28 ENTOMOLOGY 



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 of 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 m the typical manner, but is 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 



