in animals, organs capable of emitting light for the purposes of showing recognition of another member of the species, attracting a member of the opposite sex, gathering herds and other groups (signal function), luring prey, and disorientating and frightening predators. Luminescent organs are specialized luciferin-synthesizing glands, mostly of cutaneous origin. They vary in structure from simple accumulations of glandular cells to very complex spherical or cubic photophores and saccular appendages containing luminescent bacteria. The organs range in size from 0.1 mm to several centimeters. Their number and arrangement on the body, as well as the intensity and spectrum of the emitted light, are different in various species and, in some species, are different in males and females.
Luminescent organs are usually most active in the adult animal, especially during sexual reproduction. The most complex are similar in structure to a projector: they have structures acting as a diaphragm, clear lens, light source (glandular photogenic cells or luminescent bacteria), underlying mirror reflector, and a mounting and insulating black or red coating. The light is emitted by photogenic cells or by slime released from the cells (autonomous luminescence), which in some species is excreted as a stream or cloud. Light may also be emitted by luminescent bacteria living in cells or in special cavities (symbiotic luminescent organs). As a rule, luminescent organs have branching blood vessels and nerves; in insects they also have tracheae. The functioning of luminescent organs is regulated by the endocrine glands and the nervous system; luminescence is often stimulated by external irritation.
Luminescent organs characterize many deep-sea and pelagic animals. They are also found in some terrestrial animals, including insects (fireflies, the cucujo click beetle, and the larvae of fungus and cave gnats), several earthworms, and myriapods.
For information on the biochemical nature of luminescence seeBIOLUMINESCENCE.
T. S. RASS