STRUCTURE 91 



palliation, or by the extreme reduction in size of some of the 

 parts. 



Besides the division of the body into consecutive segments, 

 another feature is usually conspicuous ; the upper part, in many 

 segments, being differentiated from the lower and the two being 

 connected together by intervening parts in somewhat the same sort 

 of way as the segments themselves are connected. Such a differen- 

 tiation is never visible on the head, but may frequently be seen in 

 the thorax, and almost always in the abdomen. A dorsal and a 

 ventral aspect are thus separated, while the connecting bond on 

 either side forms a pleuron. By this differentiation a second form 

 of symmetry is introduced, for whereas there is but one upper and 

 one lower aspect, and the two do not correspond, there are two 

 lateral and similar areas. This bilateral symmetry is conspicuous 

 in nearly all the external parts of the body, and extends to most 

 of the. internal organs. The pleura, or lateral regions of the 

 sac, frequently remain membranous when the dorsal and ventral 

 aspects are hard. The dorsal parts of the Insect's rings are 

 also called by writers terga, or nota, and the ventral parts 

 sterna. 



The appendages of the body are : (1) a pair of antennae ; (2) 

 the trophi, constituted by three pairs of mouth-parts ; (3) three 

 pairs of legs ; (4) the wings 1 ; (5) abdominal appendages of various 

 kinds, but usually jointed. Before considering these in detail we 

 shall do well to make ourselves more fully acquainted with the 

 elementary details of the structure of the trunk. 



In the adult Insect the integument or crust of the body is 

 more or less hard or shell-like, sometimes, indeed, very hard, and 

 on examination it will be seen that besides the divisions into 

 segments and into dorsal, ventral, and pleural regions, there are 

 lines indicating the existence of other divisions, and it will be 

 found that by dissection along these lines distinct pieces can be 

 readily separated. Each hard piece that can be so separated is 

 called a sclerite, and the individual sclerites of a segment have 

 received names from entomotoniists. The sclerites are not really 



1 The wings, by many morphologists, are not included in the category of 

 "appendages"; they apparently, however, differ but little in their nature from 

 legs, both being outgrowths of the integument ; the wings are, however, always 

 post-embryonic in actual appearance, even when their rudiments can be detected in 

 the larva. No insect is hatched from the egg in the wing-bearing form. 



