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, a , li) ; 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 ; 
