178 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
transparent, and has an intima which is thrown into longitudinal 
folds. Sections demonstrate a very thin epithelium, the nuclei of 
the flattened cells measuring 12 by 6.5 /x. Longitudinal muscles 
cannot be found. For a space immediately behind the stomach the 
circular muscles are also wanting (pi. 16, fig. 45, x ), and into this ♦ 
naked annulus the five Malpighian tubules empty. These tubules 
in every way resemble the Malpighian tubules of the perfect insect. 
The second region of the hind gut, the colon (pi. 14, figs. 26, 27, 
co ) is distinct from the ileum, but is not sharply marked off from the 
rectum. Raschke (’87) grouped colon and rectum together as 
“ Enddarm ” and Hurst’s figures include both as “ rectum/’ In the 
text, however, the colon would seem to make part of his “ileum.” 
The colon lies wholly within the eighth abdominal joint. Its walls 
have a deep epithelium of large polygonal cells and appear to be 
rather unyielding. Belts of slender circular muscles occur at regular 
intervals, but no longitudinal muscles can be demonstrated. The 
living cells of the Colon are granular. Sections show faint striations 
radiating from the region of the nucleus. The oval nuclei of the 
cells measure 12 by 16 /x. 
Posteriorly, the colon passes gradually into the rectum (pi. 16, 
fig. 44, rec) which is wholly within the last joint of the abdomen. 
The rectum has delicate walls that tear easily. Sections show a 
very thin epithelium, resembling in structure the epithelium of the 
colon, and a series of regularly placed, slender, circular muscles. 
The Pupa. 
The first absolutely diagnostic sign of the on-coming of the pupa¬ 
tion moult as observed in Culex stimulans is the appearance of two 
white spots in the prothorax, as the air chambers at the base of the 
pupal respiratory trumpets fill with air. At this moment probably 
the trumpets are evaginated beneath the cuticle. The larva be¬ 
comes increasingly turgid in aspect and intermittent peristalsis¬ 
like waves of contraction pass over its body. The turgescence is 
largely due to the evagination of the pupal thoracic appendages 
under the cuticle. The charging of the air-sacs usually precedes 
the moult by about three minutes, but in one instance eight minutes 
elapsed, during almost all of which time the larva lay passive. 
