HEXAPODA. 63 



the other segments, and it may also differ greatly in form 

 from them ; under such circumstances it is sometimes desig- 

 nated the metatarsus (met-a-tar'sus) (Fig. 77, ///). The last 

 segment of the tarsus usually bears one or two claws. 



On the ventral surface of the segments of the tarsus in 

 many insects are cushion-like structures ; these are called 

 pulvilli (pul-vil-li). The cuticle of the pulvilli is traversed by 

 numerous pores which open either at the surface of the cuticle 

 or through hollow hairs, the tenent hairs, and from which ex- 

 udes an adhesive fluid that enables the insect to walk on the 

 lower surface of objects. 



The wings. — The two pairs of wings are borne by the 

 mesothorax and metathorax ; the prothorax never bears 

 wings. 



In form, an insect's wing is a large membranous append- 

 age, which is thickened along certain lines. These thickened 

 lines are termed the veins or nerves of the wing ; and their 

 arrangement is described as the venation or neuration of the 

 wings. The thin spaces of the wings which are bounded by 

 the veins are called cells. When a cell is completely sur- 

 rounded by veins it is said to be closed ; but when it extends 

 to the margin of the wing it is said to be open. 



The wings of different insects vary greatly in structure, 

 and thus afford excellent distinctions for the purposes of 

 classification. The various parts of the wing have, there- 

 fore, received special names. There is considerable lack of 

 uniformity among entomologists as to the names applied to 

 these parts ; but we have adopted the set of terms defined 

 below as representing the best usage. 



An insect's wing is more or less triangular in outline ; it 

 therefore presents three margins : the costal margin, or costa 

 (Fig. 78, «, b) ; the outer margin (Fig. 78, b, c) ; and the inner 

 margin (Fig. 78, c, d). 



The angle at the base of the costal margin (Fig. 78, a) is 

 the humeral angle (hu'me-ral) ; that between the costal mar- 

 gin and the outer margin (Fig. 78, b) is the apex of the wing ; 



