Hugh Dingle

Migration Strategies of Insects

Physiological and ecological results from a variety of species are consistent with what seem to be valid general statements concerning insect migration. These are as follows: (i)During migration locomotory functions are enhanced and vegetative functions such as feeding and reproduction are suppressed. (ii) Migration usually occurs prereproductively in the life of the adult insect (the oogenesis-flight syndrome). (iii)Since migrant individuals are usually prereproductive, their reproductive values, and hence colonizing abilities, are at or near maximum. (iv) Migrants usually reside in temporary habitats. (v)Migrants have a high potential for population increase, r, which is also advantageous for colonizers. (vi)Both the physiological and ecological parameters of migration are modifiable by environmental factors (that is, phenotypically modifiable)to suit the prevailing conditions. Taken together, these criteria establish a comprehensive theory and adumbrate the basic strategy for migrant insects. This basic strategy is modified to suit the ecological requirements of individual species. Comparative studies of these modifications are of considerable theoretical and practical interest, the more so since most economically important insects are migrants. No satisfactory general statements can as yet be made with respect to the genotype and migration. Certainly we expect colonizing populiations to possess genotypes favoring a high r, but genotypic variation in r depends on the heritabilities of life table statistics, and such measurements are yet to be made (10, 53). The fact that flight duration can be increased by appropriate selection in Oncopeltus fasciatus, and the demonstration of additive genetic variance for this trait in Lygaeus kalmii, suggest that heritability studies of migratory behavior would also be worth pursuing. Most interesting of course, will be possible genetic correlations between migration and life history parameters. Also, migration often transports genotypes across long distances with considerable mixing of populations. An understanding of its operation therefore carries with it implications for population genetics, zoogeography, and evolutionary theory. Finally, at least parts of the above general theory would seem to be applicable to forms other than insects. Bird and insect migrations, for example, are in many respects ecologically and physiologically similar. Birds, like insects, emphasize locomotory. as opposed to vegetative functions during long-distance flight; the well-known Zugenruhe or migratory restlessness is a case in point. Further, many birds migrateat nigt at a time when they would ordinarily roost(vegetative activity). Because their life spans exceed single seasons, bird migrants are not prereproductive in the same sense that insect migrants are, and hence reproductive values do not have the same meaning(but note that some insects are also interreproductive migrants). The situaion is complicated further by the fact that in many birds adult survivorship is virtually independent of age so that colonizing ability tends to be also (10, 54). Nevertheless, birds arrive on their nesting grounds in reproductive condition with the result that migration is a colonizing episode. It is also phenotypically modifiable by environmental factors, some of which, for example, photoperiod, influence insects as well (55). The similarities between birds and insects thus seem sufficient to indicate, at least provisionally, that the theory developed for insects applies also to birds with appropriate modifications for longer life span and more complex social behavior; comparisons between insects and fish (56) lead to the same conclusion. In birds especially, and also in other forms, various functions accessory to migration such as reproductive endocrinology, energy budgets, and orientation mechanisms have been studied extensively (55, 56). But there is need in vertebrates for more data andtheoy on the ecology and physiology of migratory behavior per se in order tobetter understand its evolution and its role in ecosystem function (5, 57). Migration in any animal cannot be understood until viewed in its entirety as a physiological, behavioral, and ecological syndrome.

Ecology and evolution of agonistic behavior in stomatopods

Stomatopods are marine crustacea which occupy cavities and burrows on most of the worlds tropical reefs and beaches. They are highly aggressive and defend their burrows using a raptorial appendage capable of inflicting lethal blows. Morphologically, the raptorial appendages of stomatopods can be divided into two types, spearing and smashing. Those species possessing the smashing appendage, which is the more potent of the two forms, show heavier telson armor, agonistic repertoires which are more intense and complex, and tend to occupy homesites such as cavities in rock or coral which are scarce as compared to the availability of burrow sites for spearers. Within a group of species from one habitat, the most aggressive is usually the most abundant, suggesting contest competition.

