NO. 12 FLYING APPARATUS OF BLOW-FLY — RITTER 35 



part during the downstroke in consequence of its flexibility and the 

 resistance of the air. 



17. Toward the end of the upstroke the torsion is at its minimum. 



18. The folds of the surface and the hairiness of the wing affect 

 and probably heighten its mechanical effect on the air. 



19. The numerous small folds of the anal portion of the wing, 

 which is poor in veins, serve to strengthen it. 



