Page:Popular Science Monthly Volume 79.djvu/539

Rh of the year, of the month, and of certain hours of the early evening, during which the male is attracted from below to the luminous female at the surface. Various species of earth worms have also been reported to be photogenic, and it seems probable that the usefulness of the luminosity in this case is somewhat the same as in the Odontosyllid mentioned above.

The bivalve Pholas dactylus presents another anomaly, however, for it lives a rather sedentary life, and is certainly not poisonous, at least to man. Yet it possesses definite photogenic organs. Although it is possible that in this case the luminosity is protective, we probably have here one of the cases of the limited use of photogenicity, not yet discovered.

Among the Crustacea there are several interesting cases of photogenicity, and in regard to them, and indeed to the whole subject of the use of "phosphorescence" in sea-forms, Alcock's interesting book "A Naturalist in Indian Seas," is well worth reading. Certain shrimplike crustaceans throw out from glands, corresponding to kidneys, a substance which in contact with sea-water produces clouds of bluish light. There seems but little doubt that this is defensive in nature, and acts in much the same manner as the "ink" of the cuttle-fish. Some of these prawns are provided with enormous eyes, others with only rudimentary ones, and some with none at all. Alcock also mentions a large spider-crab, which, although completely blind, "shone like a star." Here we may readily conceive the light is alluring in function, serving to attract the creatures on which the crab feeds.

Among the insects we find the most widely known cases of photogenicity, and probably, also, the greatest field of usefulness. "With the true fireflies, the Lampyridæ, the evidence that has been collected tends to show that the possession of the photogenic function is primarily a secondary sexual character. It has long been known that if the female of the European glowworm (Lampyris noctiluca) were exposed by night, a male would shortly come to it. The use of the photogenic function as a secondary sexual character has also been shown for the Italian luciola, and for certain of the fireflies common in the eastern United States (e. g., Photinus pyralis), and it appears probable that the same thing applies to the entire family. Curiously enough, the true "lightning bugs" show but little tendency to come to ordinary lights, though in Photinus pyralis either sex will respond to a small electric bulb operated in imitation of the light of the opposite sex.

Among the Pyrophores, the Elaterid fireflies of the tropics, such as the cucuyo of Cuba, the luminosity very probably plays the same role as in the Lampyrids. These insects give a light which is continuous, though of varying intensity, instead of a flashing light as is emitted by