686 
J. DAVIDSON. 
The ventral branch from each of the prothoracic spiracles 
passes towards the median line, and both fuse to form a 
continnous, stout, transverse trachea {t. t.), which extends 
across the median line joining the two spiracles. Several 
smaller tracheal tubes arise from it, and aerate the muscles 
in the head and prothorax. A branch also passes into the 
first pair of legs. 
The prothoracic spiracle is connected with the metathoracic 
spiracle by a small trachea (c. tr.), which arises near the 
former spiracle, and passing posteriorly, bifurcates, one branch 
passing into the second pair of legs, and the other joining the 
second tracheal trunk near the metathoracic spiracle. 
The ventral tracheal branch from each of the metathoracic 
spiracles also fuse in the median line, and form a stout 
trachea, which lies transversely across the thorax, connecting 
the two spiracles, and giving off a number of smaller tracheae, 
which aerate the muscles of the thorax. A small trachea 
arises from the tracheal trunk of this spiracle, and passes to 
the tracheal trunk of the first abdominal spiracle, giving off 
a branch to the third leg. A branch also passes forward into 
each of the second pair of legs. 
D. The Circulatory System. 
A tubular heart or dorsal vessel has been described by 
Witlaczil (1882, p. 35) as occurring in Aphis, and this 
author described its development in his later work (1884, 
p. 652).^ Mordwilko (1895, p. 356) also refers to the presence 
of a chambered dorsal vessel in Tram a. Dreyfus (1894, 
p. 238), on the other hand, was unable to find any trace of 
this structure in Phylloxera; and Grove (1909, p. 26) 
failed to find it in Siphonophora rosarum. I have been 
* Ich fand bei Aphis dass das Herz und die Aorta aus einem Strang 
von Mesodernizellen gebildet werden, welcher Anfangs solid, durch 
Theilung seine zellen vermehrt und indein er sicli ausbblt, wahrscbeinlicli 
die Blutkorpcben ensteben liisst. Die venosen Ostien fand icb bier’an 
der Grenze je zweier Segmente des Abdomens. 
