoustic groups, possess only three known mechanisms of acoustic signaling: tegminal stridulation, in the vast majority of species; and femoro-abdominal stridulation (Richards, 1973) and antennofrontal stridulation (Menon and Parshad, 1960) in one or two species each.
Some reasonable guesses can be made as to why grasshoppers should be so diverse. First, when they became diurnal and long-range vision was made possible, the stage was set for two classes of movements to evolve into visual signals: tactile signal movements such as jerking and repelling movements of the hind femora; and noncommunicative movements already closely associated with interindividual interactions, such as locomotary and orientation movements. Second, a specialized sound-receiving system (abdominal tympanum) was apparently acquired early and evidently only once by grasshoppers, perhaps in the context of predator avoidance; furthermore, it probably preceded most or all sound-producing mechanisms found today. Practically all members of large acridid subfamilies, even some silent subfamilies (Catantopinae), possess a tympanum. This indicates that the tympanum is ancient and that it could have evolved independently of specialized sound-producing mechanisms, since it can be maintained in their absence. Third, while the tympanum is very similar among widely divergent taxa, sound-producing mechanisms of the same taxa are very different, suggesting that they were not present in a common ancestor and must have been acquired independently. With a hearing mechanism already in existence, visual signals could repeatedly become acoustic with only slight modification of movements. The existence of a variety of sound-producing mechanisms in grasshoppers, some of them confined to a single species, suggests that visual signals and perhaps also various tactile movements became acoustic signals independently when the parts of the body involved came to rub against one another, probably accidentally at first. Subsequent specialization of these body surfaces into scrapers and files resulted in the mechanisms indicated in Table 1.
The oedipodine Encoptolophus sordidus alone possesses four mechanisms (A2, Bl, B2, and C2).
Once evolved, why did some crickets lose their songs? I suggested above that songs might have become superfluous when another mode of communication became more important. For example, olfaction might be a better system of signaling in subterranean forms, or vision may supercede hearing in diurnal forms, and so on. But Walker (1964) and Cade (1975) suggest that prédation pressure might select against signaling by song. Walker demostrated that cats orient on and capture singing crickets using phono-taxis, and Cade showed that females of the tachinid fly Euphasiopterx ochracea locate male crickets (Gryllus integer) by their song and deposit larvae. The larvae burrow through the exoskeleton and feed internally.
There are various stages evident in the loss of calling songs. Cade showed that in Gryllus integer some males call while others do not. 'Tor most of the night, cricket aggregations are composed of calling as well as noncalling males. Noncalling males, termed satellites, walk in the area occupied by calling males and attempt to intercept and copulate with females attracted by the calling males." He made the important discovery that noncalling males experience fewer parasite attacks. He also showed that if a calling male stops singing, one of the noncalling males begins to sing. In addition to being less susceptible to parasite attack, noncalling males are less likely to be attacked by the calling males themselves and may expend less energy in seeking mates by allowing calling males to attract the females (some of whom fly in to the calling male). The relative importance of these three factors (energy conservation, prevention of aggression, and avoidance of parasitic attack) in causing males to remain silent has not been assessed.
The next stage in song loss is evident in a Florida cricket Gryllus ovisopis, a relative of G.integer where the calling song is lost entirely; but males still perform courtship and aggressive songs. Why this species has lost its song while a sympatric relative retains its song is unknown. Walker and Mangold (in prep.) have attracted E. ochracea flies to mole cricket (Scapteriscus acletus) songs, and have raised adult flies from crickets infected artificially.
A third stage in song loss is evident in crickets that retain the acoustical apparatus, but do not sing (e.g. in some Metioche) (Alexander and Otte, in prep.). And a final stage is evident in some leaf-inhabiting and some burrowing species which have lost the sound-producing apparatus and, in some species, also the hearing organ (e.g., Apterogryllus, Amusurgus) (Alexander and Otte, in prep.).
What Walker's and Cade's studies suggest is that the prédation costs attached to acoustical signaling may on occasion outweigh the femaleattracting benefits. Whether prédation or mate-theft ever selects for a genetically determined and frequency-dependent singing-male vs. silent-male dimorphism is not known, but it is conceivable.
Diversity of Signals
SIGNAL DIVERGENCE AMONG SPECIES
In a number of papers Alexander (1957, 1960a, 1962a), Walker (1957), Bigelow (1964), and others have emphasized the role of signal differences in preventing interspecific matings. Calling songs designate an individual's mating type and are, in effect, used by females to assess the genotype of potential mates. The precise stages by which signal differences arise remains problematical. Biologists generally agree that speciation is initiated through extrinsic separation of populations, usually geographically but occasionally seasonally (Alexander, 1968), and is completed upon evolution of appropriate identifying displays that prevent reproductive interactions between the diverged units. But the question of whether signal differences develop while populations are evolving independently of one another, as proposed by Mayr (1963), or whether the divergence takes place subsequently as a result of selection against interspecific interactions (Lack, 1947; Dobzhansky, 1970) requires further exploration.
Clear cases of song divergence in areas of sympatry have not been forthcoming in acoustic Orthoptera, a group ideally suited to detecting examples of it (Alexander and Walker, pers. comm.; Walker, 1974b). A careful analysis of Australian Teleogryllus species also failed to reveal differences between allopatric and sympatric populations (Hill, Loftus-Hills, and Gartside, 1972).
