Fig. 16.—Chart showing relative intensities and durations of flashes of American fireflies (after McDermott). One cm. vertically = approximately 0.02 candle power; one cm. horizontally = approximately one second. The flash of the males (♂) is at the left; that of females (♀) at right of chart.
Some forms only produce light at certain seasons of the year. According to Giesbrecht (1895) this is true of the copepods, which only light in summer and autumn, and according to Greene (1899) in the toad-fish; Porichthys,
which can only be stimulated to luminesce during the spawning season in spring and early summer.
Some animals possess a periodicity of luminescence. They only luminesce at night and fail to respond to stimulation or are difficult to stimulate during the day. Bright light has an inhibiting effect. Perhaps correlated with this is the fact that most luminous forms are strongly negatively heliotropic. Fireflies lie hidden in the day, to appear about dusk and the ostracod crustacean, Cypridina, is difficult to obtain on moonlight nights.
The Ctenophores were the first forms in which the inhibiting effect of light was noticed. This was described by Allman (1862) and has been confirmed by a number of observers, especially Peters (1905). Massart found that Noctiluca was difficult to stimulate during the day and Ceratium, according to both Zacharias (1905) and Moore (1908), only luminesces at night, or if kept in darkness, for some little time. Crozier[4] finds a persistent day-night rhythm of light production when Ptychodera, a balanoglossid, is maintained for eight days in continued darkness. The animal is difficult to stimulate during the period which corresponds to day and luminesces brilliantly and at the slightest touch during the period which corresponds to night.
On the other hand, a great many forms are able to luminesce quite independently of previous illumination. According to Crozier[4] Chætopterus luminescence is not affected by an exposure to 3000 metre-candles for six hours.
[4] Private communication.
In the case of animals with extracellular luminescence we may speak of luminous secretions and true luminous
glands. A large number of forms possess luminous glands or gland cells, including some of the medusæ, the hydroids (probably), the pennatulids (?), the molluscs (Pholas and Phyllirhoë) (probably), some cephalopods (Heteroteuthis and Sepietta), most annelids, ostracods, copepods, some schizopods (Gnathophausia) and decapod (Heterocarpus and Aristeus) crustaceans, all myriapods, and the balanoglossids. The remaining organisms burn their material within the cell. These include the bacteria, fungi, protozoa, some medusæ (?), ctenophores (probably), most cephalopods, a few annelids (Tomopterus (?)), ophiuroids (?), some schizopod (Nyctiphanes, Euphasia, Nematocelis, Stylochiron) and decapod (Sergestes) crustacea, all(?) insects, Pyrosoma, and fishes (selachians and teleosts). It is among this latter type that the most complicated luminous organs have been developed. While a description of all the types of luminous organs and luminous structures cannot be attempted here (excellent descriptions have been given by Dahlgren and Mangold) it is necessary to understand the structural conditions in a few of the forms whose physiology has attracted most attention.
Luminous bacteria are so small that the light from a single individual cannot be seen. It is almost impossible to make out structural differences within the cell and we cannot definitely state in just what special region, if any, the luminescence is produced. We do know that the light is intracellular and that filtration of the bacteria from their culture medium gives a dark sterile filtrate absolutely free from any luminous secretion.