Three hormones or hormone types are integral to growth and reproductive functions in insects. These are the ecdysteroids, the juvenile hormones and the neurohormones (also called neuropeptides).

Ecdysteroid is a general term applied to any steroid with moult-promoting activity. All ecdysteroids are derived from sterols, such as cholesterol, which insects cannot synthesize de novo and must obtain from their diet. Ecdysteroids occur in all insects and form a large group of compounds, of which ecdysone and 20-hydroxyecdysone are the most common members. Ecdysone (also called a-ecdysone) is released from the prothoracic glands into the haemolymph and usually is converted to the more active hormone 20-hydroxyecdysone in several peripheral tissues, especially the fat body. The 20-hydroxyecdysone (referred to as ecdysterone or b-ecdysone in older literature) is the most widespread and physiologically important ecdysteroid in insects. The action of ecdysteroids in eliciting moulting is well studied, and functions similarly in different insects. Ecdysteroids are produced also by the ovary of the adult female insect and may be involved in ovarian maturation (e.g. yolk deposition) or be packaged in the eggs to be metabolized during the formation of embryonic cuticle.

Juvenile hormones forma family of related sesquiterpenoid compounds, so that the symbol JH may denote one or a mixture of hormones, including JH-I, JH-II, JH-III and JH-0. The occurrence of mixed-JH-producing insects (such as the tobacco hornworm, Manduca sexta) adds to the complexity of unravelling the functions of the homologous JHs. These hormones are signalling molecules, and act via lipid activation of proteins that play a diversity of roles in development and physiology. Lipid-based signalling systems are known to have diverse modes of action and generally do not require high-affinity binding to receptor sites. Insect JHs have two major roles– the control of metamorphosis and the regulation of reproductive development. Larval characteristics are maintained and metamorphosis is inhibited by JH; adult development requires a moult in the absence of JH . Thus, JH controls the degree and direction of differentiation at each moult. In the adult female insect, JH stimulates the deposition of yolk in the eggs and affects accessory gland activity and pheromone production .

Neurohormones, the largest class of insect hormones, are peptides (small proteins) with the alternative name of neuropeptides. These protein messengers are the master regulators of all insect physiological processes, including development, homeostasis, metabolism and reproduction, as well as the secretion of the JHs and ecdysteroids. Over a hundred neuropeptides have been recognized, many existing in multiple forms encoded by the same gene but resulting from gene duplication, mutation and selection giving rise to closely related signalling systems. Some important physiological processes controlled by neurohormones in some or all insects are summarized in Table 1. Thediversity and vital co-ordinating roles of these small molecules are characterized increasingly via peptide molecular biology  combined with the availability of the complete Drosophila genome. Neuropeptides regulate by both inhibitory and stimulatory signals, and reach action sites (receptors) via nerve axons or the haemolymph. Others control indirectly via their action on other endocrine glands (corpora allata and prothoracic glands). Receptors are high-affinity binding sites located in the plasma membrane of the target cells, with most classified as G protein-coupled receptors (GPCRs). Exceptions include prothoracicotropic hormone (PTTH) and insulin-like peptides that activate by binding to a receptor tyrosine kinase. Some ligand-receptor pairs are very specific in responding to a single neuropeptide type, whereas other receptors respond to several types of ligand.

table1. Examples of some important insect physiological processes mediated by neuropeptides; note that usually only one, of often multiple, function of each neuropeptide is listed. (After Keeley & Hayes 1987; Holman et al. 1990; Gäde et al. 1997; Altstein 2003.)