PHYLUM ARTHROPODA 



637 



The head presents no trace of segmentation, but the history of 

 its development indicates that it may be looiied upon as composed 

 of a prostomium and about five segments, intimately united 

 together. It varies a good deal in shape, but always presents the 

 regions that have already been described in the case of the Cock- 

 roach. Of these the epicraninm is the most extensive ; the clypeus, 

 situated in front of it, supports the labrum ; the gcncc are situated 

 laterally, and a median piece, the gxda, occupies the middle of the 

 ventral surface. Some- 

 times the head is sunk 

 within the anterior part 

 of the thorax ; sometimes 

 it is free from the latter ; 

 and there may be, as in 

 the Cockroach, a short 

 narrow region or ncclv, 

 covered with soft skin, 

 supported only by iso- 

 lated cervical sclerites, on 

 the ventral aspect. 



The three segments of 

 the thorax — pro-, nicso-, 

 and mcta - thorax — are 

 usually firmly iinited to- 

 gether ; but in some 

 Insects the prothorax is 

 movable upon the other 

 segments : it is usually 

 the smallest of the three 

 segments. In each the 

 exoskeleton consists of 

 dorsal or tergal and ven- 

 tral or sternal elements, 

 sometimes separate from 

 one another laterally, 

 sometimes united to- 

 gether in such a way as 

 to form complete rings 

 round the segments. 

 Laterally projecting processes or pleura are sometimes developed. 



The abdomen contains from seven to eleven segments, enclosed 

 in tergal and sternal shields. In some Insects the first abdominal 

 segment is united with the thorax so as to appear to belong to 

 the latter region. _ , 



The appendages of the head are four pairs, as in the Cock- 

 roach ; but a considerable variation is observable in the different 

 orders, especially as regards the jaws. In certain of the Aptera 



Fir;. .ilC— A, mouth parts of the Honey-bee (Apis 

 mellifica) ; B. the two pairs of maxilto. cm. eye ; 

 a. antenna ; c. cardo ; ep. epipharynx ; Ihr. labrum ; 

 li ligula ; m. nientum ; mm, j/u-i, first pair of maxilLw ; 

 md. mandible ; -pi. labial palpi ; pm. palp of the first 

 pair of maxillai ; prg. parafclossa ; sm. submentum ; 

 stm. stipes of the first maxillae. (From Lang.) 



