36 
BRITISH APHIDES. 
rule the younger individuals have the greatest endur- 
ance of such treatment. 
I examined, for a quarter of an hour, the motion of 
the dorsal apparatus of D. platanoides under paraffin 
oil. There was a regular pulsation of about 40 beats 
in a minute. The constriction appeared to begin from 
the thoracic region, to proceed down the back, and 
then to end at the last abdominal ring, where there 
appeared to be a transparent sac, which repeatedly 
contracted, and probably forced a colourless liquid 
over the body. I have in vain looked for particles 
analogous to the oat- shaped corpuscles to be seen in 
the wing nervures of Chrysojpa and other insects. The 
oil- globules, so freely distributed over the bodies of 
Aphides do not circulate, though they slightly change 
their position during the inhalation of air through the 
stomata. 
A remarkable circumstance is connected with the 
circulation in Aphides, viz. the presence of a pulsating 
sac at the knee-joint of each tibia. Under petroleum 
spirit I have watched these contractions for twenty 
minutes, and counted them at from 120 to 180 in a 
minute, according to the activity of the insect. Under 
a 2 ^ 2 " objective, I have noticed a current of fluid pass 
down one side of the leg and a return up the other. 
This was indicated by the movement of two or three 
globules (oily ?) which occasionally came in sight. 
This observation of moving particles in the blood of the 
living insect, however, is by no means easily made. 
By treatment with dilute chromic acid after death, 
the walls of this sac may be traced. 
Cuvier and Leon Dufour have recorded their opinions, 
that when there is a circulation of air through the body 
of an animal for respiration, we ought not to expect a 
circulation of the nourishing fluid or blood. The latter 
author doubts even that the ‘‘ cordum dorsale ” of 
insects acts at all as a circulatory vessel. He says, 
that after twenty years’ practice in dissecting insects, 
he cannot prove it to be either a secreting or a circu- 