A statistical and information analysis of aggressive communication in the mantis shrimp Gonodactylus bredini Manning

Abstract 1. 1. The aggressive communication system of the mantis shrimp Gonodactylus bredini was studied. The data from 1-hr observation periods involving interactions between homosexual pairs were used to analyse the distribution of acts within individuals and the distribution of acts by one animal following the execution of a particular display by the other animal. 2. 2. When distributions following particular acts were compared with the overall distribution of patterns, most were found to be significantly different. This indicates that particular sequences of behaviour were likely to occur. In the case of inter-individual sequences most behaviour patterns elicited a specific change in the behaviour of a recipient animal; communication was thus taking place. 3. 3. The frequency with which aggressive interactions took place decreased with time. There were also changes in the overall distributions of the various acts. Both these changes occurred concurrently with the establishment of a dominant-subordinate relationship. 4. 4. In the inter-individual sequences specific acts were statistically ‘directive’ or ‘inhibitive’ toward other acts. Changes in the acts toward which these were directive or inhibitive also occurred over time and were correlated with the formation of a dominant-subordinate relationship. The establishment of such a relationship, therefore, seems to involve at least partially changes in response to specific behavioural acts. 5. 5. Values for information present per act, information transmitted per act, and information per interaction per individual were calculated for each time interval using the data from the interindividual sequences. Information present (mean 2·73 bits per display) is similar to that of hermit crab acts, but information transmitted per act (mean 0·78 bits) and information transmitted per interaction (mean 1·82 bits) were higher. The higher value for information transmission may be the result of adaptation to cavity living and to potential injury (from strikes) during aggressive interactions. 6. 6. The rate of information transmission ranged from 0·013 to 5·46 bits per sec, a range similar to but somewhat broader than that of hermit crab transmission rates. The maximum rate is over twice that of the honey bee and three times that of the fire ant. Whether or not this reflects either the type of information transmitted or the means of transmission is not known.

THE NATURE OF GENETIC VARIANCE INFLUENCING PHOTOPERIODIC DIAPAUSE IN A MIGRANT INSECT, ONCOPELTUS FASCIATUS

Female Oncopeltus fasciatus in long-day photoperiods lay eggs 15-20 days after their adult molt. Individual bugs transferred to short-day photoperiods extend age at first reproduction to about 80 days, but populations maintained in that same short-day condition show rapid decrease in extent of reproductive delay. Results from breeding experiments suggest that this decrease cannot be avoided, and those from parent-offspring comparisons for extent of reproductive delay in newly encountered short-day conditions indicate heritability of about 0.70. We suggest that the high level of additive genetic variance influencing female sensitivity to photoperiod functions as a genetic rheostat and results in eggs of the fall field population being optimally distributed geographically in the face of varying association between photoperiod and temperature. Genetic variance for the response is probably maintained by seasonally reversing selection pressures and by substantial genetic interchange among populations of this long-distance migrant.

The aggressive and territorial behaviour of the mantis shrimp Gonodactylus bredini manning (crustacea: stomatopoda).

In the stomatopod Gonodactylus bredini when two individuals of the same sex encounter each other, they exhibit marked aggressive interaction. One animal usually becomes dominant over the other in approximately 10-20 minutes, and the frequency of aggressive acts declines steadily during the course of an hour. Dominance is influenced, although not necessarily determined, by size, stage in the reproductive cycle (females), stage in the moult cycle, and individual differences in level of aggressiveness. Aggressive behaviour involves a variety of fixed motor actions including spreading of the raptorial meri, antennular flicking, lunging with the meri spread, coiling of the body, and attacking and striking with the raptorial second maxillipeds. The meral expansion reveals conspicuous white spots on the inner surfaces of the raptorial meri and also silver streaks along the borders of the small first maxillipeds while the remaining maxillipeds are extended to form a rosette. This posture seems to serve as a threat. Because of structural modifications of the cuticle, especially on the dorsal surface of the telson, and of the frequent assumption of a coiled position during aggression, the strikes of attacking animals are seldom severely injurious. In nature G. bredini occupies cavities in rocks in shallow water from just below low tide line, and the aggressive behaviour is well suited to the defence of these cavities. The meral spread, for example, effectively fills the entrance and hence blocks encroachment by an intruder. The occupant may leave briefly to strike another animal or attack prey, but quickly returns to its home. The behaviour with respect to cavities thus seems to be territorial with the territory including the occupied rock plus a small area surrounding it. A resident animal undertakes several housekeeping activities including cleaning its chamber and closing the entrances at night with small bits of debris and reopening them the following morning. Only a particularly aggressive and dominant animal is capable of dislodging a cavity occupant, but eviction takes place rapidly when it does ocrur. Aggressive displays from outside alone seem insufficient to cause a def ender to leave a chamber ; takeover occurs when the attacker enters the cavity and drives the defender out.

