LUMINOUS INSECTS. 511 



With regard to the immediate source of the luminous properties of in- 

 sects, 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 differing, except in its yellow colour, from the interstitial sub- 

 stance (corps graisseux} 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 intersti- 

 tial 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 permament 



Joseph Simpson, a fisherman, at Frieston near Boston, all strongly corroborating the 

 above statements as to the probability tbat at least some ignes fatui are caused by 

 luminous insects. George Wailes, Esq., on the other hand, has given in the Entom. 

 Mag. i. 351. the result of his father's observations and his own, and has also quoted 

 those of Major Blesson, from Jameson's Edinb. New Phil Journ. for Jan. 1833, in proof 

 " that the moving ignis fatuus of this country always owes its origin to the sponta- 

 neous ignition of gaseous particles " (meaning, I presume, phosphuretted or carbu- 

 retted hydrogen gas), and consequently cannot be an insect. Without pretending to 

 deny that these gases may be a cause of stationary ignes fatui, I confess myself quite 

 unable to conceive of a small mass of these inflammable materials "about the size of 

 the hand " moving at the height of " three feet from the surface of the ground " and 

 " for the distance of fifty yards nearly parallel with the road," as in the instance 

 seen by Mr. Wailes's father, and being luminous all the time. A mass of hydrogen 

 gas and its compounds, as is well known, whether large or small, when once inflamed 

 (and if not inflamed it cannot be luminous), burns but for an instant except renewed 

 by a fresh supply. In passing the Apennines between Bologna and Florence in 1827, 

 my two sons and nryself amused ourselves the night we slept at Pietramala, in ob- 

 serving 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 distances are actually luminous insects as above supposed, how- 

 ever 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 phosphuretted 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 argu- 

 ment 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 ofMa- 

 mestra oleracea may sometimes, though so rarely, be luminous, and if, as Dr. Bois- 

 duval 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 genera, and even orders, are said 

 to have exhibited this phenomenon. 



