Hormonal control of invertebrate behavior

Invertebrates show a wide variety of behaviors that are influenced by hormones. In insects the involvement of hormones at a particular life stage is directly correlated with the complexity of the behavioral repertoire at that stage. In larval stages, the steroid hormone, ecdysone, when present with juvenile hormone, apparently causes the behaviors observed during the periodic molts. At the end of larval life, ecdysone in the absence of juvenile hormone triggers the onset of premetamorphic behaviors such as wandering behavior and cocoon-spinning behavior. In insects having complete metamorphosis, the emergence (eclosion) of the adult from the pupal case is accomplished by a stereotyped program of movements that are triggered by a peptide hormone. In moths, injection of this “eclosion hormone” into competent recipients will cause the release of the eclosion program. Also this program can be elicited by the hormone from the isolated abdominal central nervous system (CNS). The onset of reproductive behavior in females of various species requires juvenile hormone. In addition, certain peptides are then involved in the transition from virgin to mated behaviors. Also, pupatitive peptide factors trigger specific stereotyped behaviors such as those involved in mate attraction and in oviposition. In males, the control is simpler. Juvenile hormone is required for the maturation of sexual behavior in only a few species. But in at least one insect group, the cockroaches, a neurosecretory hormone serves to release directly copulatory behavior. Social behavior and migratory behavior in certain insects are also under hormonal influence. Hormones play a prominent role in regulating the behavior of gastropod mollusks. The best studied examples involve the hormonal stimulation of egg-laying behavior by CNS peptides. Also, peptide hormones cause stereotyped changes in specific identified neurons in the CNS of various gastropods. In at least some cases, these latter changes are related to arousal from aestivation.With their simple nervous systems, invertebrates are especially suited for studies on the mode of action of hormones on the nervous system. In most cases the behavioral effects of these hormones appear to be due to their direct action on the CNS. Indeed, the isolated moth CNS will respond to the eclosion hormone by generating the motor program that gives rise to the emergence behavior, and various isolated molluscan preparations will respond to hormones with stereotyped neural responses. By the direct application of hormone to the surface of identified nerve cells in mollusks it has been possible to localize target cells for specific hormones. Little is known of the mode of action of ecdysone or juvenile hormone in altering behavior. Peptide hormones appear to have effects which long outlast the actual presence of the hormone. In at least two cases, cyclic AMP has been implicated as a mediator of the hormonal response.