Hormone

Definition

A chemical signaling molecule produced by endocrine organs or neurosecretory cells and transported via the haemolymph to target tissues or organs, where it regulates physiological processes and development. In arthropods, hormones coordinate moulting, metamorphosis, reproduction, diapause, and other metabolic functions. Unlike pheromones, which act externally between individuals, hormones act internally within the same organism.

Etymology

From Greek hormōn, present participle of hormān 'to set in motion, impel', from hormē 'impulse, onset'.

Example

In the tobacco hornworm (Manduca sexta), ecdysone triggers the moulting process, while juvenile hormone determines whether the moult produces another larval instar or initiates pupation; declining juvenile hormone levels permit the final metamorphic moult to the adult.

Synonyms

• endocrine
• chemical messenger

Related Terms

• Ecdysone
• Juvenile hormone
• neuropeptide
• haemolymph
• neurosecretory cell
• Endocrine gland
• Pheromone
• moulting
• Metamorphosis

Usage Notes

Distinguished from pheromones by site of action: hormones are internal signals, pheromones external. In insects, the major endocrine centres include the corpora cardiaca, corpora allata, and prothoracic glands. Juvenile hormone and ecdysteroids are the best-studied insect hormones, though many others regulate digestion, osmoregulation, and colour change. The term is sometimes used loosely for any regulatory chemical, but specialists reserve it for substances with demonstrated endocrine function and specific target receptors.