LUMINOUS INSECTS. 611 
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 (conps graisseuv) 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 trache, 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 that at least some ignes fatui are caused by 
luminous insects. George Wailes, Esq., on the other hand, has given in the Mntom. 
Mag. i. 351. the result of his father’s observations and his own, and has also quoted 
those of Major Blesson, from Jameson’s Hdinb. 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 myself 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 fayour of the supposition that ignes fatui which flit about and 
travel considerable distances are Penis 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 (Wag. 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” ( Zipula 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 of Ma- 
mestra oleracea may sometimes, though so rarely, be luminous, and if, as Dr. Bois- 
duval suggests, and is yery 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, 
