THE RESPIRATORY SYSTEM. 
35 
wliite silvery threads through the skin, and are so 
disposed that they often give an appearance as if here 
the skin of the insect were cut up into dorsal plates. 
I have failed to trace in other insects the spiral 
thread so commonly seen to traverse the interior of 
these tracheae. 
The removal of carbonic acid and the oxygenation 
of the animal juices is probably in great measure 
effected by diffusion of the gases. The trachem 
speedily empty themselves of air after the death of the 
insect. Air would appear to be here in some condition 
of solution, for it is a curious fact, that the insect 
plunged into ether disengages a stream of bubbles 
from each stoma, the volume of which, if collected, 
would equal the size of the whole body. This expul- 
sion of air has been many times seen on the stage of 
the microscope. 
Sir John Lubbock has shown how the tortuous form 
of the trachem permits perfect freedom in the motions 
of an insect, and allows of considerable extension and 
contraction of its body. 
THE CIRCULATORY SYSTEM. 
Up to the present time no distinct dorsal vessel with 
definite walls has been discovered in Aphis. That some 
such vessel exists is very probable, from the slow vermi- 
cular motion, often to be traced down the medial line of 
the upper side in several transparent species. I have 
seen this motion distinctly in Gallipterus coryli^ and yet 
better in Drepanosiphum platanoides. It is remarkable 
that whilst submergence in water speedily kills most 
Aphides, some will live many minutes under the ordi- 
nary paraffin oil now so much used for illumination. 
They will also live for half an hour under weak glyce- 
rine, and thus their circulation may be watched on the 
microscope stage under either of these liquids. As a 
