LUMINOUS INSECTS. 



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the power of voluntarily extinguishing, not by retracting it 

 under a membrane, as Carradori imagined, but by some inscrut- 

 able 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 unin- 

 terruptedly for several hours. Mr. Macartney conceives, 

 from the radiated structure of the interstitial substance 

 surrounding the oval yellow masses immediately under the 

 transparent spots in the thorax of Elater noctilucus, and the 

 sub transparency of the adjoining crust, that the interstitial 

 substance in this situation has also the property of shining — 

 a supposition which, adverting to the luminous patch under 

 its elytra, and the fact that the incisures between the abdomi- 

 nal segments shine when stretched, may probably be extended 

 to the whole of the interstitial substance of its body. 1 What 

 peculiar organisation contributes to the production of light in 



1 The following interesting facts, in addition to those of Mr. Macartney, have 

 been observed by M. Morren, Professor of Botany in the University of Liege. 

 The corneous transparent cap (calotte), which covers the sac enclosing the 

 luminous matter in each luminous point of the penultimate abdominal segment 

 of Lampyris noctiluca, presents on its exterior surface a network of hexagonal 

 facets, convex above and concave below, constituting an apparatus absolutely 

 similar to that invented by Fresnet for increasing the diffusion of light, and 

 when this exterior portion of the cap is removed, the luminous matter loses a 

 great portion of its lustre, which mainly depends on this curious and beautiful 

 contrivance for augmenting it. The central facets are larger and more regular 

 than those of the margins, and each facet has in the middle a corneous hair bent 

 backwards, which hairs M. Morren conceives are intended to prevent the ad- 

 hesion of dust. The luminous masses contained in the two sacs are intersected 

 in every part with a vast multitude of trachean ramifications, which compose 

 also their common envelope, the whole proceeding from a large trachea, which 

 issues from a spiracle situated immediately at the side of the luminous mass, with 

 which it communicates by a small round lateral orifice near the margin of this 

 last ; thus fully confirming the opinion of those physiologists who conceive that 

 the luminous power under consideration is essentially connected with the act of 

 respiration. In fact, M. Morren found that when the spiracle next to the lu- 

 minous material is closed, the light is immediately extinguished, and re-appears 

 when it is opened. If the luminous sac be removed with its accompanying 

 trachea, it continues to shine ; but if this trachea be taken away or compressed 

 so as to hinder the access of air, the sac becomes obscure. This fact explains 

 how, in the insects of the genus Lampyris, as well as those of Elater ( Pyrophorus), 

 the light is not constant, but becomes more feeble at intervals, and why it is 

 increased during the flight or other energetic movement of the insect, and 

 diminished when it is in repose. It is, in fact, always in proportion to the 

 energy of the respiration of the insect, which, having the power of opening or 

 closing its spiracles at will, can thus also increase or diminish its light at 

 pleasure, though whenever it respires it cannot prevent it from shining. Some 

 differences excepted, the luminous apparatus of Lampyris splendidula is similar 

 to that of L. noctiluca above described ; and it is probable that a similar organis- 

 ation exists in the genus Pyrophorus. 



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