92 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 82 



The fourth axillary (fig. 47 A, B, 4AX) is a plate of the dor.sal 

 wing membrane only. It is small in each wing and serves merely as a 

 connective between the edge of the tergum and the third axillary. 

 It is probably a detached lobe of the tergum, since it is usually absent 

 in insects that have a posterior notal wing process. 



Beneath the base of each wing are the epipleurites, or small scler- 

 ites derived from the pleuron, which are intimately associated with 

 the wing mechanism in the adult insect. In Dissostcira there are in 

 each segment two episternal epipleurites, or basalarcs, (fig. 48, iBao, 

 2Ba2, iBa.i, 2Ba:^), and a single epimeral epipleurite, or suhalare 

 (Sa). The basalares are hinged to the upper edge of the episternum 

 (Eps), and are connected with the subcostal region of the wing base 

 by a ligamentous thickening (a) of the ventral wing membrane. The 

 subalare (Sa) lies free in the subalar membrane behind the wing 

 process, but it is connected with the ventral plate of the second ax- 

 illary (2Ax) by a thickening (b) of the intervening membrane. 



THE WING MECHANISM 



Flying insects are uncjuestionably descended from wingless an- 

 cestors. When paranotal lobes were first evolved on the thoracic seg- 

 ments, the insect was already organized for terrestial locomotion — 

 there was no provision for future organs of flight. When movable 

 wings were evolved from the paranotal lobes, they had available for 

 their purposes only a motor mechanism developed for other pur- 

 poses. It needed but an area of flexibility at the base of each paranotal 

 extension to convert the lobe into a movable flap. The dorsal ends of 

 the pleura, previously supporting the bases of the paranotal lobes, 

 easily became fulcra on which the wing flaps could rock up and down. 

 A contraction of the longitudinal muscles of the dorsum could now 

 give a down-stroke to the wing flaps by producing an upward curva- 

 ture in the tergal plates of the wing-bearing segments, and probably 

 at first the elasticity of the terga sufficed to produce the up-stroke. 

 Thus, apparently, by the simple device of becoming flexible at their 

 bases, the paranotal lobes became wings- that could be weakly flapped 

 up and down by the simple motor equipment already at hand. 



Modern insects, however, have added much to the primitive wing 

 mechanism. In each of the wing-bearing segments there are powerful 

 tergal-depressor muscles, which, since they do not occur in the pro- 

 thorax or in the segments of the abdomen, are probably specially 

 developed wing muscles, though they may be supposed to have been 

 evolved from small, lateral tergo-sternal muscles such as are usually 



