Group effects attributable to the presence of chemical primers have been demonstrated for the mealworm beetle Tenebrio molitor L. Both oocyte development and rate of sex pheromone emission are increased if isolated adult females are exposed to airstreams containing the scents of either mature males or females (Happ et al. 1970). The male scent accelerates both oocyte growth and the rate of sex attraction liberation at the end of three days, whereas the female scent affects egg development only after seven days. These results demonstrate that beetles, like locusts, temporally regulate reproduction in local populations by utilizing primer pheromones elaborated by both sexes.
In an elegant series of experiments, McFarlane (1964) reported that the cricket Acheta domesticus (L.) would inhibit the growth of another cricket, Gryllodes sigillatus (Walk.); however, extracts of Acheta domesticus inhibited the growth of both species. Subsequently, McFarlane and Henneberry (1965) observed that the methyl esters of fatty acids inhibited the growth of Gryllodes sigillatus nymphs, presumably after entry through the external body wall. Group-reared nymphs of G. sigillatus exhibit reduced survival in the presence of methyl palmitate but this ester does not affect the survival of singly-reared individuals.
On the other hand, methyl laurate increased the body weight of both group-reared and singly-reared males ofAcheta domesticus but this effect is concen- tration dependent so that higher dosages of the ester had no effect on group-reared individuals (McFarlane 1966b). Significantly, the effects of a free fatty acid and a methyl ester on Gryllodes sigillatus nymphs appear to result from different inhibit- ory mechanisms and to affect different developmental systems during sensitive growth periods (McFarlane 1968). The results of these investigations emphasize the possible primer effect which compounds may mediate as cuticular penetrants and indicate that this phenomenon should be considered when nutritional studies are undertaken.
Eusociality: the social imperative
The various phylogenetic lines of Hymenoptera that have become eusocial have done so through the evolution of a variegated series of communicative stimuli to which they are programmed to respond adaptively. These insect societies exhibit a complex of behavioral interactions which, in their totality, surpass those that are encountered in any of the species of presocial insects. The very nature of the eusocial state dictates that a high degree of organization be maintained in order to optimally integrate the activities of the ‘colonial members. Different degrees of eusociality are present among the Hymenoptera and Isoptera but in all cases, they possess three fundamental qualities in common: 1) individuals of the same species cooperate in caring for the young; 2) a reproductive division of labor which in- cludes more or less sterile adults caring for reproductives; and 3) overlapping generations resulting in offspring functioning alongside their parents in colonial maintenance. This trio of conditions set the stage for a multitude of interactions between immatures, adult workers and reproductives, which are fundamental to the maintenance of a viable eusociality. Signiﬁcantly, it now seems evident that chemic- all signals are the principal vehicles governing speciﬁc behavioral interactions be- tween individuals. Therefore, eusociality became possible only after a finite number of adaptively favorable patterns of interaction, initiated chieﬂy by chemical stimuli, had been established by natural selection in the colonial milieu.