The glow-worm, remaining in concealment through the day, crawls abroad at night, when it appears surrounded by a beautiful radiant light of considerable intensity, and of a greenish colour. It is most brilliant two or three bow s be fore midnight ; and an elegant and interesting spectacle is presented by collecting several together in a glass vessel. More powerful emanations illuminate the animals ; some times the light is suddenly extinguished, sometimes it shines at protracted intervals, while the motion of the in sects produces perpetual variety : but it always grows fain ter and fainter, and last it almost totally disappears. The observer, in the course of his examination, discovers, that the exhibition and intensity of the light are partly un der the controul of the glow-worm ; that the place from which it emanates is in the last three rings of the abdomen ; and that it ceases entirely with the death of the animal, though a severed portion will continue to be illuminated, and after extinction the light will at a moderate interval be renewed.
The three rings, while illuminated, are of a pale yellow ish colour, and their internal surface is spread over with a layer of peculiar soft yellow substance, whose consistence resembles paste. The whole interior, however, is not covered, as it is more or less deficient along the inner edges of the rings, where it forms an irregular waving outline. We compare this substance to paste ; but, when magnifi ed, it is found to be organised like the common interstitial matter of the animal's body, except that it is of a closer texture and paler yellow ; and the segments of the abdo• men, behind which it is situated, are thin and transparent, on purpose to expose the internal illumination. Several years ago, Count Razoumousky, a learned Polish natural ist, discovered that, beides the rings, there were luminous points in the abdomen, giving out a more permanent light ; and the ingenious experiments of Mr Macartney prove, that two minute bodies, endowed with this property, are lodged in two slight depressions in the shell of the last ring. By the microscope, these are found to consist of two sacs, containing a substance similar to that lining the inner sur face of the rings ; and the membrane composing the sacs is so strong and elastic, as to resume its figure after the contents are discharged. The light thence proceeding is less under the controul of the insect than that of the lumin ous substance disseminated over the rings : it is rarely dis tinguished even through the clay, during the season that the glow-worm gives out light. The presence of these two bodies seems alluded to by M r Waller, in the Philosophical Transactions, so long ago as the year 1684, when speaking of those species of lampyri•es, both male and female of which are winged ; and Thunberg mentions one found in Japan with too vesicles on the extremity. The former
describes -their site as under the termination of the tail ; and observes, that the winged insects are extremely rare in England.
The luminous substance, when extracted from a dead glow-worm, gives out no light, though the sacs, n hen cut from the living animals, shine several hours after separa tion; and, if put into water, they will emit light uninter ruptedly for forty-eight hours. Whether they contain greater heat than the other parts of the animal, as a suffi cient number have never yet been accumulated to prove the fact by experiment, is not altogether free of doubt. John Templer, one of the earliest English observers of the na ture of the glow-worm, says," If my senses fail me not, she emits a sensible heat in her clear shining." Mr Macart ney also thought that, when shining brilliantly, the luminous rings communicated the sensation of warmth to the hand ; but, front remarking that the heat of the surrounding at mosphere, 69°, was not raised more than to 75° or 77°, on introducing a very sensible thermometer among several glow-worms, he concluded, that the actual difference of heat was insufficient to warn him of its presence. Some times, however, the luminous portion of the tail seemed to raise the thermometer more quickly than other parts of the body ; and on cutting it off several animals, he found, that if the thermometer were immediately applied, it would rise one or two degrees ; but that no effect whatever was pro duced after these parts were dead, though they continued to give out light. It ought to be kept in view, that many erroneously believe insects are entirely destitute of heat, whet eas its presence is sufficiently demonstrated by ac cumulating a number together in a limited space.
Besides the glow-worm properly so called, an insect wanting wings, there are several of the genus provided with them, and then generally denominated fire flies. In the hot climates, thousands shoot across the eye, and sparkle in the woods and bushes during night, with the most beau tiful and brilliant illumination. Some, resembling so many specks, are not larger than the common house-fly ; others are above half an inch long ; and several, collected in a glass vessel, emit sufficient light for reading a book. All are of the coleopterous tribe : both sexes have wings, and the emanations proceed from the last segments of the tail.