Walker (1974) discusses several possible instances of sympatric song divergence but finds the evidence too weak to conclude that divergence has occurred. He advances the following possible explanations for the rarity of this phenomenon:
(1) Sympatric divergence in song characteristics does not occur either because songs are not important, i.e., they are not under sufficient selection to produce divergence (he considers this unlikely) or because calling songs diverge sufficiently in allopatry that "when newly speciated populations become sympatric the songs are different enough so that no additional divergence occurs." He then cites examples of entirely allopatric species which have very similar songs and other pairs in which the songs are quite different.
(2) Sympatric divergence is difficult to detect because the critical song characters that diverge have not been examined, or because not enough songs have been analyzed to show a statistical difference between populations.
While the existing song patterns in relation to geography seem superficially to support the Mayr model, i.e., that most species have already acquired song differences before they reestablish contact, Wallace (1970) presents a special case of the Dobzhansky model that might explain why few cases of character displacement are evident. According to this model one species arises from another as a small border population. Following contact with the parent population all individuals of the daughter population quickly come to possess song differences because they interact rather directly with members of the parental population. Subsequently the new species expands to become largely allopatric. Hill et al. (1972) advanced a similar hypothesis.
DIVERSIFICATION OF SIGNALS WITHIN SPECIES
In orthoptera the usual method by which a signal serving one function gives rise to several new signals, each with a different function, appears to be somewhat as follows: At an early stage a signal occurs in one context only; later the signal occurs in two contexts but remains structurally unchanged. Still later it becomes structurally distinct in the two functional contexts. The process may repeat itself as shown in Table 2. In crickets the calling song is probably ultimately derived from the courtship song. Some species have calling and courtship songs that remain structurally similar (stage 2), while others have signals that are very different (stage 3) (Alexander, 1962a). Alexander also postulates that aggressive songs originated as outgrowths of the calling song.
Source: After Alexander, 1962b.
Rates of signal evolution and signal diversification seem to depend on signal function. Grasshopper courtship and calling signals have undergone extensive evolutionary changes, as though subject to particularly strong selection for change, while agonistic signals are more conservative (Table 3) (Otte, 1970, 1975). The table shows a net decrease in the number of signals in some taxa. Because reproductive signals are involved in species identification, unique codes are required for quick recognition of appropriate mating types. Cricket calling songs are more susceptible than courtship signals to change, evidently because selection operates more strongly on signals that promote early recognition (Alexander, 1967).
Many orthopteran species may possess several very different signals in certain contexts, while other species possess only one signal. For example, according to Spooner (1964), the katydid Scudderia texensis begins singing in the afternoon and stops late at night. Males sing different songs at different times of the day. Spooner shows some functional differences between signal types, but one wonders why some species manage to get along without them. Similar multiple signals are practically nonexistent in the Gryllidae but relatively common in Tettigoniidae and Acrididae.
Contexts of Signaling
Male and female Orthoptera form temporary pairs that last at best for several copulations. The precise sequence by which pairing is achieved varies from group to group, depending on the method of signaling and on whether one or both partners signal during the process. Some of the principal pair formation sequences known are shown in Fig. 3. The figure emphasizes acoustic signaling because much is known about it. The main categories are: a. and b. A male signals and a female is attracted to him. c. Similar to b., but the female answers the male just prior to visual contact; the male then closes the gap. d.-g. Females answer signaling males, who then approach the female (in some phaneropterine katydids [Spooner, 1968] and in some pamphagid and pneumorid grasshoppers [Otte, 1970] males fly about in the field signaling and listening for female answers), h. Male and female wander about looking for one another in likely spots; they encounter one another by chance, and final recognition is achieved through visual and olfactory cues. Conditions clearly intermediate between these categories also exist.
Courtship behavior differs considerably among orthopteran families. Gryllids usually have distinct and elaborate courtship signals, especially in ground-dwelling groups, while tettigoniids do not (Alexander and Otte, 1967a; Spooner, 1968). Congeneric acridids have species-specific courtship behavior, sometimes elaborately developed (Otte, 1972a), but congeneric cricket species have quite similar patterns. The difference between crickets and grasshoppers in this regard may be due to the fact that accidental pairing in gryllids is less common. Since they are nocturnal for the most part, an accident in pair formation must involve actual contact. In grasshoppers accidental pairing is common because individuals can perceive moving individuals twenty five feet or more away and may be attracted to one another; but they are unable to distinguish between conspecific and heteros-pecific individuals at that distance. Thus, when accidents in pairing are common, courtship signals would be under stronger selection to become species-specific.
The considerable variation existing in courtship among subfamilies of crickets is described by Alexander(1962a), Alexander and Otte (1967a), and Otte (1970). In these papers the ecological significance of certain patterns is explored.
AGONISTIC AND AGGRESSIVE CONTEXTS
Most Orthoptera posess signals that promote spacing of individuals. Such signals are more elaborately developed in males than in females (for reasons outlined by Trivers, 1972). In some groups aggressive signals are mere elaborations of calling songs (e.g., gryllids: Alexander, 1962a; gomphocerine grasshoppers: Otte, 1970; Jacobs, 1953). In others, agonistic and pair-forming signals may be quite different (e.g., phaneropterine katydids: Spooner, 1968; oedipodine grasshoppers: Otte, 1970). In field crickets (Alexander, 1961), conocephaline katydids (Morris, 1971), and acridids (Otte and Joern, 1975) agonistic signals may act as threats that are backed up by physical attack, but in most Orthoptera the signals have at best a sex-identifying or spacing function.
