Because they may secure themselves from becoming the prey of the nightingale or other nocturnal birds. Mr. White, indeed, thought that they regularly extinguished their torch between the hours of eleven and twelve. The light of the glow-worm may perhaps, occasionally deter its insect enemies from making an attack on it, as the wolf and other ravenous beasts of prey are deterred from making an approach on travellers by night, when encircled by fire. - John Murray.
Because the head is margined with a horny band, or plate, under which the eyes are situate. This prevents all upward vision ; and blinds, or winkers, are so fixed at the sides of his eyes, as greatly to impede the view of all lateral objects. The chief end of this creature in his nightly peregrinations, is to seek his mate, always beneath him on the earth; and hence his apparatus appears designed to facilitate his search, confining his view entirely to what is before him or beneath him. The first serves to direct his flight, the others present the object of his pursuit: and, as we commonly, and with advantage, place our hand over the brow, to obstruct the rays of light falling from above, which enables us to see clearer an object on the ground, so must the projecting hood of this creature converge the visual rays to a point beneath. Knapp.
Because, this distinction being especially noticebale at the season of love, it is probable that the light serves the purpose of directing the male to her. Some time after the female has laid her eggs, which also shine in the dark, this light disappears in both sexes. - Blumenbach.
This theory of the light of the glow-worm has, it is feared, more poetical prettiness than truth. Mr. Ren-nie, (whose observant genius has raised him to the Professorship of Natural History, in King's College, Lon-dan,) has lately communicated such facts to the Journal of the Royal Institution, as must convince the reader of the fallacy of the above. Unfortunately, the grubs, - which being in a state of infancy, are therefore incapable of propagating - exhibit a no less brilliant light than the perfect insect. De Geer says the light of the grub was paler. He also remarked the same light in the nymph state, which he describes as " very lively and brilliant;" and in this stage of existence it is still less capable of propagating than in that of larva. " Of what use then," he asks, " is the light displayed by the glow-worm ?
It must serve some purpose yet unknown. The authors who have spoken of the male glow-worms say positively that they shine in the dark as well as the females."
We must devote a page or two to the economy of this very interesting creature, which every body knows by name, but comparatively few have seen. A correspondent of the Philosophical Magazine describes it as follows: " The female deposits her eggs in the month of June or July, among moss, grass, etc. These eggs are of a yellow colour, and emit light. After remaining about five or six weeks, the larvae break their shells and make their appearance ; at first they appear white, and are very small, but they soon increase in size, and their colour changes to a dark brown, or nearly black. The body of the larva is formed of eleven rings. It has six feet, and two rows of reddish spots down the back. It emits light in the dark ; this light arises from the last ring of its body under the tail, and appears like two brilliant spots when attentively examined. The larvae are seen creeping about and shining daring the fine nights of autumn, and the light they emit is to direct them to their food. They feed on small snails, the carcases of insects, etc. They frequently cast off their skins : after the expiration of about one year and nine months from their birth, they arrive at their perfect size. They then cease to eat, cast off their skin, and assume another appearance. The form of the perfect insect may be discovered through a thin skin that covers them. After remaining in this state two or three weeks, (scarcely ever moving,) they throw off their last skin, and arrive at perfection. The male then appears a perfect beetle, with wings and cover, to the same. The female, on the contrary, has neither wings, nor wing-cases; she is larger than the male, and of a lighter colour. It is the female that principally shines in a perfect state. Her light is far superior to that emitted by the larvae, and arises from the three last rings of the body on the lower side."* Numerous opinions have obtained on the proximate cause or source of this curious illumination; and several experiments and observations to determine this point will be found collected in a small volume of Experimental Researches, by John Murray, F.S.A. F.L.S., etc. We have not space for the details, but quote the corollaries deduced from them by this ingenious observer:
1. Light, as connected with the glow-worm, is a subtle, evanescent principle, perhaps connected with a peculiar organized structure, or attached to a substance circumfused round the vitellula of the ovum, or integrating with it; unsupported by any chemical action, and confinable by the transparent film, or capsule, which imprisons it. "2. This light is permanent, and independent of any power possessed by the insect over it, except in so far as it can withdraw the luminous matter from the window, or transparent medium, through which it is discerned, burying it in the interstitial matter, or secreting it under an opaque shell. 3. The light is not connected with any of the functions of animal life as to its support or continuance, as with the spiracula, or breathing apparatus, and even the extinction of life itself does not extinguish the power and property of emitting light. 4. The luminous matter is not adherent exteriorly, but included in a capsule, which preserves it from extrinsic agency and contingency. 5. The light seems connected with peculiar organization, which elevated temperatures destroy, perhaps by decomposition, but which low temperatures only suspend temporarily. This very suspension, indeed, by cold, and restoration by warmth, and by a temperature equal to that of animal heat, goes far to prove a peculiar function, inherent in the capsule, and capable of educing and sustaining the phenomenon." Mr. Murray thus concludes: " The use to which it is subservient in the animal economy, it is difficult to ascertain - ' we see but in part.' Its very existence, however, proves that it is a condition indispensable to its being. Providence has tipt the insect with living fire - a non-material ignition - burning, yet not consumed - even extinguished by a temperature which the animal system, with which it is so singularly interwoven, cannot withstand. It may be a ' lamp to its path,' to guide it to its food, subserving the additional purpose of warding off its enemies - while it may also be the luminous point that directs the nightingale to its proper prey."
* Phil. Mag. vol.lviii, p. 53.