346 



LUMINOUS INSECTS. 



might be less sensible to the stimulus of the gas than in the 

 former, in which perhaps their irritability was 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 hydrogen 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 sulphureted hydrogen gases. 2 Possibly some of 

 these contradictory results were occasioned by not adverting 

 to the faculty which the living insect possesses of extin- 

 guishing its lights at pleasure. At the same time, however, 

 it may be here observed, that as this luminous substance can 

 be collected in considerable quantities, there can be no 

 difficulty in deciding by chemical analysis whether it is really 

 phosphoric or not ; and that till this analysis has been made 

 it is premature to build any hypothesis on the assumption of 

 its being so, or to apply this epithet to it, as is so generally 

 done. 



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 pro- 

 bably have a different object. Thus in the lantern-flies 

 {Fulgord), whose light precedes them, it may act the part 

 that their name imports, enabling them to discover their 

 prey, and to steer themselves safely in the night. In the 

 fire-flies (Elater), if we consider the infinite numbers, that in 

 certain climates and situations present themselves every 

 where in the night, it may distract the attention of their 

 enemies or alarm them. And in the glow-worm — since 

 their light is usually most brilliant in the female ; in some 

 species, if not all, present only in the season when the sexes 

 are destined to meet, and strikingly more vivid at the very 

 moment when the meeting takes place 3 — besides the above 

 uses, it is most probably intended to conduct the sexes to 

 each other. This seems evidently the design in view in 



i Phil. Trans. 1810, p. 287. 2 p^H Trans. 1801, p. 483. 



3 Miiller in Illig. Mag. iv. 178. 