In acridids, agonistic signals are evoludonarily more conservative than courtship or pair-forming signals, particularly in subfamilies where courtship and agonistic signals are quite different from one another and perhaps where they are not neurally linked.
Orthopteran defense is of two kinds (Robinson, 1969a): primary defense, in which animals attempt to prevent attacks from being initiated, through hiding, cryptic behavior, and cryptic morphology (Fig. 4); and secondary defense, in which animals attempt to stop an attack that has been initiated by direct counteroffensive measures such as poisonous tissues, toxic sprays or* liquids, or dangerous weapons such as spines or biting mouthparts. Secondarily, displays such as aposematic coloration and conspicuous and intimidating postures evolve, which warn predators of impending danger (Fig. 5).
Romaleine and pyrgomorphid grasshoppers are poisonous and discharge poisonous odors or froth, which deter attacks from predators. These groups are also characteristically brightly colored and tend to form tight aggregations, especially in the nymphal stages. In Argentina nymphs of the romaleine Chromacris speciosa are black with small red spots. When a tight cluster is disturbed the nymphs scatter, but reassemble within minutes. The members of a cluster are probably typically siblings, but clusters composed of two very different size classes are common. This species feeds on the poisonous solanaceous plant Cestrum kunthi in northern Argentina. Similar aggregations are known in the desert romaleine ( Taeniopoda eques: Alcock, 1972).
In the African pyrogomorphid Poekilocerus bufoni, nymphs can eject poisonous secretions up to 60 cm by arching the back and spraying over their heads (von Euw et al., 1967). In adults the secretions run down the side of the body and over the spiracles. Air forced out of the spiracles is mixed with the gland secretion and forms a pungent-smelling froth. Toxic compounds are derived from their diet of Asclepiad plants, which are known to be rich in cardiac glycosides. These insects have several lines of defense: bright coloration, which deters predators from attacking; ejection of a jet or foam of defensive fluid containing cardenolides and histamine; possession of a penetrating and disagreeable odor perceived at several meters; and possession of cardenolides in their body tissues. Quite possibly, the second and last characteristics are functionally linked, and bright coloration evolved secondarily as a warning device.
The walking stick Anisomorpha buprestoides ejects a toxic spray toward birds in its vicinity even before being attacked. Once they have been sprayed, birds strongly avoid walking sticks (Eisner and Meinwald, 1966).
SOME GROUP-RELATED ACTIVITIES
Migratory locusts display various activities mediated by visual, tactile, and olfactory signals or cues that seem to promote formation and maintenance of large cohesive migratory swarms. Contact between developing locust nymphs promotes aggregation of individuals and leads to the development of conspicuous (black and yellow) coloration. The latter may further enhance aggregation through visual attraction (Ellis, 1963, 1964; Ellis and Hoyle, 1954). Black coloration is evidently not aposematic, but may be an energy color, which promotes faster development through greater absorbtion of solar energy (Hamilton, 1973). Locust odorants also have the effect of hastening or slowing development, resulting ultimately in nearly simultaneous maturation to the adult stage, an important requirement for the formation of mixed swarms in which mating takes place during or after migration. Nymphs exposed to the odors of adults grow more slowly in the presence of younger animals (Norris, 1954, 1964, 1970). Odors may affect gamete formation as well, causing changes in chiasma frequency and ultimately in the production of a more variable batch of offspring (Nolte, 1968). Ovipositing females of Schistocerca gregaria are also strongly attracted to one another during oviposition and lay their eggs in a group (Norris, 1970). Similar attraction of females can be achieved with immature and even dead animals. These studies indicate that it is important for Schistocerca females to lay eggs where other females lay them; this ensures that developing nymphs will group with numerous other individuals. Their chances of surviving and successfully migrating from unsuitable to suitable areas may be enhanced when they move as a group. Similar group effects have been demonstrated in house crickets (Chauvin, 1958).
Signaling individuals can interact in many ways (Alexander, 1975). Here I will concentrate on acoustic interactions among males attempting to maximize their female quota. The main categories are outlined in Table 4.
RANDOM ACOUSTIC ACTIVITY
The signals of males may be temporally and spatially relatively independent of one another. Yet, indirect interactions result when two males attempt to attract the same female.
Female-attracting signals of many orthopteran species are t males in the field and in emporally and spatially interdependent, resulting in the production of bursts of activity or in aggregations of males. Thus, in several species of oedipodine grasshoppers with loud, conspicuous flight displays, a number of males may become active all at once, and a hillside that has remained silent for many minutes suddenly becomes noisy with the loud buzzing sounds of dozens of flying males. In August 1973, a wheat field on the plains of Colorado was silent for more than thirty minutes after I arrived, then gave forth to flight displays by hundreds of males of the oedipodine grasshopper Aerochoreutes carlinianus over the entire field. A few minutes later all were silent again.
NOTE: The categories are not mutually exclusive. Symbols indicate possible relationships between interacting males. C = reproductive competition, AP = antipredator or defensive tactic, CP = cooperative, IR = interference reduction.
