As to luminous or phosphorescent (often called "luminescent") insects and other animals, there are a great many curious and interesting facts known. There are luminescent bacteria (common on old meat bones and dead fish and in the sea generally), animalcules of various species, jelly-fish, star-fish, worms, shell-fish, and crustaceans and true fishes. Inhabitants of the great depths of the ocean of all kinds are usually luminescent. The light is caused by the oxidation of a peculiar fatty substance. Without free oxygen there is no luminescence, and yet no heat is produced but merely light, as when a stick of damp phosphorus glows. The luminescence of living things (often, but undesirably, called phosphorescence) is a process differing greatly from that called "phosphorescence" in minerals and crystals, such as the emission of light by a lump of white loaf-sugar when crushed. You may see that kind of phosphorescence by standing in front of a looking-glass in a dark room and crushing a lump of loaf-sugar with the teeth, keeping the lips raised. It seems that in many organisms luminescence occurs without any consequent use or service to the organism. But in higher forms the power of emitting light has been seized upon by natural selection having become of value in attracting the individuals of a species to one another, or in attracting prey, or again in scaring enemies. The luminescent matter is concentrated in certain definite organs, and the access to it of oxygen and even its formation are controlled by the nervous system.
Among insects far better known than the rare luminescent May-flies, are the glow-worms, a family of beetles of which several species are known besides our own familiar one, called Lampyris noctiluca. The fire-flies of Southern Europe—Luciola italica—are small beetles allied to the glow-worm, but both sexes fly and both are luminous, whilst in the common glow-worm the female is wingless, and the flying male, who is guided to the female by her light (which she can vary in intensity), gives but a feeble light. The swarms of Italian fire-flies consist of as many as a hundred males to one female, and the males are far more brilliant than the females. My fellow-student Moseley showed some in oxygen gas at the Royal Society's soirée many years ago. The gas greatly increased their brilliancy. Many valuable experiments in search of an explanation of the brilliance of the male Luciolæ and their excess in number could be carried out in North Italy. A peculiar grub-like female glow-worm, three inches long, is found in South America, which produces a red light at each end of the body and numerous points of green light on each side of it. It is called the "railway-beetle" in Paraguay.
Another family of beetles besides the Lampyrids, or glow-worms, is celebrated for the brilliant luminescence of some of its species. These are the click-beetles, or Elaterids [(see Fig. 22, C)]. In South America there are upwards of a hundred species of this group, showing various degrees of luminosity. The "Cucujos" (Pyrophorus noctilucus) of tropical America is one of the most abundant and largest. It is as much as an inch and a half long, and has three "lamps," or luminous organs, one on each side of the body and one below the tail. The light given off is extremely beautiful, and the live insects are used by the women for ornament and by the country-folk as lamps on nocturnal excursions. Erroneously the term "fire-fly" is applied to these beetles; it should be reserved for the little Italian Luciola, which swarms, as countless thousands of dancing lights, in the nights of early summer over the marsh lands of North Italy. I have seen it at the end of June as far north as Bonn, on the Rhine. In Australia a small true "fly"[7]—that is to say, a two-winged fly or Dipteron like our gnats, midges, and house-flies—is known, the maggot of which is luminous. And in New Zealand there is another of which both the maggot and the perfect insect are luminous. The grub is called the New Zealand glow-worm.[8]
[7] Known to entomologists as Ceroplatus mastersi.
[8] Boletophila luminosa of entomologists.
There are grounds for believing that the luminescence of some of these insects serves them not to attract one another, but to scare would-be predatory foes, such as birds, bats, and reptiles. I have heard a story (which I should like to have confirmed) that in some part of tropical Asia a certain kind of bird collects half a dozen or so of a species of glow-worm and places them at the entrance to its nest, so as to scare nocturnal animals which might attack its eggs or its young. It is a noteworthy fact that a point of light in the dark may act in two opposite ways on animals which see it—either it attracts or it repels them. The physiologist calls this positive and negative "photo-taxis" (light-guidance). And we have the similarly positive and negative influence of chemical taste and smell, called "chemo-taxis," and a similarly contrasted positive and negative "hygrotaxis," or directive influence of moisture upon the movements of animals and plants.
CHAPTER XXVI
FROM APE TO MAN
THE recent discoveries of the actual bones of very early races of man raise again a general interest in the inquiry as to what are the actual differences of structure between men and apes, and what were probably the steps by which, as the result of "survival of the fittest," some early man-like apes became ape-like men. The question also arises as to how long ago the transition actually took place, and whether it was a very gradual or a rapid one. We are to-day in possession of some important facts bearing upon this inquiry which were unknown to Huxley when he wrote his ever-memorable essay on "Man's Place in Nature," and triumphantly closed the controversy between himself and Sir Richard Owen. That was nearly fifty years ago.