of a Species of Musca. 129 
posed of two hemispheres. In the acephalous larva it cannot be included 
in the head. In the fully-formed nymph, notwithstanding the existence 
of a large vesicular head, it is still on the outside of the lattcr, not enter- 
ing within it till the second transition, and in the fly. A rudimentary 
retina is first observed in the mature nymph, and it is not till the follow- 
ing age, and particularly in the fly, that we see this retina developed, and 
forming the pigment of the choroid, both to the eyes and ocelli. In the 
larva and two first ages of the nymph, the brain and thoracic ganglion 
seem confounded in one and the same mass ; while in the second transi- 
tion and in the fly, the thoracic ganglion is separated from the encephalic 
by a very distinct rachidian cord. The latter is simple in the Diptera, a 
discovery I can claim for myself. It is double in all the other orders of 
insects. The thoracic ganglion of the larva, like that of many other dip- 
terous forms, is furnished with numerous pairs of peculiar bodies of an 
ambieuous nature, which have not been previously mentioned, and which 
I have designated by the name of ganglionoidal bodies. 
Respiration in insects is a true circulation of air, and the vascular ap- 
paratus through which it takes place is the seat of the two most import- 
ant functions of the animal. To this part of the subject I shall again te- 
vert. In the larva, the stigmata, or respiratory orifices, consist of two 
pairs. Each of the anterior pair is in the form of a moveable fan with 
fifteen digitations; the posterior, lodged in what I have called the stig- 
matic cavity, the structure of which is worthy of admiration, are rather 
large, rounded, somewhat reniform, placed near each other, and incapable 
of motion, each having three small linear openings. When the trans- 
formation into a nymph takes place, the two pair of stigmata are re= 
jected by the larva, and left adhering to the walls of the pupal capsule, 
which is mérely the hardened and coloured tegument of that state. At 
the same time, the nymph, notwithstanding its embryo condition, and 
apparent death, is not destitute of a respiratory apparatus. We find in 
it a single pair of stigmata, namely, the anterior pair ; but it is very likely 
that they cease to exercise any active functions. After the fly is evolved 
from the nymph, it is provided with eight pair of simple stigmata, two 
bivalvular ones with hairy valves on the thorax, and six on the abdomen 
which are small, rounded, and encircled with a ring. 
The trachez, the only vascular system in insects, are, in the larva, 
wholly of a tubular or elastic description, and constitute a perfectly sym- 
metrical apparatus. The latter consists, in each half of the body, of a 
wide dorso-lateral canal, which deserves the name of the tracheal artery, 
continued in a direct line from the posterior to the anterior stigma, and 
emitting on the righ{ and left a determinate and regular number of nutri- 
tive branches. This arrangement incontestably proves that the inhala- 
tion of air or respiration takes place by the posterior stigmata. The two 
tracheal arteries communicate with each other anteriorly by a transverse 
canal. The tracheal system of the nymph closely resembles that of the 
VOL, XXXII, NO. LXI1I.—Janvuary 1842, I 