Both crepitating and stridulating species chorus. In the gomphocerine grasshopper Syrbula admirabilis males in the field and in the laboratory tend to sing at the same time so that silent periods alternate with bursts of activity (Otte, 1972a). The adaptive significance of such choruses is poorly understood. Experiments carried out on S. admirabilis suggest that such interactions are the result of intermale competition. Experiments show that two males singing at the same time and within hearing range of females divide the females between them, with the leading male having a slight advantage over the following one. It was also shown that two songs emanating simultaneously from the same loudspeaker are not any more or less effective in attracting females than the song of one male emitted by another speaker at the opposite end of the arena. Thus, males may attempt to interfere with other males' songs by making it more difficult for females to orient. The adaptive value of such simultaneous singing may lie in the ability of a male to cause females to remain available a little longer.
Snowy tree crickets (Oecanthus fultoni) synchronize their chirps in such a way that a tree full of males can be perceived as a single rhythmically pulsating unit (Walker, 1969). The adaptive value of synchrony appears to be that a male by synchronizing reduces the interference of a neighbor with his chirp rhythm, the component of the song most significant to females searching for males. Whether or not synchrony also facilitates attraction of males from outside the chorus is problematical. If several males singing in unison increase their quota of females as against solitary or nonsynchronizing males, they can be viewed as cooperating with one another.
Synchrony between pairs of males depends on auditory stimuli, and the lead between males may change frequently. The mechanism of establishing synchrony involves either a temporary lengthening or shortening of both chirp intervals and chirp lengths by the individual attempting to synchronize with another male or with a chorus (Walker, 1969). The occurrence of lengthening or shortening depends on the phase relations between his song and that of the chorus (Fig. 6).
Alexander (1975) observed synchrony in two katydid species, but in both species it is evidently rare. On the Ohio State University campus he observed a very dense population of Orchelimum vulgare synchronizing in the same bed of tiger lilies day after day. Evidently the species synchronizes only under conditions of high density. In Neoconocephalus ensiger synchrony is also rare. In this species, males only synchronize at low temperatures when the chirp rate is greatly slowed down. Alexander (1960) reasons that:
It scarcely seems likely that these males have been selected to synchronize when the conditions under which they can do so effectively are rarely encountered. Rather, their songs appear to become synchronizable at very low temperatures as an incidental effect of their structure at more usual singing temperatures.
What songs are synchronizable? According to Alexander (1960), for songs to be synchronizable they should contain “a precise or highly uniform chirp or phrase rate within the range of two to five per second. . .”
There may be some interesting causal relations between synchrony (temporal clustering) and aggregation of individuals (spatial clustering) which need to be explored. Dense aggregations of males may be environmentally imposed (high overall population densities or the forced aggregation of males on resource patches), or males may voluntarily form aggregations. Synchrony is only possible when interindividual spacing is greatly reduced, but under those conditions it is expected to arise only when song elements are synchronizable and when there is some clear advantage to the participating members to retain a conspicuous rhythm. It is even conceivable that low synchronizability would sometimes impede the evolution of voluntary aggregation if the temporal rhythm were an important component of the signal. (Readers should consult Alexander, 1975, for a lenthy discussion of this and related points.)
In Goiania, Brazil, I listened to a species of tree cricket whose solitary song was superficially similar to that of O. fultoni. Adjacent males did not synchronize their chirps, but alternated instead. And, by roughly halving their chirp rate, they nearly maintained their original chirp rhythm. Shaw (1968) examined in detail a mechanism of chirp alternation in the katydid Pterophylla camellifolia, where males singing alone have a faster chirp rate than do males alternating with one another (Fig. 7). Most interactions consist of the entrainment of each katydid to a slower chirp rate because of inhibition by the other individual, plus intermittent escapes from entrainment. Alternation can be disrupted if the leader begins to solo before the termination of the follower's chirp.
In some grasshoppers, alternation occurs between aggressive songs of two males. When the pulse rate is very rapid (on the order of twenty or more per second), alternation occurs between successive songs, but when the gaps between pulses are great, alternation between pulses is possible (Otte, 1970).
Signals that promote spacing of individuals are known in crickets (Alexander, 1961), conocephaline katydids (Morris, 1971), and Acrididae (Otte and Joern, 1975). Among gryllids, territorial defense and the signals that result in spacing are selected only in certain ecological situations (Alexander, 1961). The most strongly territorial species are those that live in burrows or on the ground. No arboreal species is known to defend territories, but some spacing may occur. Territorial defense is more prevalent among species that are sedentary and located in defensible regions, and it may be more likely to develop in predaceous species that are already equipped for attacking other insects. Morris (1971) describes aggressive interactions in conocephalines, where males do not restrict themselves to a given site, but attempt to clear their surroundings of other singing males.
The only territorial acridid known is a gomphocerine species, Ligurotettix coquilletti, which defends bushes of Larrea divaricata (creosote) in the Sonoran Desert (Otte and Joern, 1975) (Fig. 8). Proximate resources over which males fight are medium to large bushes. Ultimately such bushes are probably preferred because it is to these that females are most strongly attracted. Males of Ligurotettix begin singing in the morning and continue all day and into the night. Males that enter an occupied bush are sought out and attacked by the resident male, especially at low grasshopper densities, when the ratio of males to suitable bushes is low. By behaving territorially a male can, through defensive behavior, prevent mate theft and thereby increase his quota of females. Under high densities some abatement in territorial defense seems to set in, and it is possible to find bushes with several singing males. (See Otte and Joern, 1975, for a discussion of density-dependent aggressiveness).
