548 



LUMINOUS INSECTS. 



tapers^ is incapable of inflammaiion if applied to the flame of a candle 

 or red-hot iron ; and when separated from the body exhibits no sensible 

 heat on the thermometer's being applied to it — rejects the preceding 

 hypotheses as unsatisfactory, but without substituting any other explana- 

 tion ; suijf^esting, however, that the facts he observed are more (livorable 

 to the supposition of light being a quality of matter than a substance.- 

 Lastly, Dr. Todd finding that the luminous substance of Lampyris con- 

 tinues to shine when detached, sometimes for a longer and at others a 

 shorter period, but never exceeding twenty minutes, and that under mer- 

 cury, various gasses, water, and in vacuo, considers it solely as an effect 

 of vitality.^ 



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 

 luminous 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 asserts 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 accumu- 

 lated by a longer abstinence : but it is not so easy to reconcile the experi- 

 ment 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 extinguished by the same gas, as well as by 

 carbonic acid, nitrous and sulphureted hydrogen gases.^ 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 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 n)eans of 

 defence against their enemies. In different kinds of insects, however, it 



' "And lor nighi-lapers crop iheir waxen ihiglis, 



And light ihem at the fiery glow-worms' eyes." 



* Some experiments made by my friend the Rev. R. Sheppard on the glow-worm are 

 worthy of being recorded. — One of the receptacles lieing extracted with a penknife, con- 

 tinued luminous ; but on being immersed in camphorated spirit of wine, became immediately 

 extinct. The animal, with one of its receptacles uninjured, being plunged into the same 

 spirit, became apparently lifeless in less than a minute ; but the receptacle continued lumi- 

 nous for five minutes, the light gradually disappearing. — Having extracted the luminous 

 matter from the receptacles, in two days they were healed, and filled wiih luminous matter 

 as before. He found this matter to lose its luminous property, and become dry and glossy 

 like gum, in about two minutes; bni it recovered il again on beins moistened with saliva, 

 and again lost ii when dried. When the matter was extracted from two or three glow- 

 worms, and covered with liquid gum-arabic, it continued luminous for upwards of a quarter 

 of an hour. 



' F/iU. Trans. 182-1. « Phil- Tram. 1810. p. 287. * Phil. Trans. 1801, p. 483. 



