550 



HISTORY OF INSECTS. 



shops also do the same. The largest in these 

 parts are about an inch long, and as much in 

 circumference, but others are not above three 

 quarters of an inch. Some are of a pure azure 



subject, has ascertained that in the common glow-worm, 

 and in Elater noctilucus and ignitus, the light proceeds 

 from masses of a substance not generally differing, except 

 in its yellow colour, from the instertitial substance (corps 

 graisseux) of the rest of the body, closely applied under- 

 neath those transparent parts of the insect's skin which 

 afford the light. In the glow-worm, besides the last- 

 mentioned substance, which, when the season for giving 

 light is passed, is absorbed, and replaced by the common 

 instertitial substance, he observed on the inner side of the 

 last abdominal segment two minute oval sacs formed of 

 an elastic spirally-wound fibre similar to that of the 

 tracheae, containing a soft yellow substance of a closer 

 texture than that which lines the adjoining region, and 

 affording a more permanent and brilliant light. This 

 light he found to be less under the control of the insect, 

 than that from the adjoining luminous substance, which 

 it has the power of voluntarily extinguishing, not by re- 

 tracting it under a membrane, as Carradori imagined, 

 but by some inscrutable change dependent upon its will ; 

 and when the latter substance was extracted from living 

 glow-worms, it afforded no light, while the two sacs in 

 like circumstances shone uninterruptedly for several 

 hours. Mr Macartney conceives, from the radiated 

 structure of the instertitial substance surrounding the oval 

 yellow masses immediately under the transparent spots 

 in the thorax of Elater noctilucus, and the sub-transpar- 

 ency of the adjoining crust, that the instertitial substance 

 in this situation has also the property of shining a sup- 

 position which, if De Geer and other authors be correct in 

 stating that this insect has two luminous patches under its 

 elytra, and that the incisures between the abdominal seg- 

 ments shine when stretched, may probably be extended 

 to the whole of the instertitial substance of its body. What 

 peculiar organization contributes to the production of 

 light in the hollow projections of Fulgora laternaria and 

 candelaria, the hollow antennae of Puusus spheerocerus, 

 and under the whole integument of Scolopendra electrica, 

 Mr Macartney was unable to ascertain. Respecting this 

 last he remarks, what I have myself observed, that there 

 is an apparent effusion of a luminous fluid on its surface, 

 that may be received upon the hand, which exhibits a 

 phosphoric light for a few seconds afterwards ; and that 

 it will not shine unless it have been previously exposed 

 for a short time to the solar light.* 



With respect to the remote cause of the luminous 

 property of insects, philosophers are considerably divided 

 in opinion. The disciples of modern chemistry have, 

 in general, with Dr Darwin, referred it to the slow 

 combustion of some combination of phosphorus secreted 

 from their fluids by an appropriate organization, and 

 entering into combination with the oxygen supplied in 

 respiration. This opinion is very plausibly built upon 

 the ascertained existence of phosphoric acid as an animal 

 secretion ; the great resemblance between the light of 

 phosphorus in slow combustion and animal light ; the 

 remarkably large spiracula in glow-worms ; and upon 

 the statement that the light of the glow-worm is ren- 

 dered more brilliant by the application of heat and oxy- 

 gen gas, and is extinguished by cold and by hydrogen 

 and carbonic acid gases. From these last facts, Spallan- 

 zani was led to regard the luminous matter as a com- 

 pound of hydrogen and carbonated hydrogen gas. Car- 

 radori having found that the luminous portion of the 

 belly of the Italian glow-worm (Lampyris italica) shone 



* Phil. Trans. 1810, p. 281. Mr Macartney's statement on 

 this point is not very clear. He probably means that the insect 

 will not shine in a dark place in the daytime, unless previously 



ixiAvd to the solar IL-iit : for it is often seen to shine at ni/ht 

 couid have had no recent exposure to the sun. 



colour, others of a pure gold, and others again 

 have a mixture of pure gold and azure colours ; 

 but they are all very brilliant, and extremely 

 beautiful. These insects, as is well known, 



in vacuo, in oil, in water, and when under other circum- 

 stances where the pressure of oxygen gas was precluded, 

 with Brugnatelli ascribed the property in question to the 

 imbibition of light separated from the food or air taken 

 into the body, and afterwards secreteo hi a sensible form. 

 Lastly, Mr Macartney having ascertained by experiment 

 that the light of a glow-worm is not diminished by immer- 

 sion in water, or increased by the application of heat ; 

 that the substance aflbrding it, though poetically em- 

 ployed for lighting the fairies' tapers, is incapable of in- 

 flammation 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 hypothesis as unsatisfactory, but 

 without substituting any other explanation ; suggesting, 

 however, that the facts he observed are more favourable 

 to the supposition of light being a quality of matter than 

 a substance. 



Which of these opinions is the more correct, I do not 

 pretend to decide. But though the experiments of Mi- 

 Macartney seem fairly to bear him out in denying the exis- 

 tence of any ordinary combination of phosphorus in lumin- 

 ous insects, there exists a contradiction in many of the 

 statements, which requires reconciling before final deci- 

 sion can be pronounced. The different results obtained by 

 Forster and Spallanzani, who assert that glow-worms 

 shine more brilliantly in oxygen gas, and by Becker- 

 heim, Dr Hulme, and Sir H. Davy, who could perceive 

 no such effect, may perhaps be accounted for by the sup- 

 position 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 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 ol 

 the glow-worm not to be sensibly diminished in hydro- 

 gen 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 sulphurated 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 plea- 

 sure ; or diflerent philosophers may have experimented 

 on diflerent 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 (Futgoru) 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 con- 

 sider 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 besides the 

 above uses, it is most probably intended to conduct the 

 sexes to each other. This seems evidently the design 

 in view in those species in which, as in the common 

 glow-worm, (L. noutiluca, L.) the females are apterous. 

 The torch which the wingless female, dooired to crawl 

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

 beacon which unerringly guides the vagrant male to her 