Males of Goniatron planum, a close relative of Ligurotettix, are variably territorial in much the same way. In west Texas (near Marathon) host bushes (Florensia cernua) were small and males readily flew from the bushes when approached. In this region a single male sings in any one bush, but bushes frequently contain one to four silent males as well. We are uncertain of the tactics of these silent males, but it is clear they are in a good position to intercept females attracted to the bush. By remaining silent they might either reduce the chances of provoking attacks by the signaling male or they may be attempting to locate females without themselves having to expend energy in calling. Singing males failed to attack silent ones even though they seemed to be aware of them. The lower aggressiveness may have been due to the high density of males. In northern Mexico a sparse population of Goniatron was found in large bushes. Here males attacked one another when placed into the same bush, and no satellites were found.
Spooner (1968) describes attempted theft in several species of phaneropterine katydids. Males of this group are particularly susceptible to being robbed because females answer male songs during pair formation. Scudderia texensis males possess two female-stimulating songs, slow-pulse songs (sps) and fast-pulse songs (fps). Females respond to the low-intensity fps by approaching the male without answering, and to loud fps by stopping. When quite near the male, females respond to the sps by answering. The singing male then searches and approaches the female. When females answer males, other non-singing males approach and attempt to steal copulations. However, singing males may have evolved a mechanism for circumventing theft. After the female has answered, but is still some distance from him, the singing male reduces the intensity of fps, thus causing the female to become silent and to approach still further, where he is more likely than the nonsinging males to find her.
In Syrbula admirabilis nonsinging males become highly excitable when they hear a female answering the song of another male. They rush about searching for her, occasionally reaching her before the calling male does. Theft in S. admirabilis and S. fuscovitta also occurs during courtship, which in these species is quite prolonged (Otte, 1972a). Courting males inadvertently attract other males, who assemble about the courting pair. When the female signals receptivity, noncourting males make a sudden rush, attempting to mount the female. Courting males sing very softly, perhaps to reduce the chances of theft to a minimum. Males also court much more vigorously when other males are about. In S. fuscovittata, wing flipping is usually absent when a male courts a female alone but is prevalent when other males are nearby. One can speculate that increased intensity advertises the fitness of the performing male and insures that the female perceives who the real performer is.
Cade (1975) has shown that singing cricket males (Gryllus integer) are frequently surrounded by silent (satellite) males, who may intercept females attracted to the calling males. He feels that such theft is at least partially accounted for by the fact that singing males are more prone to parasite attack (see p.337). Of course the selective effect of the parasitoids would depend on how soon they incapacitate a male. One might even predict that it would be advantageous for the first batch of larvae to be deposited on a male to silence him, thus ensuring the larvae of a greater share of the resource. Such a mechanism may operate in a cicada parasitized by the sarcophagid fly Colcondamyia auditrix, where the larvae silence singing males (Soper, Shewell, and Tyrell, in Cade, 1975).
Clustering in Space and Time
Dispersion of signaling animals in space varies from highly dispersed to strongly clustered. Likewise, signals themselves may be highly independent of one another or clustered in time. Describing an animal's position with respect to these two axes is of some interest (Fig. 9). I have placed no animals near the horizontal axis be cause some degree of mutual attraction should always be beneficial. An individual who finds that he is the only male displaying in an area may take this to mean that females are also absent, and consequently he may seek the location of other males. Lowering the density may have the effect of moving all species toward the origin. The models presented below are attempts to account for clustering in time (models 1, 2, and 3) and in space (models 4 and 5). Recall that clustering in time is of two kinds: unsynchronized chorusing and synchrony.
THE INTERFERENCE REDUCTION MODEL
The precise synchrony in tree cricket songs may serve to reduce song interference. A male in effect synchronizes with another because in this fashion the other male interferes only minimally with the species-specific rhythm of his own song. Animals that synchronize for this reason would not be expected to form spatial aggregations, but if they are spatially aggregated for other reasons, they may be under greater pressure to synchronize.
THE INTERFERENCE OR INTERLOPING MODEL
Suppose the following conditions are met: ( 1 ) Alone male in the presence of receptive females attracts all receptive females capable of hearing him. (2) Each of two matched males singing independently of one another gets half the females(or has a 50 percent chance of attracting any given female). (3) Singing during the call of another male reduces the effectiveness of that particular call. (4) A male that occasionally sings alone, but interferes every time the other male sings, attracts more females than the other male. The following strategies by males might then obtain: (1) It is best to be alone among females (a tactic successfully employed by territorial species). (2) If other calling males are nearby, it would be advantageous for a male to call during silent periods, so that the location of his call remained clearly defined. (3) It would also be advantageous to call during the call of another male so as to interfere with that male's ability to attract females. Two males interacting and utilizing the same tactics will each attempt to sing alone and, whenever possible, to interfere with each other's singing. The inevitable result in a closely matched pair of males is an overlap of songs with perhaps some alternation in who sings first (Fig. 10).
THE ANTIPREDATION MODEL
This model simply says that a predator might have greater difficulty in locating one individual when many become active simultaneously than it has in locating an isolated individual. If prédation were the primary force causing chorusing, males would not be expected to aggregate. The case of Aerochoreutes carlinianus discussed above fits this model.
