OF THE TRACIIE.E IN INSECTS. 
9 
which enters, over the carbonic acid which escapes, is in all cases so necessary to expan 
the trachea? as Dr. Graham suggests, because I hare found the small branch Ws 
of Musca still full of air after it had been drowned by immersion for some 
m a 
g< i r m 
water 
On putting some larva? of Ilelolontha ioto water, I was surprised to see a considerable 
formation of bubbles on the skin, especially at a point below each spiracle, while no 
bubble ever appeared from the spiracles themselves. If, however, the water is first boiled, 
then no bubbles are produced, and the larvae very soon, say in about a quarter of an hour! 
become motionless, though, if pricked, they still contract a little. Their flesh is then 
quite soft and flabby, while generally it is tolerably firm to the touch. I expected to 
have found the trachea? free from air, or nearly so; but this was not the case. Like 
other insects, these larva) readily recover from their suffocation when they are taken out 
of the water. 
^ The larvae of flies are also naked fleshy grubs ; and I expected them to behave in a 
similar manner; this, however, is by no means ihe case. They live much Ion 
water. When they are placed in it, no bubbles form on their skin; nor does it sc 
make any difference to them whether the water is boiled or not : I put four into some 
boiled water, and the same number into water which had not been boiled ; and at the end 
of forty-four hours they still moved a little, gently turning their heads from one side to 
the other. These facts seem to me to prove that the larvae of Melo/ontha breathe partly 
by means of their skin, and that those of Musca do not. 
Yet these opposite states of the skin may be necessary for these larva?, living as they 
do under such different circumstances *. 
This result was quite unexpected by me ; yet it throws much light on the intermediate 
stages which, upon the principle of natural selection, must have existed between ordinary 
larvae, respiring principally through spiracles, and those which, like the larva? of Botys, of 
Dragonflies, Ephemerae, &c, breathe by means of foliaceous expansions of the skin. 
In most insects the air will be found, after death, filling the fine ends of the trachea?. 
In some cases, however, as in many parts of Carabus, Ifelolontho, Acheta, mpparchia, &c, 
the smaller branchlets are generally, even very soon after death, filled with fluid, and can 
therefore scarcely be distinguished, or they even become quite invisible. Th is happens very 
frequently in Lepidoptera, Coleoptera, and Orthoptera ; but I have not noticed it so often 
in Hymenoptera, Diptera, or Neuroptera. It is not, however, constant in the first three 
orders; and in Necrophorus, for example, the very fine tubules may be beautifully seen. 
The larvae, at least of Coleoptera and Lepidoptera, do not in this respect resemble the per- 
fect insect ; but in Acheta all the three forms are alike. 
* M. Lyonet, writing before the observations of Dr. Graham had thrown so much light upon the subject, and mis- 
led principally by the absence of special respiratory movements in many insects, doubted whether the tracheae were 
organs of respiration, and suggested that one at least of their uses might be " de concourir avec les nerfs, a la contrac- 
tion des muscles, pour ope'rer les mouvements," though I confess that I do not quite understand in what way the 
with the nerves. In support 
nil 
M. Lvonet deduces from it : and 
This experiment, however, 
of movement in the same way as M. Lyonet himself would, had he been treated in a similar manner. 
