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 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 behind 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