THE LEK MODEL
Many animals display in groups (leks) rather than individually (Alexander, 1975). Such leks are aggregations of males mutually attracted to one another (in contrast to passive aggregations discussed below). Since degrees of aggregation vary widely, and since at some level all organisms are aggregated, one may have difficulty deciding in any particular case whether individuals have been attracted to one another. The following model might explain why males sometimes aggregate and sing simultaneously: Suppose that in an imaginary field receptive females search randomly for displaying males. Suppose also that once they are perceived by females several males acting together attract more than their share of females because of one or more of the following factors: (1) Their area of influence is larger and is also more likely to be encountered since it subtends here a larger angle (Fig. 11). (2) The surface of attraction is greater. (3) The larger area of influence is less likely to be overshadowed by a smaller area. (4) Two animals together constitute a supernormal stimulus and hence have a greater probability of attracting mates. Under these assumptions, one can construct the relations shown in Fig. 12. If a group of males attract more than their share of females, it becomes advantageous for males to associate with other males. When numerous males are attracted into the group, then the number of females attracted per male drops back to a low level, but it continues to be advantageous to aggregate if females refuse to approach lone males. If different females are restricted to different parts of the field several clusters of males could develop.
PASSIVE AGGREGATION MODEL
Males of some species are aggregated not because they are attracted to one another but because some aspect of the environment forces aggregation. Spooner (1968), for example, finds that groups of Inscudderia strigata males may be aggregated on their food plant, Hypericum fasciculatum. That the aggregation is passive is suggested by the fact that nymphs are also found aggregated on these plants. It is clear, however, that when forced together males behave differently than when they are alone.
Control of Signaling
NEUROMUSCULAR CONTROL OF EMISSION
Neuromuscular aspects of signaling have been analyzed rather extensively in crickets and grasshoppers. I can merely outline some of the findings of several lines of research.
Experimental work indicates that sound-producing movements are the expression of interaction between a series of thoracic muscles whose activities are coordinated centrally in ventral nerve ganglia. The motor activity is evidently generated by a small number of neurons that control the basic rhythm and coordinate the stridulatory muscles and that are little influenced by phasic feedback. In some species, the motor program is extraordinarily complex. Species in the European genera Gomphocerippus and Myrmeleotettix and the North American genus Syrbula utilize five to seven body parts and perform ten to eighteen recognizably different movement patterns (Fig. 13). The functional aspects of complex courtship are discussed elsewhere (Otte, 1972a, 1975). In G. rufus hind femora, palpi, and antennae move synchronously, while the forepart of the body is raised and the head is moved from side to side periodically (Loher and Huber, 1966; Eisner, 1971). Courtship in G. rufus is normally released when a female is detected visually or acoustically. Blinded males may court after hearing a female's stridulation, but blind and deaf males may also court after tactile stimulation or "in vacuo." Thus different inputs can trigger the mechanisms that produce the motor output (Eisner, 1971). Several experimental techniques, including removal and immobilization of body parts, making central and peripheral lesions, and implantation of electrodes, have yielded what appear to be reasonably comprehensive pictures of neural control. It has been possible to implant as many as sixteen electrodes into freely moving grasshoppers without significantly affecting their behavior (Eisner, ms.).
In G. rufus coordination between the hind legs and the head changes in different subsequences. The motor systems of the head and hind legs are strongly coupled, and each chirp is accompanied by a burst of head muscle activity (Eisner, 1971). The motor pattern underlying courtship behavior in this species is programmed mainly in the CNS. Peripheral input has a minor influence on the quality of muscular activity. During the whole behavioral sequence, supra- and sub-esophageal ganglia and all three thoracic ganglia send coordinated motor commands to the muscles of the head, antennae, palpi, and legs. Participating interneurons are distributed over the cephalic and thoracic part of the CNS and synapse in all ganglia. These interneurons may determine not only the course patterns, i.e., the onset of different subsequences, but also the timing of chirps. The fine pattern, i.e., the pulse pattern within single chirps, is thought to be organized by local thoracic networks, which are driven by those interneurons. The hypothesis that individual command fibers time the start of the different parts of the motor patterns is appealing because gradual transitions between subunits cannot be observed.
The overall synchrony between body parts displayed by Syrbula and Gomphocerippus suggests how complex courtship patterns might have evolved. Increased complexity might have been produced by increasing the influence of single command fibers or various motor units. It has been postulated that the process may have occurred as follows: Initially only the hind legs were employed in signaling, but slight movements of other appendages were produced, perhaps because nervous commands loosely coupled with other motor units indirectly affected other motor patterns as they traveled from their origin to their destination in the third thoracic ganglion. Coupling between command fibers and various motor units that control palpi, antennae, and wings could have been under selection to improve if individuals displaying more movement were favored by females over individuals displaying less (Otte, 1972a).
In crickets, brain commands may initiate or trigger song production, but the sequence and intensity of brain stimuli are variable. Commands affecting calling, aggression, and courtship are transmitted via separate fiber systems. Sound patterns themselves appear to be organized in the thoracic nervous system, since the calling song can still be generated when the head is removed (Otto, 1971). The calling song in Gryllus campestris, comprising a series of chirps, could result from the activities of a slow (3-4 Hz) thoracic oscillator that determined the chirp rate and a fast (30 Hz) oscillator that determined the pulse sequence. The structure of the oscillators is not known, but they appear to be located in the pro- and metha-thoracic ganglia. The rhythm of the slow oscillator coincides in G. campestris with the respiration cycle and with muscles involved in flight and walking; hence they are also believed to be influenced by these oscillators (Kutsch, 1969). But chirp and pulse rate vary widely among crickets, so the relationship between chirp and respiration cycles may be fortuitous or at least of no great consequence.
