Some small insects, like fire flies and glow worms, give out their own light. They are luminous.
Many insects are possessed of a luminous preparation or secretion, which has all the advantages of our lamps and candles, without their inconveniences; which gives light sufficient to direct our motion; which is incapable of burning; and whose lustre is maintained without needing fresh supplies of oil, or the application of snuffers.
Of the insects thus singularly provided, the common Glow-worm (Lampyris noctiluca) is the most familiar instance. -This insect in shape somewhat resembles a caterpillar, only it is much more depressed; and the light proceeds from a pale-coloured patch that terminates, the underside of the abdomen.
It has been supposed by many, that the males of the different species of lampyris do not possess the property of giving out any light; but it is now ascertained that this supposition is inaccurate, though their light is much less vivid than that of the female. Ray first pointed out this fact with respect to (L. noctiluca.) Geoffrey also observed, that the male of this species has four small luminous points, two on each of the two last segments of the belly: and his observation has been recently confirmed by Miller. This last entomologist, indeed, saw only two shining spots; but from the insects having the power of withdrawing them out of sight, so that not the smallest trace of light remains, he thinks it is not improbable that at times two other points, still smaller, may be exhibited, as Geoffrey has described. In the males of L. splendidula, and of L. hemiptera, the light is very distinct, and may be seen in the former while flying. The females have the same faculty of extinguishing or concealing their light; a very necessary provision to guard them from the attacks of nocturnal birds. Mr. White even thinks that they regularly put it out between eleven and twelve every night, and they have also the power of rendering it for a while more vivid than ordinary.
Though many of the females of the different species of lam pyris are without wings, and even elytra, fin Coleoptera,) this is not the case with all. The female of L. Italica, a species common in Italy, and which, if we may trust to the accuracy of the account given by Mr. Waller, in the Philosophical Transactions for 1684, would seem to have been taken by him in Hertfordshire, is winged; and when a number of these moving stars are seen to dart through the air in a dark light, nothing can have a more beautiful effect. Dr. Smith says, that the beaus of Italy are accustomed in an evening to adorn the heads of the ladies with these artificial diamonds, by sticking them into their hair; and a similar custom prevails amongst the ladies of India.
Besides the golden species of the genus Lampuris, all of which are probably more or less luminous, another insect of the beetle tribe, Elater noctilucus, is endowed with the same property, and that in a much higher degree. This insect, which is an inch long, and about one-third of an inch broad, gives out its principal light from two transparent eye-like tubercles placed upon the thorax; but there are also two luminous patches concealed under the elytra, which are not visible except when the insect is flying, at which time it appears adorned with four brilliant gems of the most beautiful golden-blue lustre: in fact, the whole body is full of light, which shines out between the abdominal segments when stretched. The light emitted by the two thoracic tubercles alone is so considerable, that the smallest print may be read by moving one of these insects along the lines; and in the West India islands, particularly in St. Domingo, where they are very common, the natives were formerly accustomed to employ those living lamps, which they called cucuij instead of candles, in performing their evening household occu pations. In travelling at night, they used to tie one to each great toe; and in fishing and hunting, required no other flam beau. - Pietro Martire's Decades of the New World, quoted in Madoc, p. 543. Southey has happily introduced this insect ir. his "Madoc," as furnishing the lamp by which Coatel rescued the British hero from the hands of the Mexican priests.