Page:Popular Science Monthly Volume 79.djvu/541

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both fish and cuttlefish the explanation of these body-organs is probably the same. In the depths of the sea colors are practically indistinguishable. The dim light of the stationary photogenic forms there would be insufficient to differentiate colors. But in the more than semidarkness of these depths, a creature with a row of luminous dots along its sides would show up like an electric sign in a dark street. Tbe arrangement of the light-giving spots could very easily be followed, even for considerable distances through the water. It is probable, therefore, that these rows of photogenic organs on the bodies of these creatures serve the purpose of plumage and pigmentation on land, a welcome for friend and a warning for foe. No doubt one species of fish, seeing a luminous streak some distance away through the water, could readily tell whether its pattern of light—its "electric sign "—spelled the same as those upon its own body, or the legend of a foe. Many of the fish that possess these rows of luminous organs, while insignificant in size, are obviously raptatorial, and are armed with vicious teeth.

Of course another expression of the luminosity of deep-sea as well as surface fish is that it is alluring. This is especially true in those species which are provided with luminous barbels, or baits, like those of the angler-fishes. Alcock mentions a blind angler-fish with a luminous barbel, in which the alluring significance is scarcely to be doubted.

An objection which seems to have been urged against the "pattern" theory of photogenicity in marine forms is that different specimens of the same species occasionally show variations in the number and distribution of their lights. It seems probable, however, that the reason for this variation may be the age of the specimens of the fish in question, the number or position of the spots varying with age. Almost any of the various books and papers on the deep-sea fish will illustrate the effect of this "pattern" arrangement as seen, for example, in Cyclothone and Astronesthes.

Still another type of organ of light-production among fishes is illustrated in Carl Chun's book "Aus den Tief en des Weltmeeres"; in this case the luminous apparatus is set in a pit in the foremost part of the head, before the eyes. Here there can be little doubt, again, that the usefulness to the animal consists in the illumination of the "road ahead," as the searchlight does for the automobile.

A large number of other forms might be mentioned, which emit light in more or less characteristic ways, but what has gone before will serve to illustrate the majority of the points in interest. Some considerations as to the phenomenon in the fireflies may, however, be of interest, especially as deductions therefrom will doubtless hold true for many other luminous forms also.

It has been observed that the various species of fireflies (Lampyrids) emit lights of slightly differing tone, and in decidedly different man