LUMINOUS INSECTS. 427 
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 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, where pos- 
sibly their irritability was, as Brown would say, ac- 
cumulated 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 extinguished by the same 
gas, as well as by carbonic acid, nitrous and sulphuret- 
ted hydrogene gases’. Possibly some of these contra- 
dictory results were occasioned 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 object. ‘Thus 
in the lantern-flies (Fulgora), 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 (Eater), if we con- 
sider the infinite numbers that in certain climates and 
@ Philos. Trans. 1810, p. 287. b Tid. 1801, p. 483. 
¢ See above, p. 228. 
