546 LUMINOUS INSECTS. 



With regard to the immediate source of the luminous properties of 

 insects, Mr. Macartney ascertained that in the common glow-worm, and 

 in Elater noctilucus and ignitus, the light proceeds from masses of a sub- 

 stance not generally differing, except in its yellow color, from the intersti- 

 tial substance (corps grassicux) of the rest of the body, closely applied 

 underneath those transparent parts of the insects' skin which afford the 

 light. In the glow-worm, besides the last-mentioned substance, which, 

 when the season for giving light is passed, is absorbed, and replaced by 

 the common interstitial substance, he observed on the inner side of the last 

 abdominal segment two minute oval sacs formed of an elastic spirally-wound 

 fibre similar to that of the tracheae, containing a soft yellow substance of a 

 closer texture than that which lines the adjoining region, and affording a 

 more permanent 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 cir- 

 cumstances shone uninterruptedly for several hours. Mr. Macartney 

 conceives, from the radiated structure of the interstitial substance sur- 

 rounding the oval yellow masses immediately under the transparent spots 

 in the thorax of Elater noctilucus, and the subtransparency of the adjoin- 

 ing crust, that the interstitial substance in this situation has also the pro- 

 perty 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.^ What peculiar organization con- 

 neither saw nor heard of any thing of the kind. On the whole, iherefore, the evidence up 

 to this time would seem to be in favor of the supposition that ignes fatui which flit about 

 and travel considerable distances are actually luminous insects as above supposed, however 

 rarely they may have come under the notice of entomologists. In the ignes fatui observed 

 by M. Weissenborn {Mag. of Nat. Hist. N. S. i. 553.), which were clearly caused by the 

 explosion of phosphureied hydrogen, there was " a succession of flashes" extending for 

 perhaps half a mile, but ihey passed over this distance "in less than a second," — an ap- 

 pearance entirely different from those leisurely movements mentioned by Mr. Chambers and 

 Mr. Wailes, or that by Mr. Main (3Iag. of Nat. Hist. N. S. i. 549.), in which the farmer, 

 who said he had knocked the luminous object down, described it as exactly like a "Maggy 

 long-legs" (Tipula oleracea), the very same insect with which Mr. Sheppard compared the 

 luminous appearance he witnessed. I will conclude this long noie with observing that a 

 very strong argument for ihe possibility of some flying insects being occasionally luminous 

 is afforded by the facts above stated of luminous caterpillars having been within these few 

 years observed for the fir.st time since entomology has been attended to, and that by observers 

 every way competent. If caterpillars so very common as those of Mamestra ohracea may 

 sometimes, though so rarely, be luminous, and if, as Dr. Boisduval suggests, and is very 

 probable, this appearance was caused by disease, it is obvious that flying insects may be 

 also occasionally (though seldom) luminous from disease, — a supposition which will at once 

 explain the rarity of the occurrence, and the circumstance that insects of such different 

 "enera, and even orders, are said to have exhibited this phenomenon. 



> 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 lumi- 

 nous point of the penuliimaie abdominal segment oi Lampyris noctilitca, pref^ems on its 

 exterior surface a network of hexagonal facets, convex above and concave below, consti- 

 tuting an apparatus absolutely similar to that invented by Fresnet for increasing the diffu- 

 sion 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 con- 

 trivance 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 



