Insects respond to crowding in a variety of ways that are usually exemplified by rapid changes in behavior and culminate in enduring long-term morphological and/or chromatic responses. A common feature of both short-term and long-term effects is that they are graded, dependent not only on density but also on the duration and on phase history of the maternal generation. Because of their exoskeletons, which are persistent for the duration of each instar and endure throughout adult life, overt changes in morphology or coloration are restricted to the molting period and shortly afterward, when cuticular hardening and pigmentation are expressed. Changes in internal organs or metabolism elicited by population density, being independent of integumental constraints, are not restricted to the molting period, but the temporal difference between internal and external responses is not of fundamental significance. Intraspecific responses to the presence of sibling insects are of apparent ecological significance and often involve directional movement and/or migration. They are mediated via the sensory system, involve signal transduction, and elicit downstream biochemical and physiological changes.

Mating elicits two well-defined reactions in sexually matured females of many insects: reduction of receptivity and increased oviposition. These post-mating responses have been shown to be induced by factors synthesized in the reproductive tract of the adult male and transferred in the seminal fluid to the female during copulation. One of these factors, named sex-peptide (SP), has been identified in Drosophila melanogaster. Using an in vitro radiochemical assay, we show that synthetic sex-peptide considerably activates juvenile hormone III-bisepoxide (JHB3) synthesis in corpus allatum (CA) excised from Days 3 and 4 post-eclosion virgin females. Base levels are significantly lower at emergence (Day 0) than on subsequent days, and only weak stimulation is obtained on Day 1, while none is obtained on Day 2, where maximal basal synthesis occurs. The CA of mated females cannot be stimulated further for at least 7 days, but regain responsiveness by Day 10 after mating. Synthesis of JHB3 stimulated by SP in vitro persists for at least 4 h after removal of the peptide. Development of responsiveness of the CA to SP in vitro is compared with development of the post-mating reactions of sex-peptide injected virgin females. Our results suggest that the CA is a direct target for SP in vivo and that sexual maturity is established separately for the two post-mating reactions.

The in vitro production of juvenile hormone (JH) was investigated by using isolated ring glands from third instar Drosophila melanogaster. A JH-like molecule is secreted that comigrates with a synthetic sample of methyl 6,7;10,11-bisepoxy-3,7,11-trimethyl-(2E)-dodecenoate (JHB3) during TLC, liquid chromatography, and GC analysis. Purified product from farnesoic acid-stimulated ring glands was analyzed by electron impact GC/MS and gave a mass spectrum identical to synthetic JHB3. Additional structure confirmation was obtained following conversion of product from unstimulated biosynthesis to a derivative that comigrated on liquid chromatography with the derivative prepared from synthetic JHB3. Physiological studies revealed that JHB3 is produced solely by the corpus allatum portion of the ring gland in vitro. Isolated ring glands from other cyclorrhaphous dipteran larvae also produce JHB3 almost exclusively in vitro. Corpora allata from mosquito larvae, however, produce only JH III, indicating that JHB3 production may be restricted to the higher Diptera. Topically applied synthetic JHB3 caused developmental responses in newly formed D. melanogaster white puparia similar to those obtained with JH III. The data suggest that JHB3 is a fly juvenile hormone.

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