514 LUMINOUS INSECTS. 



Which of these opinions is the more correct I do not pretend to decide. 

 But though the experiments of Mr. Macartney seem fairly to bear him out 

 in denying the existence of any ordinary combination of phosphorus in lu- 

 minous insects, there exists a contradiction in many of the statements, 

 which requires reconciling before final decision can be pronounced. The 

 different results obtained "by Forster and Spallanzani, who assert that glow- 

 worms shine more brilliantly in oxygen gas, and by Beckerheim, Dr. Hulme, 

 and Sir H. Davy, who could perceive no such effect, may perhaps be 

 accounted for by the supposition that in the latter instances the insects 

 having been taken more recently, 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 l , with those of Spallanzani and Dr. Hulme, 

 who found it to be extinguished by the same gas, as well as by carbonic 

 acid, nitrous and sulphuretted hydrogen gases. 2 Possibly some of these 

 contradictory results were occasioned by not adverting to the faculty which 

 the living insect possesses of extinguishing 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 hy- 

 pothesis 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 (Fu/gora), 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 everywhere 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 con- 

 duct the sexes to each other. This seems evidently the design in view in 

 those species in which, as in the common glow-worm (L. noctiluca), the 

 females are apterous. The torch which the wingless female, doomed to 

 crawl upon the grass, lights up at the approach of night, is a beacon which 

 unerringly guides the vagrant male to her " love-illumined form," however 

 obscure the place of her abode. It has been objected, however, to this 

 explanation, that since both larva and pupa, as De Geer observed 4 , and 

 the males shine as well as the females the meeting of the sexes can 

 scarcely be the object of their luminous provision. But this difficulty 

 appears to me easily surmounted. As the light proceeds from a peculiarly 

 organised substance, which probably must in part be elaborated in the larva 



i Phil Trans, 1810, p. 237. * Phil. Trans. 1801, p. 483. 



3 M-Uller in Illig. Mag. iv. 178. 4 iv. 49. 



