LUMINOUS INSECTS. 427 



results obtained by Forster and Spallanzani, who as- 

 sert that glow-worms shine more brilliantly in oxygene 

 gas, and by Beckerheim, Dr. Hulme, and Sir H. Davy, 

 who could perceive no such effect, may perhaps be ac- 

 counted for by the supposition that in the latter in- 

 stances the insects having- been taken more recently, 

 might be less sensible to the stimulus of the gas than 

 in the former, where possibly their irritability was, as 

 Brown would say, accumulated by a longer abstinence : 

 but it is not so easy to reconcile the experiment of Sir 

 H. Davy, who found the light of the glow-worm not 

 to be sensibly diminished in hydrogene gas^, with those 

 of Spallanzani and Dr. Hulme, who found it to be ex- 

 tinguished by the same gas, as well as by carbonic 

 acid, nitrous and sulphurated hydrogene gases'*. Pos- 

 sibly some of these contradictory results were occa- 

 sioned by not adverting to the faculty which the living 

 insect possesses of extinguishing its lights at pleasure; 

 or different philosophers may have experimented on 

 different species of Lampyris. 



The general use of this singular provision is not 

 much more satisfactorily ascertained than its nature. 

 I have before conjectured — and in an instance I then 

 related it seemed to be so — that it may be a means of 

 defence against their enemies'. In different kinds of 

 insects, however, it may probably have a different ob- 

 ject. Thus in the lantern-flies (Fulgora), whose light 

 precedes them, it may act the part that their name im- 

 ports, enabling them to discover their prey, and to steer 

 themselves safely in the night. In the fire-flies (^/r/^er), 



« PhUos. Trans. 1810, p. 287. " Ibid. 1801, p. 483. 



' See abovp, p, 22S. 



