FLOTSAM AND JETSAM 217 



A clue to their use is to be found in the fact that 

 they show a remarkable sexual difference, the males 

 having them on the upper side of the tail base, and 

 the females on the lower side. Of course in my 

 necessarily brief and sporadic researches, when no 

 fish lived longer than thirty-six hours, there was 

 no chance to observe courtship or any such use 

 which these lights might subserve. But when a fish 

 exerted itself unduly to get out of the way of an- 

 other, either of its own or another species, these 

 lights would flash and die in quick succession. 

 Three separate times in unusually strong, vigor- 

 ous fish when the body luminescence was very dim, 

 these scale search-lights flashed like heliographs, 

 being much stronger than the combined, steadier 

 glow of all the others. This luminescence was of 

 a much deeper green than that of the ventral lights. 

 If continuously alight, a single fish would enable 

 one easily to read fine print. 



In the dark it was thus possible to distinguish 

 species of lantern fish by the lateral hieroglyphic 

 heliographs and the sexes by the upward or down- 

 ward direction of the tail lights. I have never seen 

 the latter illumination given out by a fish swim- 

 ming alone in an aquarium. Although it is very 

 evident that the caudal flashes have some sexual 

 significance yet another very important function 

 seems that of obliteration. It certainly was to my 

 eyes, and I have no reason to think that a natatory 

 enemy might not also be frustrated. When the 

 ventral lights die out they do so gradually, so that 

 the eye holds the image of the fish for a time after 



