200 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
already noted, from which part of the rectum comes, and the fact 
that the degenerative and regenerative processes in the epithelium of 
the ileum commence and are completed near the Malpighian tubules 
before they involve the rest of the hind gut, recall the positions of 
the anterior and posterior imaginal rings in the postgut of many 
Hies. Even the peculiar metamorphosis of the colon may be com¬ 
pared with the formation of the hind gut in the specialized Cyclor- 
rhaplia by a forward growth from the anal and a backward 
growth from the Malpighian regenerative rings, whether the older 
epithelium is destroyed completely as in Gastrophilus, or partly as 
in Musca. The alterations of the midgut also are precisely the same 
as those which are found in all hies, even the most specialized. 
Moreover, comparisons may be made for other organs than those 
of the alimentary canal. For example, on the basis of the anten¬ 
nal buds, a series of genera may be formed, beginning with Culex > 
where the larval — and pupal — eye is independent of the antennal 
bud, through hies where the eyes develop from the same invagi¬ 
nations as the antennal buds, up to Musca, a species in which the 
whole head is formed from a pair of large invaginations. But all 
such comparisons must be taken to mean no more than this, that 
Culex and the other hies present in varying degree and under vary¬ 
ing conditions, tendencies common to the insect group of which 
they are representatives. 
