5 
(o/e) 
EN TOMOLOGY 
into the question of the origin of the segmented from the un- 
segmented “ worms.” Cope, Packard and others give the me- 
chanical explanation which is here summarized. In a thin- 
skinned, unsegmented worm, the flexures of the body initiated 
by the muscular system would throw the integument into 
folds, much as in the leech, and with the thickening of the 
integument, segmentation would appear from the fact that the 
deposit of chitin would be least at the places of greatest flex- 
ure, i. e., the valleys of the folds, and greatest at the places 
of least flexure, i. e., the crests of the folds. This explana- 
tion, which has been elaborated in some detail by the Neo- 
Lamarckians, applies also to the segmentation of the limbs, as 
well as the body. 
Head.—In an insect several of the most anterior pairs of 
primary appendages have been brought together to co-operate 
as mouth parts and sense organs, and the segments to which 
they belong have become compacted into a single mass—the 
head—in which the original segmentation is difficult to trace. 
The thickened cuticula cf the head forms a skull, which 
serves as a fulcrum for the mouth parts, furnishes a base of 
attachment for muscles and protects the brain and other 
organs. 
While the jaws of most insects can only open and _ shut, 
transversely, their range of action is enlarged by movements 
of the entire head, which are permitted by the articulation 
between the head and thorax. 
As a rule, one segment overlaps the one next behind; but 
the head, though not a single segment of course, never over- 
laps the prothorax in the typical manner, but 1s usually re- 
ceived into that segment. This condition, which may possibly 
have been brought about simply by the backward pull of the 
muscles that move the head, has certain mechanical advantages 
over the alternative condition, in securing, most economically, 
freedom of movement of the head and protection for the artic- 
ulation itself. 
The size and strength of the skull are usually proportionate 
