LUMINOUS INSECTS. 4.23 
in a direct line; but this was not the case. It had the 
same motions as a Tipula, flying upwards and down- 
wards, backwards and forwards, sometimes appearing 
as settled, and sometimes as hovering in the air.— 
Whatever be the true nature of these meteors, of which 
so much is said and so little known, it is singular how 
few modern instances of their having been observed are 
on record. Dr. Darwin declares, that though in the 
course of a long life he had been out in the night, and 
in the places where they are said to appear, times with- 
out number, he had never seen any thing of the kind: 
and from the silence of other philosophers of our own 
times, it should seem that their experience is similar. 
With regard to the immediate source of the luminous 
properties of these insects, Mr. Macartney, to whom 
we are indebted for the most recent investigation on the 
subject, has ascertained that in the common glow-worm, 
and in Elater noctilucus and ignitus, the light proceeds 
from masses of a substance not generally differing, ex- 
cept in its yellow colour, from the interstitial substance 
(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 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. 
