OF INSECTS. 323 



ocelli are present, which is very frequently the case* 

 they are usually three in number, and placed on the s 

 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 but little space 

 for the others, the prothorax 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 two 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 halteres, 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 



