Some general conclusions have been drawn regarding the nature of luminous animals, most of which apply to the glow-worm and first, that this property is not constant, but exists only at certain periods, and in particular states of the animal's body. The site of its emanations, also, re sides in a particular substance, compared to paste ; and the light is differently regulated while that substance is in the animal's living body, or separated from it. In the former case, it is intermitting : it is commonly produced or aug mented by a muscular effort, and is sometimes absolutely dependent on the will of the insect. In the latter case of separation, the luminous exhibition is usually permanent, until becoming extinct, after which it may be restored di rectly by friction, concussion, and the application of warmth. No diminution of the substance follows the ex hibition of light, however long it may be protracted. It does not require the presence of pure air, nor is it extin guished by other gases. The luminous appearance of living animals is not exhausted by long continuance, or fre quent repetition, nor is it accumulated by exposure to na tural light. From these principles, it is inferred, that this property not being dependent on any foreign source, is in herent in a peculiarly organized animal substance, and is regulated by the same laws which govern all the other functions of living beings.
Many conjectures have been indulged regarding the use of the light exhibited by luminous animals. Some have boldly declared, that it is for no specific purpose ; others, more prudently, affirm that it is utterly unknown : while a third class, substituting opinion for experiment, at once de cide that, in the glow-worm at least, it is a wise provision of nature for promoting the concourse of the sexes. Un
doubtedly this reason at first sight is plausible ; but it ought to have been previously ascertained, that the male, while flying through the air, is sensible of the luminous emana tions from the body of his grovelling mate. We know little of the different senses of insects ; hearing seems al together denied to certain species, and others are void of any external organs of sight : hut what is more extraordina ry, neither the number nor size of the eyes apparently in crease the acuteness of vision. It ought constantly to be preserved in remembrance, that the beautiful structures of the bee are erected, and all its complicated operations performed in the dark ; but that other organs, and especial ly the antenna, are constantly resorted to by them as a guide. Emanations undoubtedly proceed from the bodies of fe males at certain periods, which produce a lively impression on the sensations of the male, and we should be inclined to admit that there might be some analogous concomitant of the luminous exhibition of the female glow-worm ; but the probability of this hypothesis is greatly diminished, on considering, that the emanations of light belong to the earliest period, even when the insect is in the larva state. We must therefore necessarily conclude, that naturalists have not yet discovered the real uses of this remarkable property.
See Bartholinus de lace 9nimaliuzn ; Philosonhical Transactions, vol. vi. p. 2178 ; vol. xv. p. 841 ; De Geer, Memoires sur les Insectes, torn. iv. p. 29 ; Aldrovandus, De Insectis, p. 494 ; Geoffroy, Mem. sur les Insect. torn. i. p. 166. (c.)