THOMPSON: ANATOMY OF MOSQUITO. 
167 
the abdomen. In fresh material under transmitted light the cyto¬ 
plasm is steel gray in color, the nuclei are transparent, and the cells 
are filled with refringent granules. Sections show a definite struc¬ 
tureless border along the free edges of the cells. This stains deeply 
with “ orange G ” but I am not certain whether it is to be regarded 
as a cuticle. 
The ileo-colon is a canal of small diameter and its walls are thin. 
The cytoplasm of the epithelial cells as seen in sections is often 
vacuolated, especially toward the base of the cells. A thin, dis¬ 
tinct, cuticular lining is visible, and for a short space immediately 
behind the entrance of the Malpighian tubules this is roughened 
by bristle-like chitinous papillae which point caudad (pi. 14, fig. 21, 
^7c). This hirsute belt cannot be regarded as characterizing a 
“region” in the gut of the imago and the area does not correspond 
to any distinct region in the hind gut of the larva. The longitu¬ 
dinal and circular muscles of the ileo-colon are fairly well developed 
and like the stomach this part of the alimentary canal has an exten¬ 
sive tracheal supply. 
Posteriorly, the ileo-colon passes imperceptibly into the rectum. 
The depth of the epithelial cells of the rectum is variable and the 
muscle coats are less developed than in the ileo-colon. There are 
six rectal papillae, each consisting of a cone of large epithelial cells 
arranged about a common axis. The tracheal supply to the rectum 
is generous, especially to the papillae. Tracheae derived from the 
main longitudinal trunks that follow the alimentary canal on either 
side run to each papilla, emit branches that course over the base of 
the cone and adjacent walls of the rectum, and then one or more 
branches enter the axis of the papilla. These axial tracheae ascend 
to the apex of the cone and divide, their branches curving back in a 
somewhat tortuous course among the component cells. 
The Larva. 
The larger and broader head and the elongate respiratory siphon 
on the dorsum of the eighth joint of the abdomen are perhaps the 
most obvious of the external differences that distinguish the wrig¬ 
glers of Culex from those of Anopheles. Otherwise the ground plan 
is similar: the three thoracic segments are consolidated into a single 
