Adaptation, compromise, and constraint: the development, morphometrics, and behavioral basis of a fighter-flier polymorphism in male Hoplothrips karnyi (Insecta: Thysanoptera)

Summary

Males of the colonial, wing-polymorphic thrips Hoplothrips karnyi (Hood) fight each other with their forelegs in defense of communal female oviposition areas. In this study, males were reared individually under varying conditions of food deprivation to investigate the developmental cues used in morph determination and the relationships between wing morph, developmental time in each instar, propupal weight, and five adult morphological characters associated with fighting ability and dispersal ability. Males deprived of food for five days midway through the second (final) larval instar had smaller propupal weights and were more likely to develop wings than males deprived of food in the first instar or control males. However, the mean propupal weight of all males that developed wings was not significantly less than that of wingless males. Wing morph of female parents had no measurable effect on this character in the offspring. Wingless males possess relatively larger fore-femora and prothoraces than do winged males, but winged males possess relatively larger pterothoraces (Fig. 1). Behavioral observations of wingless and winged males of similar weight as propupae showed that wingless males won fights and became dominant in oviposition areas. Thus, a trade-off exists between characters associated with male fighting and dispersal ability. The cost of wings, in terms of fore-femora size and prothorax size, increased with propupal weight. Wingless males that developed in the experimental treatment that produced a high proportion of winged males were relatively small in size, and were intermediate in body shape with respect to winged males and other wingless males (Fig. 2). This shape intermediacy indicates that there may be developmental constraints on alternative tactics of resource allocation. Total developmental time varied between wing morphs, but was not correlated with propupal weight or adult morphological characters of winged or wingless males. For wingless males that developed in the treatment that produced a high proportion of winged males, adult morphological characters were negatively correlated with the duration of the second instar. This correlation suggests that the development of small wingless males involves a compromise between the benefits of large adult size and the costs of prolonging the second instar to increase the probability of becoming larger.