MALE COURTSHIP BEHAVIOUR, FEMALE RECEPTIVITY SIGNAL, AND SIZE DIFFERENCES BETWEEN THE SEXES IN (HYM., CHALCIDOIDEA ), AND COMPARATIVE NOTES ON OTHER CHALCIDOIDS

by

J. VAN DEN ASSEM (Department of Zoology,University of Leiden, The Netherlands)

SUMMARY

In the evolutionary history of the Pteromalidae, the position of the courting male has moved from the rear (the area where copulation will take place) to the front (the area where an exchange of stimuli between partners may occur most efficiently). The shift of position is related to a number of other developments, one of which is the antennal receptivity signal by the female. The effect of this signal is that the male stops courting, leaves the frontal position by backing up on the female, and establishes a genital contact (VANDEN ASSEM, 1974). In the present paper I have argued that, once a frontal position for courtship has been realised, and female receptivity can be perceived by the male at the front, a further reduction of the size of the male seems advantageous. Small males are found in many groups of Chalcidoidea including the Pteromalinae. In several species of this subfamily the size differences between the sexes are considerable. In a gregarious parasite, small males take a smaller share of the host which leaves more food for their sisters whose average reproductive success seems to be pro- portional to their longivity (which, within limits, is proportional to body size as adult). Small-sized males are likewise advantageous in solitary parasites: a small host (e.g. a young instar) may be insufficient as a food source for a female offspring, to complete its development, but sufficient for a male. A discriminative parasite will lay unfertilised eggs on small hosts and fertilised eggs on large hosts. In this way the number of hosts available in a certain area to a parasitising parent becomes greater and hence searching for hosts more successful. (It is true that a small instar will become larger with time, if left undisturbed, but a delay of egg-laying will certainly result in losses due to the competitive activity of other parasites, conspecifics or otherwise). Additional advantages of small males are the more rapid development, which ensures a timely presence where females come to emerge a little later. Further, a smaller body size had no effect on the duration of a courtship sequence required to induce receptivity in virgin females. The reduction of male body size also has negative effects: a limitation of the num- ber of possible inseminations, a reduction of longivity, and an increased vulnerability to competing conspecific males (who may "steal" a copulation) are the most obvious ones. The advantages and disadvantages of size reduction were discussed. Courtship behaviour of Pteromalus venustuswas described and two other species, Pteromaluspuparum and were dealt with in detail because of their peculiarities. In P. puparum the antennal movements made by the receptive female no longer seem to serve a signal function (although they are completely 536 identical to those of other Pteromalinae). I argued that this ineffectiveness is a secondary phenomenon, due to the peculiar behaviour of the male which may be an adaptation to oppose strong competition for females. In M. uniraptormales are extremely rare, and they no longer serve a reproductive function. Virgin females lay diploid eggs which produce female offspring exclusively. Morever, males (who may arise as progeny of females who were exposed to high temperatures during oviposition) appear to have retained a normal courtship. Conspecific females, however, were never seen to respond to it, but females of a related species (M. raptorellus) did signal receptivity when courted by uniraptor males. Fertilisation did not occur because uniraptor-mated raptorellus females pro- duced only male offspring. A few courtship repertoires of Encyrtidae were described. To all appearances the evolution of courtship behaviour of Pteromalidae, Encyrtidae, and Eulophidae exhibits parallel developments.

Earlier (VAN DEN AssEM, 1974) I presented data supporting the hypothesis that in the evolutionary history of the Pteromalidae (a large family of parasitic ) the courtship position of the male moved from the rear of the female to the front (i.e., from an area where copulation will take place to an area where an exchange of specific signals probably leads to less ambiguity in the identification of the partner). The development of an antennal signal by the female to in- dicate the moment of overt sexual receptivity is related to the positio- nal shift of the male. In this respect I emphasised that there seemed to be a need for a receptivity signal in species with small-sized males. Such species are common in the Pteromalinae, the largest subfamily of the Pteromalidae. It has not been possible to categorize the Pteromalinae based on morphological characters alone (GRAHAM, 1969). I have shown pre- viously (VAN DEN AssEM, 1974) that this group can be defined unam- biguously if a set of behavioural characters is used. These characters include the position of the male during courtship, some general properties of his motor patterns, and the type of signal produced by the female when ready to copulate. Since the 1974 publication I have collected additional data relevant to these problems. Below I present some arguments suggesting that the size of the male was reduced after the shift of male position in court- ship occurred. Or better, perhaps, that size was reduced further. For reasons explained below, natural selection has probably always favoured those males who complete development in (slightly) less time than females. A (slightly) shorter developmental period is correlated with a smaller size. Males being (slightly) smaller than females seems to be the rule in the parasitic Hymenoptera. However, in those groups where the courtship position of the male shifted to the front, and female receptivity could be perceived in this frontal position, there