30 University of California Publications. [Entomology 



xnent in some cases is that they may aid in adjusting the 

 center of gravity; the inflation of air sacs in one region caus- 

 ing the body fluids to occupy another. This reason, however, 

 can not account for their distribution. 



What appears to be the most important function of tlie air 

 sacs is their utility in the recoil after the wing stroke. The 

 muscles of flight are all so attached that the body contents at 

 the end of each stroke are greatly compressed and the return 

 stroke is started as a xecoil from this tense condition. The 

 elasticity of the body wall of the thorax produces this result 

 without the aid of the body contents, but not as well as with 

 it. If the body cavity is tense, as by the inflation of the air 

 sacs, the recoil becomes to a large extent pneumatic. 



Resume. — We have thus' seen that the flight of insects is a 

 process not duplicated elsewhere in the animal series; it is 

 unique not only in the matter of the structure of the wings 

 and other flying mechanism, but likewise in the character of 

 the stroke. There is still need of investigations that shall 

 settle the question of the exact nature of the stroke in the 

 higher forms. The mechanics of flight in its simplest form is 

 not very complicated, but in more specialized types of wing 

 structure several factors contribute toward its complication. 

 Of the factors modifying the stroke the effect of the resistance of 

 the air against the surface of the wing is always in evidence, 

 causing the wing to take an oblique position during each 

 stroke. The stroke may be further influenced b}' the action 

 of the minor muscles at the wing-roots, either temporarily or 

 continuously by their synchronization with the primary mus- 

 cles. Peculiarities in the structure of the articulation may 

 also contribute to the modification of the stroke in many ways. 

 The slower, vertical stroke contrasts strongly with the more 

 steady and rapid oblique movement of the wings, and the 

 latter presents a number of details requiring further investiga- 

 tion. Finally, a possible relation of the air sacs with the 

 recoil at the beginning of the stroke is pointed out as the 

 most reasonable explanation of the development of these 

 structures. 



ARTICULATION. 



The articulation of the Avings of insects has been carefully 

 studied by only three investigators: von Lendenfeldt ('81), 

 who described with i;reat detail the structure in one of the 



