1900.] INSECTS OF THE " SKEAT EXPEDITION." 863 



pool that we discovered that the light proceeded from beetle larvae, 

 which were clinging, dorsal surface downwards, to the floating 

 fronds of a small cryptogam. The luminous points were blue in 

 colour and very brilliant, though small. They did not flicker like 

 the lights of the fire-flies which flitted in hundreds over the surface 

 of the marsh, and when they were extinguished they died away 

 gradually. In the pool they did not change their position, but 

 they became sometimes brighter and sometimes less bright slowly, 

 occasionally dying out entirely for no apparent cause. When the 

 larva was taken out of the water, its luminosity disappeared, and 

 did not reappear until it had been restored to its habitual element 

 for some minutes. The light of some specimens which were placed 

 with water and weeds in a glass jar, and brought near a lamp after 

 they had recovered from their capture sufficiently to shine again, 

 went out. After a longer or shorter interval of rest near the 

 lamp, on different trials, it reappeared again. Poking them with a 

 twig sometimes caused them to shine more brightly, but more often 

 to become entirely dark. If several individuals were in a bottle 

 and one of them became brilliant from any cause, the others 

 followed suit after a few seconds. A specimen which was put 

 into corrosive solution ceased to be luminous, but after about a 

 quarter of an hour became exceptionally bright. It was then 

 transferred to a weak solution of formalin ; whereupon its light 

 went out finally, taking several seconds to disappear. 



During the day I was unable to find any of the larva? on the 

 surface of the pool ; but the captive specimens had deserted the 

 floating weeds before morning, and were crawling slowly on the 

 bottom of the jar. I did not see them feed, though the water in 

 the jar was full of small animals of different sorts — Copepods, 

 Protozoa, and water-mites. Nor, while I was watching them, did 

 the larvae ever come to the surface to false in air or to breathe. 

 I can find no special respiratory organs in my specimens : when 

 alive no part of the body was silvery in appearance under water. 



Remarks. — The question of luminosity is one even more 

 enigmatical than that of the sounds produced by insects. It is a 

 phenomenon which is manifested right down among the Protozoa, 

 and even in the border-land between the two great kingdoms ; it 

 reaches its highest development among some of the Lampyridce. 

 In the Westmann Isles I have seen a whole village accidentally 

 lighted up by the action of putrefactive bacteria in cods' heads 

 hanging to dry on the walls of the gardens; and a dead shark upon 

 the shore was visible on the darkest night from the same cause to 

 the distance of half a mile. Noctiluca and other marine animals — 

 ccelenterates, crustaceans, tunicates, &c. — produce even more 

 astonishing luminescent effects. It is not apparent what is the 

 object of this display among these forms ; though possibly in the 

 case of the Medusae it may serve as a lure for prey, as it appears 

 to do among certain deep-sea fishes. Among the insects and 

 Myriapoda the purpose of luminescence is also obscure '. It 



1 See Dubois, Bull. Soc. Zool. France, " Les Elaterides lumineux " (1886), &c. 



