THOMPSON: ANATOMY OF MOSQUITO. 
177 
The cells of the epithelium of the eight caeca are transparent, 
polygonal, subequal in size, and show considerable intercellular sub¬ 
stance. The nuclei are large and have nucleoli. Although the 
surface of these pouches appears nodulated as if from the contraction 
of muscle fibers, none were found. Sections show that the cells are 
on the average larger than the cells of the cardia or mid-intestine 
and very irregular in depth. The caeca usually contain a dark 
ochre-colored fluid. 
The stomach is cylindrical and extends back to the fourth or fifth 
segment of the abdomen. It is abundantly supplied with tracheae 
and the cells of its walls are polygonal, with granular, yellowish 
cytoplasm, large oval nuclei (17 x 10 g ), and nucleoli. The cells 
vary greatly in size, but are of uniform depth. Like the cells of 
the cardia and caeca they have a striated border. The walls of 
the stomach are nodulated, but this seems to be due more to the 
irregular size of the component cells than to the contraction of 
muscle fibers, although scattered muscles are visible in sections. 
In preparations, the basement membrane is very distinct. Protru¬ 
sions of cells or parts of cells into the lumen, either in fresh or sec¬ 
tioned material was not observed for any part of the midgut. 
The above description would not be wholly applicable to the 
stomach and caeca at all stages. For the processes by which the 
epithelium is destroyed and replaced during the pupa stage begin 
before larval life is ended, and involve progressively more and more 
cells as the wriggler nears the moment of pupation. Small regen¬ 
erative nuclei appear in increasing numbers between the bases of the 
cells of the epithelium. The cytoplasm near the free margin of the 
cells becomes spongy. The striated border dissolves in places and 
occasionally cells protrude into the lumen of the gut. 
The hind gut .— In the neighborhood of the fourth and fifth seg¬ 
ments of the abdomen the stomach narrows and the hind gut begins. 
This portion of the alimentary canal is divided into ileum, colon, 
and rectum. The ileum (pi. 16, fig. 45, i7), extends back to the 
eighth joint of the abdomen and is not separated from the stomach 
by a valve. Probably this is in adaptation to the continuous pas¬ 
sage of bulky wastes through the gut. In the imago, in which the 
meal of blood is stored in the stomach while it is being digested, a 
valve exists between the ileum and stomach. The freshly extracted 
ileum appears as if wholly composed of circular muscle fibers, is 
