Page:Popular Science Monthly Volume 79.djvu/540

536 the Lampyrids generally, and it is of interest to note that they may be attracted by an ordinary light.

To be sure, a great many insects which are not luminous themselves are attracted to light, and there is some evidence that this is something which the insects themselves can not help. The phenomenon of attraction to light among luminous insects, however, must be regarded as of particular interest, and as being most probably voluntary.

That the photogenic function in insects may also be protective or warning in significance may scarcely be doubted. The Lampyridæ, as a group, are soft, easily crushed insects, slow of motion, and often, in the females, apterous, or of but slow and labored flight. It has indeed been observed that the flash of these insects has a tendency to discourage pursuers, perhaps frightening them in some cases, but probably more often warning them that the light-bearer is inedible. The elaters are better protected by their hard, external chitin, than the soft-bodied Lampyrids, and hence it is unlikely that their luminosity has much protective purpose. So far as its possible use as a lure for prey is concerned, this is out of the question for the elaters, and also for many of the adult winged forms of the Lampyrids; the larvæ and probably some of the larviform females of Lampyrids, however, are carnivorous, and in them it is possible, though hardly probable, that the alluring significance for the photogenic function may hold good.

A few species of myriapods are known to be luminous; perhaps more species than are now known to possess this function actually possess it for short periods during the year, probably during the height of the mating season. In some myriapods, the luminosity seems to be developed in a secretion which is ejected from pre-anal glands, while in others it is located in organs on the body of the creature, as in the insects. Mrs. Thomas has shown the almost undoubted defensive character of the first class of luminosity in myriapods, while the observations of Bruner suggest rather the sexual significance in the second class.

Among the Cephalopods we find a very peculiar class of luminous organs, occurring immediately upon or just beside the eyeball, which in these creatures is often relatively enormous. Here the significance of the luminosity seems to be interpreted by the situation of the organs as an adjunct to the function of sight, and such it very probably is. But very similar organs are found on various other portions of the body and in situations where they can not very greatly aid in vision, or illuminate the creature's path through the water. Much the same thing is true of some fish, which possess one or more large photogenic organs situated near the eye, and rows of smaller organs along the sides or abdomen; here the organs near the eye are naturally considered as an aid to vision, while the others can not possibly be so considered. In