16 



AN INTRODUCTION TO ENTOMOLOGY 



each pair or the non-development of one of them; which of these 

 alternatives is the case has not been definitely determined. 



It is clear, however, that there has been a grouping of the seg- 

 ments in pairs in the region where the appendages are doubled, for 

 corresponding with each tergum there are two sterna and two pairs of 

 spiracles. 



A few of the anterior body segments, usually three or four in 

 number, and sometimes one or two of the caudal segments remain 

 single. Frequently one of the anterior single segments is legless, but 

 the particular segment that lacks legs differs in the different families. 



The head, which is as distinct as is the head of insects, bears the 



antennae, the eyes, and the mouth-parts. The antennas are short, 



and usually consist each of seven segments. The eyes are usually 



represented by a group of ocelli on each side of the 



head; but the ocelli vary greatly in ntimber, and are '^ 



sometimes absent. The mouth-parts consist of an P 



upper lip or labrum; a pair of mandibles; and a pair 



of jaws, which are united at the base, forming a large 



plate, which is known as the gnaihochilarium. In 



the genus Polyxenus there is a pair of lobes between s 



the mandibles and the gnathochilarium, which have 



been named the maxillulcB. (paragnatha?). 



The labrum is merely the anterior part of the ^j^- '^9-— A mandi- 



11 r .1 1 -1 1 • • • Die of Julus; c, 



upper wall of the head and, as m insects, is not an cardo; d,d,teeth; 



appendage. The mandibles, in the forms in which ^' muscle; ma, 



they are best developed, are fitted for biting, and nate ' plate; s, 



consist of several parts (Fig. 19) ; but in some forms ? ^f^i^ ^ (After 



they are vestigial. The gnathochilarium (Fig. 20) is 



complicated in structure, the details of which vary greatly in different 



genera. 



Pig. 20. — The gnathochilarium or second jaws of three diplopods; A, Spirostrtp- 

 tus; B, Julus; C, Glomeris: c, cardo; h, liypostoma; Ig, linguce; m, ment'om. 

 pm, promentum; st, stipes (After Silvestri). 



