512 LUMINOUS INSECTS. 



and brilliant light. This light he found to be less under the control of the 

 insect than that from the adjoining luminous substance, which it has the 

 power of voluntarily extinguishing, not by retracting it under a membrane, 

 as Carradori imagined, but by some inscrutable 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 uninter- 

 ruptedly 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 subtransparency of the adjoining crust, that the interstitial sub- 

 stance 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 abdominal 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 the 

 hollow projection of Fulgora laternaria, the hollow antennae of Pausus 

 spkcerocerus, and under the whole integument of Geophilus electricus, Mr. 

 Macartney was unable to ascertain. Respecting this last he remarks, what 

 I have myself observed, that there is an apparent effusion of a luminous 

 fluid on its surface, that may be received upon the hand, which exhibits a 

 phosphoric light for a few seconds afterwards ; and that it will not shine 

 unless it have been previously exposed for a short time to the solar 

 light. 2 



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 cor- 

 neous 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 con- 

 cave 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 re- 

 moved, the luminous matter loses a great portion of its lustre, which mainly de- 

 pends 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 in- 

 tended to prevent the adhesion 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 luminous 

 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 organisation 

 exists in the genus Pyrophorus. 



*'Phtt. Trans. 1810, p. 281. Mr. Macartney's statement on this point is not very 



