1914} Life History and Anatomy of Siphona Plusie 305 
the Malpighian tubules, is a nearly uniform tube of considerable 
size, the anterior portion being the chyle stomach and the 
posterior portion, the intestine. 
The Malpighian tubules in this insect are particularly 
interesting in regard to their position in the body cavity. They 
arise from the alimentary canal, as shown in the drawing, as 
two lateral tubes, each of which divides into two tubes. The 
two from the right side swing forward and the two from the left 
side run towards the posterior end. This is somewhat different 
from what would be expected, and is a departure from the 
general rule. The usual number of tubes is four in the dipterous 
larve, but all four either turn and run posteriorly, keeping to 
their respective sides, as in the blow fly, or else the right and 
left branch, each sending one tube forward and one backward. 
The portion of the alimentary canal from the entrance of the 
Malpighian tubules to the anusis the metenteron. This portion 
is considerably smaller and shorter than the mesenteron and 
has a thick muscular coat. 
The dorsal blood vessel or heart; the tracheal system; the 
nervous system, and the salivary glands, are shown in plate 
XL Pip. <2. 
The heart is a thin-walled muscular tube which extends 
nearly the length of the body, lying in the pericardial cavity 
just beneath the dorsal wall. It tapers from a good-sized sac 
to a fine tube as it runs forward. 
The tracheal system is composed of two main trunks with 
large spiracles opening on the posterior segment. Branches 
are given off from the two main trunks at each body segment 
and these finer tubes wind in about the alimentary canal. The 
anterior spiracles are wanting, except in the very young larve. 
The salivary glands, which extend from the mouth, starting 
as a single narrow duct, branch out beneath the pharynx and 
extend, one on either side of the alimentary canal, for more 
than a third of the length of the body. 
The brain and body ganglion, shown in the same figure, 
compose the nervous system of the larva. The hemispheres 
encircle the oesophagus just forward of the proventriculus, and 
the main body of the body ganglion extends backward on the 
ventral side nearly the same distance that the salivary glands 
extend on the lateral sides. 
