OF INSECTS. 
323 
ocelli are present, which is very frequently the case, 
they are usually three in number, and placed on the 
vertex. 
The mesothorax is the segment of the anterior 
part of the body most developed in these insects, and 
it is so to such an extent as to leave hut little space 
for the others, the pro thorax being, in some cases, 
almost evanescent. The wings are somewhat long, 
and in general rather narrow, commonly clear and 
transparent, with simple nervures disappearing before 
reaching the apex, and crossed by a few transverse 
ones, the neuration being greatly more simple than 
in the hymenoptera. At the base of the wing, we 
very frequently find tw’o rounded membranous scales 
applied with their faces to each other, which are 
named wing- scales, alulets, or winglets. They are 
sometimes of considerable size, and doubtless aid the 
movements of the wings materially in the act of flying. 
The use of the haltercs, which have been already 
alluded to as two slender clubbed bodies placed be- 
hind the wings, can scarcely be said to be accurately 
known, but it is conjectured that they assist in giving 
a proper poise to the body in flight. Some have 
likewise supposed them to be connected with the 
function of respiration. They are often of a pale 
colour, and, when the winglets are large, partially 
covered by these appendages. 
The abdomen is attached to the thorax only by a 
small portion of its transverse diameter ; it is often 
long and narrow, sometimes oval or nearly round, 
varying in the number of its segments from five to 
