MECHANISM OF FLIGHT 95 



sist of an external set which pass somewhat obliquely downwards and 

 backwards from the dorsal to the ventral wall, and a longitudinal 

 set, which pass from the anterior end of the thorax to the meso- 

 phragma, in the upper two-thirds or more of the cavity. It will 

 be recalled that the dorsal and lateral walls of the mesothorax are 

 not welded together by continuity of their chitin, but that there is a 

 lateral area between them where the wall of the thorax is membraneous, 

 thus permitting of some alteration in the shapes of the cavity; and 

 that there is a separate mechanism, consisting of a set of chitinous 

 rods arranged as a system of levers, by means of which the wing can 

 be brought into position for flight. 



Now when the wing is in position for flight, the veins project for 

 a short distance within the cavity of the thorax, and are so arranged 

 that the muscles act on them as on a lever, the longitudinal muscles, 

 which increase the vertical diameter of the thorax, causing a downward 

 displacement, and the antero-posterior set a corresponding upward dis- 

 placement. If the wing surface presented a uniform resistance to the 

 air throughout its area nothing would result from this, but this is 

 not the case, for the veins of the wing are always stronger in its 

 anterior portion, and this part consequently yields less to the pressure 

 than the part behind it. In this way the surface becomes an inclined 

 one, and the vibratory movement is therefore accompanied by transla- 

 tion. The mode of propulsion through the air has been aptly compared 

 by Marie to that by which a boat is moved through the water by .a 

 waterman's scull. 



It is evident that, within certain limits, the more the anterior veins 

 are strengthened and the posterior ones reduced the more effective 

 will the blade-like action of the wing become, and we have here a 

 confirmation of the dictum of Williston, that flies with such a venation 

 are more highly organized and more recent than those in which the 

 wing veins are distributed evenly over the surface. We may contrast 

 the two extremes of Phlebotomus and Hippobosca, the former of which, 

 notwithstanding its large wing area in proportion to the body weight, 

 is a feeble flier, while the latter, even when it contains a full grown 

 larva, is extremely active when on the wing. 



In the abdomen the arrangement of the muscles is simpler, and 

 corresponds with the more primitive structure of the body wall. The 

 vertical muscles are only feebly developed, and connect the tergite and 

 sternite of each segment with one another, the longitudinal muscles 

 connecting adjacent terga and sterna in a similar manner. The.. two 



