INTRODUCTION. 
63 
tion to the size of the body, than in any other tribe 
of insects. The forms which they assume are very 
various ; but the most ordinary shape of the upper 
pair is triangular, with the apex of the triangle to- 
wards the body, while the outline of the under wings 
approaches to circular. They are traversed by nume- 
rous nervures, which give a great degree of strength 
to the wing, and hold in tension the thin elastic 
membrane of which it is composed. These nervures 
are tubular, and are permeated by an aerial and 
aqueous fluid, the action of which expands the wing 
when in a moist and corrugated state after the in- 
sect has emerged from the pupa. The principal 
branches rise from the point where the wing is at- 
tached to the body, and they divide towards the 
other extremity into numerous ramifications. The 
spaces into which the wing is divided by these ner- 
vures, are denominated areolets by Kirby and Spence ; 
and these authors regard the upper wings as divisible 
into three larger longitudinal sections, which they 
term areas. The costal area occupies the anterior 
margin : the anal area, a narrow space along the 
posterior margin ; and the intermediate area, all that 
part of the wing lying between the two others. The 
most conspicuous areolet in butterflies is towards 
the centre of the wing, at the base, and is usually 
closed on its outer side by transverse nervures. In 
many instances, however, there are no transverse 
nervures, and all the areolets are open towards the 
outer side. 
