32 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 56 



muscles the torsion of the wing surface is produced and its degree 

 changed as required. Apparently, in consequence of the action of 

 the pronator, the torsion is increased during the downstroke. 



The functions of the musculus anonymus and the musculus gracilis 

 could not be ascertained. 



Like von Lendenfeld (1884, opposed to Alarey, 1886), I am of 

 the opinion that the action of the direct musculature of the insect 

 and not the resistance of the air chiefly causes the complicated 

 changes in the shape and degree of torsion of the wings during 

 their movement. The direct musculature serves also for steering. 

 In several cases two muscles appear to perform nearly the same 

 function. In these cases we may suppose that one of the two 

 similar muscles is used in flight straight ahead, the second for 

 steering. If the fly wishes to turn to the right or to the left it can, 

 by means of the adductor, throw one wing back and by means of the 

 abductor direct the other forward, and so turn the side of the 

 contracted adductor. 



A few additional remarks should be made about the wing-joint. 



Doubtless an important condition for good flight is absence of 

 jerks and an elastic, unhindered, smooth movement of the wing. 

 Independent of the elasticity of the materials, peculiar devices espe- 

 cially adapted for this purpose, make sure of the movement being 

 of this kind. The processus pteralis thoracis IV, designated stroma 

 (pi. 9, fig, 26, ptIV), which is a process of the episternum, occupies 

 the axillary cavity of the wing, and is so situated that it comes to lie 

 under the terminal portion of the remigium, the processus pteralis 

 alse 3 (pi. 9, fig. 26, pts), during the downstroke. This piece, acting 

 like an elastic cushion, probably mitigates the shock which might 

 otherwise occur at the end of every downstroke. 



The pterale C (pi. 9, fig. 26, ptC), that resembles an articular 

 ball, and that fits into the socket of the processus pteralis alae 3, may 

 serve a similar purpose. It is to be mentioned concerning this that 

 the pterale C is actually in the socket of processus pterale alae 3 

 only when the wing is depressed. When the wing is raised, that 

 ball and this socket are far apart. Pterale C may, therefore, also be 

 considered as a sort of elastic buffer, serving to mitigate the shocks 

 that might otherwise ensue at the ends of the wing-strokes. 



The flexible zone also is a device serving this purpose. The broad 

 wing of Calliphora vomitoria may, in view of the rapidity of the 

 wing-beat, not be sufficiently elastic to permit an equable, unhindered 

 movement. The hovering flies, whose wings are long and narrow. 



