XI 



PHYLUM ARTHROPODA 



623 



exoskeleton consists of dorsal or tergal and ventral or sternal 

 elements, sometimes separate from one another laterally, sometimes 

 united together 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 



\TLX, 



UJ 



i:. 528. Mouth- parts of the Diptera. A, of Tabanus ; B, of Culex. Lettering as in 

 preceding figure : hp. hypopharynx ; oc. ocellus. (From Lang.) 



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

 an additional pair the so-called maxillulce occur between the 

 mandibles and first maxillae. In a few eyes are absent. Most 

 have large compound or faceted eyes, and many have simple eyes 

 or ocelli as well ; in a few groups the latter are alone present. 

 The antennae vary in shape in different groups and sometimes 

 even in the sexes of the same species. They may be tapering, 

 moniliform, club-shaped, pectinate, or plume-like. In addition to 

 functioning as tactile appendages they bear the olfactory setse, and 

 there seems reason to believe that they act also as organs con- 

 cerned in the maintenance of the equilibrium of the body. The 



