126 



INVERTEBRATE PHYSIOLOGY 



(Fig. 4A) shows that these movements actually do occur during tethered 

 flight in each stroke, up or down. The relative amount of movement is seen 

 to be different in the two strokes, showing that they are not symmetrical. 

 For the wings to operate always at the best angle of attack, the up and 

 down movements of the wings must take different paths. These paths are 

 determined by the setting of the articulation. The setting cannot be altered 

 during the course of a single cycle, for the direct muscles cannot sig- 

 nificantly change their tensions within the very brief cycle of operation of 

 the indirect muscles, 6 msec, in Sarcophaga. 



The agreement between the movements predicted from a study of CCI4- 

 treated flies and those found in flight are sufficiently good to give strong 

 support to the above analysis of the mechanics of flight in Diptera. As will 

 be indicated later, it is probably that all insects with the asynchronous mech- 

 anism have some sort of snap action at the articulation, for it plays an im- 

 portant role in assuring full amplitude of wing movement. In Fig. 4B are 

 presented results obtained on the wasp PoUstes sp. The record has been re- 

 drawn to eliminate anterior-posterior movements and the three records 



3 30Aec. 



Fig. 5. Movements of tlie scutellar lever recorded by optical levers from mirrors 

 on the scutellum. All records on fly, Sarcophaga bullata. A, normal ; B, wings re- 

 moved ; C, wings removed and articulation damaged. A, B, and C are taken on the 

 same fly, with the same magnification and time scale. Wing frequency is stated on the 

 record. D, erratic flight movements in CCl^-treated fly. E and F, parts of the same 

 record showing the development of a fast stop on the down stroke (E) and on the 

 up stroke (F). 