Phenotypic and Genetic Covariance Structure in Milkweed Bug Life History Traits

Variation in life history characteristics is critical for any species because it influences the timing and extent of population growth. When the variability is caused by gene differences among individuals, changes in population growth differ among genotypes. Gene frequencies will change at loci with effects on life history traits, and selection and microevolution of life history traits should occur. However, if many life history characteristics vary among individuals, there may be pairwise covariance caused, in part, by genetic linkage disequilibrium or by pleiotropy. Change caused by selection acting on additive genetic variance for any single character will be accompanied by change in other genetically correlated characters. Darwin (1859; see Darwin 1958) commented on this situation in reference to domestication and artificial selection, which he said “will almost certainly modify unintentionally other parts of the structure, owing to the mysterious laws of correlation” (p. 35). The importance of genetic correlations in the context of life history variables and natural selection is the primary motivation for the work reported here.

FUNCTION OF MIGRATION IN THE SEASONAL SYNCHRONIZATION OF INSECTS

Seasonal migrations of insects can be roughly divided into those within the temperate zone, those within the tropics, and those between the tropics and temperate areas. Temperate migrations often involve movements to and form diapause sites with correlated seasonal cycles in reproductive physiology. Many temperate migrants have apterous or brachypterous generations whose adaptive significance is not always clear, diapause may also be involved in tropical migrations, but the cue is likely to be food or moisture limitation rather than photoperiod. Interestingly, the capacity to diapause may be a prerequisite for migration into the temperate zone by tropical species: the two behaviours occur together in several migrants. An interesting subclass of tropical‐temperate migrants are “pied‐piper” species which apparently have no return movement: the action of natural selection in these cases is unclear. In all cases migration and diapause are intimately involved with other aspects of life histories because they allow choices of where and when to breed. The elucidation of the contributions of genetic and environmental variance to insect migration strategies is an important problem for entomologists.

Diapause in a migrant insect, the milkweed bug Oncopeltus fasciatus (Dallas) (Hemiptera: Lygaeidae)

SummaryOncopeltus fasciatus exhibits a facultative reproductive diapause which is triggered by short photoperiods. The duration of the triggering photoperiod (the “critical photoperiod”) is a function of the environmental conditions under which populations are reared. Some individuals are apparently sensitive to critical photoperiods in the early instars, but maximum sensitivity, indicated by 100% diapause, occurs in the late 5th instar during development of the pharate adult. Sixty per cent retain sensitivity into the first five days of adulthood. Temperatures of 27°C can prevent diapause. Once initiated, diapause can be broken either by raising the temperature or increasing the day length; it is broken abruptly, for once oviposition starts, it proceeds at maximum rate. Diapause contributes significantly to a migratory strategy in the temperate parts of the range of O. fasciatus and probably does so in the tropics as well.

Aggressive Behavior in Stomatopods and the use of Information theory in the Analysis of Animal Communication

The study of animal communication is one of the most active areas within the discipline of ethology. Yet in spite of a seemingly ever-accelerating rate of publication, there is still a paucity of quantitative studies. The typical analysis of communication presents a description of certain stereotyped movements or “displays” and attributes a communicatory function to these on the basis of observed or presumed changes in behavior. However, until quantitative studies are made of such movements, and changes in frequency as a result of interactions are noted, no function can be determined with certainty. Part of the problem with quantitative analysis has been that no single method has been applicable to all kinds of behavior, and the search for appropriate analysis for a given situation is often tedious. There are, nevertheless, some interesting studies available (e.g., Baerends et al., 1955; Dane et al., 1959; Wiepkema, 1961; Mittelstaedt, 1964; Stokes, 1962; Nelson, 1964, 1965; Shaw, 1968; see also Hinde, 1966), and the reader should refer to these for descriptions and discussions of the various methodologies.

Population Crosses and the Genetic Structure of Milkweed Bug Life Histories

The fitness of a given phenotype is a direct consequence of the schedule of behavior, fecundity, and mortality that constitutes a life history. For this reason life histories are major adaptations of unique importance to general Darwinism (Bell 1980). As with other complex adaptations life histories consist not of single characters, but of sets of phenotypic traits that covary and function together (Frazetta 1975). Such sets often are referred to as “strategies” or “tactics” and much theoretical and empirical effort has been devoted to attempting to understand the evolution of the complex known as a “life history strategy” (Bell 1980, Stearns 1976, 1977). Births and deaths are most closely related to fitness and have drawn most of the attention, but behavior is also an important component of life histories, especially as it confers flexibility on where and when to breed (Istock 1978 and Chapter 1, this volume, Nichols et al. 1976, Taylor and Taylor 1977, 1978). Two important elements of insect behavior are migration and diapause (Dingle 1981, Solbreck 1978), and we consider them in our discussion here, along with the more traditional life table statistics that they influence.