Hormones are chemicals produced by an organism which circulate in the blood to regulate its long term physiological, developmental and behavioral activities. They complement the nervous system, which provides shortterm coordination, and the activities of the two systems are closely linked.

General aspects of hormones are discussed in this chapter. The specific actions of hormones regulating particular functions are considered in other chapters:

molting and metamorphosis – Section 15.4.

yolk synthesis – Section 13.2.4.2.

embryonic cuticles – Section 14.2.10.

diuresis – Section 18.3.3.

mobilization of fuel for flight – Section 9.12.2.

polyphenism – Section 15.5.

diapause – Section 15.6.5.

General review of insect hormones: Nijhout, 1994

CHEMICAL STRUCTURE OF HORMONES

Apart from molting hormone and juvenile hormone, most known insect hormones are peptides. Some biogenic amines are also known to function as hormones.

Molting hormones

Molting hormones are ecdysteroids (Fig. 21.1) which, in immature insects, are produced by the prothoracic glands. In most insects these glands secrete ecdysone, but some larval Lepidoptera are also known to secrete 3-dehydroecdysone which is converted to ecdysone by enzymes in the hemolymph (Fescemeyer et al., 1995). Ecdysone is a prohormone; it is converted to the active hormone, 20-hydroxyecdysone, in the fat body or epidermis by a cytochrome P-450 enzyme. In the honeybee Apis, and in Heteroptera, the principal ecdysteroid is makisterone A.

Insects cannot synthesize steroids. Consequently, sterols, usually cholesterol or a closely related structure, are essential dietary constituents.