NEURAL CONTROL OF RECEPTION
Experiments by Stout and Huber (1972) indicated which components of male cricket chirps are transmitted to the female's brain. Recording from neurons in the cervical connectives (between subesophageal ganglion and brain) showed that several types of units are involved. Chirp coding units respond to entire chirps and therefore transmit information on chirp duration, while pulse coding units respond only to individual pulses (Fig. 14). In addition there exist units that are variably responsive to chirps and fire only during respiratory cycles.
Cricket species with slightly different song parameters frequently coexist in the same habitat. While two species rarely if ever have the same song at the same temperature, there may be some overlap in songs over a range of temperatures (Walker, 1957). Thus, a female cricket may find the song of her own species on a cooler night to be the same as that of a related species on a warmer night. Since female responses are also temperature-dependent and a female is likely to be at the same temperature as the male she hears, this does not present a problem. But, given that the connectives to the brain transmit various song parameters, how is recognition of the song that is appropriate to a given temperature achieved? According to the Stout-Huber model, respiratory cycles, which are temperature-dependent, could act as timers against which song parameters are compared. Thus, the ratio of variable bursts per respiration burst could be the important cue. The model appears attractive in the case of G. campestris but suffers in at least two regards: in many cricket species songs are continuous trills with very fast pulse rates, and are therefore tenuously coupled to respiration cycles; and the coupling between respiration cycles and decision-making central neurons in females seems on intuitive grounds rather loose: a better mechanism might obtain if the receiving template itself were temperature dependent.
NEUROENDOCRINE CONTROL OF SIGNAL EMISSION AND RECEPTION
The principal neuroendocrine elements controlling the onset of sexual activity and receptivity in Orthoptera are the neurosecretory cells (NSC) of the pars intercerebralis in the forebrain and certain cells lateral to the pars (Barth and Lester, 1973). Axons connect the NSC to the corpora cardiaca (cc), a pair of structures behind the brain. Attached to the posterior tip of the cc are the corpora allata (CA), which are also innervated by neurosecretory axons. The role of these various structures has been investigated in only a handful of species and appears variable among orthopteran taxa (Barth and Lester, 1973). In Locusta migratoria NSC comprise C and A+B cell types. C cells control sexual behavior directly by releasing hormones into the blood and indirectly through their effect on the CA, which in turn influences the intensity of mating activity (Pener, Girardie, and Joly, 1972). The influence of the CA on the behavior of Syrbula fuscovittata and Gomphocerippus rufus appears tighter than in Locusta. Allatectomy shortly after the imaginai moult prevents females from ever becoming sexually receptive (Loher, 1962, 1966).
Orthopteran neuroendocrine systems influence communication in two ways: indirectly, they influence the maturation of the gonads and accessory glands, which then cause individuals to engage in sexual activity; directly, they control the nature of behavior and chemical stimuli, which act as signals. Much variability exists between the few species examined in detail and even between the sexes of one species, making it difficult to set forth generalizations valid for large groups. In ovoviviparous cockroaches, female receptivity is correlated with oocyte maturation, but the CA appears to have far less influence on female receptivity than it does in grasshoppers. Allatectomy in the early stages in the grasshopper G. rufus results in females that never become receptive (it does not influence sexual behavior of males), but in some cockroaches allatectomy merely delays the onset of receptivity (Roth and Barth, 1964). Pheromone production in roaches is controlled by CA juvenile hormone, which stimulates female pheromone production.
Sperm of orthopteran males is generally transferred to females in a packet, the spermatophore. Insertion of the spermatophore into the genital tract may cause females to become unreceptive to male signals. In the grasshopper G. rufus, females become unreceptive to male signals immediately after mating. Cutting the nervous connection to the duct that receives the spermatophore causes females to copulate repeatedly, indicating that mechanical stimulation of the duct inhibits receptivity (Loher and Huber, 1966). In contrast, in the oedipodine grasshopper Chimarocephala pacifica the first male to copulate successfully leaves a spermatophore, but females remain receptive after copulation. Subsequent males are prevented from mating by the previous spermatophore, which acts as a block to further copulation. Between twenty-four and four hours before a female is to oviposit she becomes unreceptive, and an hour after ovipositing she is ready to mate again (Loher and Chandrashekaran, 1970). Stimuli that inhibit receptivity in roaches are also apparently mechanical in nature. Insertion of the spermatophore into the female bursa copulatrix inhibits further receptivity. Inhibition can also be produced artificially by inserting glass beads into the bursae of unmated females (Roth, 1962, 1964).
