Several organs in an insect are known to produce hormones, the principal functions of which are the control of the reproductive processes, molting, and metamorphosis. Chemicals similar to hormones of vertebrates, including androgens, estrogens, and insulin, have been detected in insects, but their function is yet unknown.
The neurosecretory cells in the brain are neurons that produce one or more hormones that play a role in growth, metamorphosis, and reproductive activities. One of these, commonly called the brain hormone or prothoracicotropic hormone (PITH), plays an important role in molting by stimulating a pair of glands in the prothorax to produce the hormone ecdysone that causes apolysis. Other hormones produced by the brain may have other functions. For example, entomologists believe that a brain hormone plays a role in caste determination in termites and in breaking diapause in some insects.
Ecdysone (Figure 1A) initiates growth and development and causes apolysis. This hormone occurs in all insects groups that have been studied, in crustaceans, and in arachnids, and it is probably the molting hormone of all arthropods. It also plays a role in the differentiation of the ovarioles and accessory reproductive glands in females and in several steps in the process of egg production (oogenesis). Ecdysone, in fact, is also produced within the ovaries of insects.
Fig1. Structure of two insect hormones. A, Ecdysone; B, Juvenile hormone.
The corporaallata produce a hormone called the juvenile honnone QH) (Figure 1B), the effect of which is the inhibition of metamorphosis. Various substances, particularly terpenes such as farnesol, show considerable juvenile hormonelike activity. JH also has effects on other processes besides the inhibition of metamorphosis. It is involved in vitellogenesis, accessory reproductive gland activity, pheromone production, and sexual behavior (see Raabe 1986).
Substances chemically related to ecdysone and juvenile hormone occur in certain plants and may protect the plants from feeding by insects. Chemical analogs of ecdysone and juvenile hormone are being studied to see whether they can function as new kinds of insecticides.
