STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 531 
action of this posterior cardiac region of the dorsal vessel. 
Lowne describes other openings in the wall of the “ heart” 
of the blow-fly larva, but I have been unable to find others 
than those already described in this larva; it has three pairs 
only. 
The dorsal aorta is the anterior continuation of the dorsal 
vessel, which gradually diminishes in diameter. When it 
reaches the fifth segment and les above the ganglion, it ter- 
minates in a peculiar cellular structure (fig. 24, c.7.), which 
in the blow-fly has a circular shape and was called by 
? In the larva of M. domestica it 
has not so pronounced a ring-like appearance, but is more 
elliptically compressed and rather A-shaped. The cells of 
which it is composed have a very characteristic appearance, 
Weismann the “ ring,’ 
fo) 
and are rather similar to a small group of cells lying on the 
neck of the proventriculus and at the anterior end of the 
dorsal vessel of the fly. From the lower sides of this cellular 
structure (fig. 28, c. 7.) the outer sheaths of the major cephalic 
imaginal discs depend, and extend anteriorly to the pharyngeal 
mass, enclosing between them the anterior portion of the great 
ventral blood sinus. | 
The pericardium lies in the four posterior segments of the 
body, and is delimited ventrally from the general body-cavity 
by a double row of large characteristic pericardial cells. ‘These 
cells have a fine homogeneous structure and are readily dis- 
tinguished from the adjacent adipose tissue cells, whose size 
they do not attain. The pericardial cavity contains a profuse 
supply of fine tracheal vessels which indicates a respiratory 
function. A similar condition occurs in the blow-fly larva, and 
Imms (1907) has described a rich pericardial tracheal supply 
in the larva Anopheles maculipennis, as also Vaney (1902) 
and Dell (1905) in the larva of Psychoda punctata. The 
adipose tissue cells (fig. 28, fc.) form the very prominent 
“fat-body.” They are arranged in folded cellular lamine 
that lie chiefly in the dorso-lateral regions of the body, and in 
section have the appearance shown in the figure. The cells 
have a similar structure to those of the adult fly; they are 