GENETICS AND DEVELOPMENT OF CRICKET SONG AND RESPONSE TO SONG
Bentley and Hoy (1970) have examined the appearance of song and flight motor patterns during development in the cricket Teleogryllus commodus. At hatching, chirpeliciting neural circuits are not yet functional, but elements of the motor patterns gradually emerge in an ordered sequence over the course of the later nymphal stages. The circuit is completed before the molt to the adult stage. The last instar nymphs are able to generate nearly complete motor patterns for aggressive and courtship songs and portions of the calling song, but inhibition from the brain prevents the patterns from being elicited until after molting. Song patterns appear to be under genetic control and to be well isolated from environmental influences (Bentley and Hoy, 1972). In general, when parental song characteristics differ significantly, the hybrid characters are about intermediate between parental types. Each song parameter is evidently controlled by several genes. Also, song parameters may be sex-linked, since reciprocal hybrid songs (of cT A x 9 B and cf B x $ A crosses) are quite different (Fig. 15). With each characteristic the song of the hybrid is more similar to the male of the maternal species than of the paternal species. Since some characteristics are sex-linked and others are not, genetic control of song is also multichromosomal.
Hoy and Paul (1973) have also examined the genetic control of female responsiveness to male calls. The genetic differences that cause changes in male songs also appear to alter female responsiveness. Female responses were measured using a tethered female walking along a Y-maze globe held suspended beneath them. Recordings of the males of the parent species and of hybrid males were played through speakers to the right and left of the suspended females. The behavior of the females at choice points with respect to the sound source was measured by their turning tendency. Results indicated that female hybrids are more strongly attracted to the hybrid song than to the parental songs. Thus, it appears that the genetic coupling of the male's song generator and the female's sensory template is indeed close (see Alexander, 1962b).
A Partial Guide to Orthopteran Communication
Systematics. Alexander, 1957, 1960a, 1962a, 1967; Bigelow, 1960, 1964; Blondheim and Shulov, 1972;Jacobs, 1953; Leroy, 1966; Otte, 1970; Perdeck, 1958; Rentz, 1973; Shaw and Carlson, 1969; Walker, 1957, 1962.
Evolution. Alexander, 1960a, 1962a, 1975; Alexander and Otte, 1967a; Hill, Loftus-Hills, and Gartside, 1972; Jacobs, 1953; Otte, 1970, 1972a, 1975a; Perdeck, 1958; Spooner, 1968; Walker, 1957; Walker, 1962, 1974a; West and Alexander, 1963; Cade, 1975.
Comparative Ethology. Alexander, 1961, 1967, 1968, 1975; Alexander and Otte, 1967b; Barth, 1970; Barth and Lester, 1973; Busnel, 1954; Dumortier, 1963; Faber, 1953; Heiligenberg, 1966; von Hörman-Heck, 1957; Jacobs, 1953; Leroy, 1964; Loher and Chandrashekaran, 1970; Morris, 1971; Nielsen, 1971; Nielsen and Dreisig, 1970; Otte, 1970, 1972, 1975a; Roth, 1962, 1964; Shaw, 1968; Spooner, 1968; Willey and Willey, 1969, 1970, 1971; Young, 1971.
Defense. Alcock, 1972; Eisner and Meinwald, 1966; von Euw et al., 1967; Regen, 1913; Robinson, 1965, 1968a, 1968b, 1968c, 1968d, 1969a, 1969b; Rowell, 1967; Walker, 1964; Cade, 1975.
Environmental Control and Rhythms. Alexander and Meral, 1967; Cloudsley-Thompson, 1953; Cymborowski, 1973; Loher, 1957; Walker, 1962.
Neuromuscular Control. Bentley, 1969a, 1969b; Bentley and Kutsch, 1966; Busnel and Burkhardt, 1962; Dathe, 1972; Dietmar, 1971; Elder, 1971; Eisner, 1971; Huber, 1960, 1962, 1963; Kutsch and Huber, 1970; Kutsch, 1969; Leroy, 1964; Loher, 1966; Loher and Huber, 1966; Moss, 1971; Nocke, 1972; Otto, 1968, 1971; Shaw, 1968; Stout, 1970, 1971; Walker, 1969.
Physiology of Hearing. Adam, 1969;Busnel, Dumortier, and Pasquinelly, 1955; Haskell, 1956; Lewis, Pye, and House, 1971; McKay, 1969, 1970; Michelsen, 1966, 1968; Nocke, 1972; Popov, 1971; Regen, 1913; Rowell and McKay, 1969; Shaw, 1968; Stout and Huber, 1972; Suga, 1966; Suga and Katsuki, 1961; Zaretsky, 1971.
Hormonal Control. Barth, 1965, 1968, 1970; Barth and Lester, 1973; Blondheim and Broza, 1970; Highnam and Haskell, 1964; Loher, 1962, 1966; Loher and Huber, 1966; Pener, 1972; Pener, Girardie, and Joly, 1972; Pickford, Ewen, and Gillott, 1969; Roth, 1962; Roth and Barth, 1964.
Mechanisms of Signaling. Alexander, 1960b; Bailey and Broughton, 1970; Broughton, 1964; Kevan, 1954; Menon and Parshad, 1960; Morris and Pipher, 1967; Nocke, 1971; Richards, 1973; Otte and Cade, 1976.
Genetics. Bentley and Hoy, 1970, 1972; Fulton, 1933; Hoy and Paul, 1973; Leroy, 1965; Nolte, 1968.
Group Effects. Chauvin, 1958; Ellis, 1963, 1964; Ellis and Hoyle, 1954; Norris, 1954, 1964, 1970; Thomas, 1970.
Development. Bentley, 1969b; Bentley and Hoy, 1970; Loher 1957.
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