Page:Popular Science Monthly Volume 79.djvu/542

538. For instance, the observer would soon notice that the lights of Photinus pyralis and Photinus scintillans were decidedly more yellowish than those of Photuris pennsylvanica and Lecontea lucifera, and that the latter were distinctly greenish in tone. A little further observation would soon enable one to distinguish these two latter insects from the two former ones also through the different method of lightemission; the flickering flashes usually given by Photuris and Lecontea differing markedly from the long flash of the Photini. Thus it Incomes probable that different species can recognize their own kind through the color and manner of emission of the light.

Another interesting circumstance is that the majority of Lampyrids have their luminous apparatus on the ventral side, the greater part of the light being directed downwards and sideways, and but very little passing upward. The effect of this ventral arrangement so far as the sexes is concerned is that a female resting on a leaf or on the ground illuminates by her flash a considerable portion of the supporting surface, and a male flying above her would see not merely a flash, but a silhouette of his mate against an illuminated background. The green color of the light would, of course, be of special advantage on foliage. Moreover, the flash of the flying male would illuminate most particularly an area immediately below him and ahead of him, as these insects fly with the body inclined, the head being highest. In species where the male is non-luminous, or but slightly so, this last service does not exist, while in those like Photuris, where both sexes are about equally active, the manner of applying the luminous property may be entirely different; it is perhaps significant in this latter group, that the light is easily visible from the dorsal side between the elytra. The cucuyo (Pyrophorus noctilucus) in which both sexes are equally active, have lights both above and below.

We have been considering the purpose of biophotogenicity so far as its application to the creatures possessing the function is concerned, and to a more limited extent, to their enemies. A few words may well be given regarding its relation to man. The use of the cucuyos as decorations and as night-lamps in the tropical countries is quite well known, and a number of instances have been recorded where travelers have owed their safe passage and even their lives to the light given by a collection of these Elaterid beetles. Several naturalists have written and read by the light of vessels filled with Noctiluca and other sea-organisms, and Chun has photographed a Cephalopod by means of its own light. The luminous bacteria have been put to a number of uses, mainly in the laboratory; flasks coated on the interior with fresh cultures of some of these organisms give light which appears of considerable intensity when the eye becomes accustomed to it, and Dubois and Molisch have taken quite a number of photographs by bacterial light. It has