342 



LUMINOUS INSECTS. 



With regard to the immediate source of the luminous pro- 

 perties of insects, Mr. Macartney ascertained that in the 

 common glow-worm, and in Elater noctilucus and ignitus, the 

 light proceeds from masses of a substance not generally differ- 

 ing, except in its yellow colour, from the interstitial substance 

 {corps grassieux) of the rest of the body, closely applied under- 

 neath 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 sub- 

 stance 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 



and Florence in 1827, my two sons and myself amused ourselves the night we 

 slept at Pietramala, in observing the well known miniature volcano of hydrogen 

 gas, near to that place, which has been burning for centuries; but though there, 

 if any where, as it is probable that hydrogen gas rises more or less from 

 crevices in the whole adjoining district, there ought to be travelling or flitting 

 lights, if such be possible, we neither saw nor heard of any thing of the kind. 

 On the whole, therefore, the evidence up to this time would seem to be in favour 

 of the supposition that ignes fatui which flit about and travel considerable dis- 

 tances 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 phosphureted hydrogen, there was " a succession of flashes " 

 extending for perhaps half a mile, but they passed over this distance " in less 

 than a second," — an appearance entirely different from those leisurely move- 

 ments mentioned by Mr. Chambers and Mr. Wailes, or that by Mr. Main {Mag. 

 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 note with observing that a 

 very strong argument for the 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 first time since entomology has 

 been attended to, and that by observers every way competent. If caterpillars 

 so very common as those of Mamestra oleracea may sometimes, though so rarely, 

 be luminous, and if, as Dr. Boisduval suggests, and is very probable, this ap- 

 pearance 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 genera, and even orders, are said to have exhibited this phe- 

 nomenon. 



