56 



THE BIOLOGY OF INSECTS 



free to move up or down, to spread so as to project outwards 

 from the axis of the body, or to be drawn inwards so that 

 jj p its length is approxi- 



mately parallel to 

 that axis as well as 

 to be partly turned 

 on its axis so that 

 its lower aspect is 

 directed backwards. 

 The spreading and 

 indrawing move- 

 ments are brought 

 about respectively 

 by extensor and 

 flexor muscles in- 

 serted at the wing 

 base. The forward 

 turn of the wing- 

 surface is due to the 

 action of pro?iator 

 muscles (Fig. 19, 

 B, pr) inserted into 

 a sclerite (anterior 

 parapterum) at the 

 front costal wing- 

 base, and also to the 

 resistant action of 

 the air on the flexible 

 wing-membrane. 

 But the large and 

 powerful muscles 

 which by their con- 

 traction move the 

 wings up and down 

 are in the vast 

 majority of insects 

 attached not directly 

 to the wing-base but to regions of the thoracic wall. The 



Fig. 19. — Principal Wing Muscles of male 

 Honey Bee {Apis tnellijica), as seen in 

 median section through thorax (A), and 

 internal view of right pleuron of meso- 

 thorax (B). el, elevators ; dp, depressors ; 

 pr, pronator : Jfl, flexor muscles ; pa, para- 

 pterum ; a, axillary sclerites. C2, C3, haunches 

 of second and third legs. X 15. After 

 R. E. Snodgrass. 



