THE CAUDA OR TAIL. 
25 
wliich assigns to these siphuncles the task of rapidly 
aerating the circulating juices 5 I believe the theory 
breaks down under the following facts. Mo anatomist 
has yet shown any real connection between the 
trachece and the tube of the cornicle. Morren, it is 
true, gives a figure to show the lacing of the tracheae 
or air-vessels close to the lease of the. cornicle, but he 
found no anastomosis whatever with the siphuncular 
canal. I have repeatedly myself sought for such a 
communication, but without success. 
Again, as the sixth abdominal ring has its own pair f 
of stomata placed in the immediate vicinity of the 1 
cornicles, it is not likely that both these apertures ' 
should have the same function. 
Once more, it is a little remarkable, that the highly | 
refractive oil-like globules seen within the cavities of ■ 
the cornicles should have been mistaken by authors 
for air-bubbles. These globules may be readily 
expressed from the tubes, when their true nature 
becomes obvious {vide Plate B, fig. 6 ; Plate A, figs. 
15, 16). The cornicles must be regarded as the ^ 
external terminations of excretory ducts, which do not 
permit the regurgitation or recession of air into the 
body of the Aphis. 
THE CAUDA OH TAIL. 
The presence or absence of this adjunct to the 
abdomen gives a good distinctive character, and affords 
material help in cutting up into sections such exten- 
sive genera as Aphis proper^ as now restricted. The 
Cauda occurs in various dav ate ^ acuminate^ 
recurved-^ and ensiform. It is exserted from the ninth 
abdominal ring, immediately above the anal plate, 
between which and itself the anus is situated. Occa- 
sionally it has an imbricated exterior, and almost 
always is furnished with thr ee o r four pairs of stout 
bristles. It is to be noticed that when the nectaries 
