37 



This would lead us to suppose that other common and well-known insects may be 

 found to possess the property on investigation. 



Kirby and Spence suggest that in some cases phosphorescence may be caused by 

 disease, they also think that light giving insects may in part explain the mysterious 

 phenomena of ignus fatui. 



With regard to the use of the light to its possessor opinions are divergent. In the 

 case of the Elateridce, one theory is that it may serve to defend them from their enemies 

 by alarming them,* and this may not be far from the truth, as an animal would require 

 a great deal of pluck to swallow one of these specks of living flame. 



Its purpose in the lantern-flies it would be rash to conjecture until the fact of their 

 luminosity is fully proven. 



Even of its use in the Lampyridce little is known. The most satisfactory explana- 

 tion, more particularly applicable to Noctiluca, is that it serves to direct the male insect 

 to the abode of its wingless mate. 



But with our American species this explanation hardly holds good, as both sexes 

 can fly, besides being equally luminous. 



I have seen it stated f that the Lampyrida are distasteful to many insectivora. 

 May not the light then serve as the brilliant colours of some caterpillars are supposed 

 to do, as a warning of their offensiveness to creatures that would devour them ? thus 

 deterring the insect eater from swallowing an unpalatable morsel, and at the same time 

 protecting these soft-winged beetles better than the shelly armour of most other members 

 of their order. 



The light in the Lampyrid® and Elateridce has been ascertained to proceed from a 

 soft, yellow substance, thickly permeated with air tubes, which is closely applied to the 

 transparent parts of the body through which it is seen. 



In the glow-worm, besides this substance, there are two small oval sacs, formed of 

 a fibre wound spirally, as in the tracheae, which contain a yellowish matter capable of 

 affording a more powerful and permanent light than that of the adjoining parts. 



Until lately the cause of insect luminosity was supposed to be the combination of 

 some phosphoric matter in combination with the oxygen inspired by the animal. But 

 latterly investigators have ascribed it to physical rather than chemical action, and it is 

 now thought to be explained by the vibratory or undulatory theory of light. Rapid 

 molecular vibrations, set up by the action of light-waves in the particles of a phosphor- 

 escent body, are supposed to be communicated to the theoretical ether which is thought 

 to fill all space. The principle being that these vibrations produce light as the vibra- 

 tions of air generate sound. 



To enter more fully into this question is not in the scope of the present paper, nor 

 is it, strictly speaking, in the domain of the Entomologist, still the subject is one that 

 may profitably occupy our leisure moments during the coming summer, and those of 

 our members who possess powerful microscopes may do good service by examining the 

 phosphorescent organs of insects and giving us the result of their investigations. The 

 field is almost unexplored, and as yet we have to confess our ignorance of the causes 

 which produce many of the phenomena that I have but touched upon. 



* Kirby and Spence— p. 549. 



1 1 think by Mr. A. R. Wallace, in The Contemporary Review. 



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