536 THE POPULAR SCIENCE MONTHLY 



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 some- 

 thing 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 Lampyridae, 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 discour- 

 age 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 con- 

 cerned, this is out of the question for the elaters, and also for many 

 of the adult winged forms of the Lampyrids; the larva? 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 obser- 

 vations 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 



