294 



ARTHUR D. WHEDON. 



from the head they are often torn loose at their insertions and 

 withdrawn. The origins are upon a chitinous semi-circle near the 

 base of the submentum ; the insertions on the dorsal surface (i.e., 

 towards the buccal cavity) of the extreme proximal region of the 

 mentum, one immediately on either side of a slight median carina- 

 like thickening. 



The secondary (accessory) extensor muscles. These small 

 muscles lie transversely on the floor of the apex of the submentum, 

 one on either side ventral to the middle region of the primary and 

 secondary flexors. They are of a depressed cone shape, about 

 40 in diameter at the origin. The origin is upon a chitinous 

 semi-circle projecting from the ventral wall of the distal end of 

 the submentum midway between the median line and the lateral 

 margin. The insertion is upon a short tendon from the extreme 

 proximo-lateral angle of the squame. Without careful focusing 

 with a 1 6 mm. objective this seems on the lateral margin of the 

 submentum ; practically, the pull is upon both at once. 



The primary flexor muscles are probably the strongest pair in 

 the labium. They are heavy and cylindrical, and lie between the 

 primary extensors and the adductors medially and the secondary 

 flexors laterally, dorsal to the transverse secondary extensors. 

 They originate on semi-circles at the base of the submentum with 

 their inner sides close together. The insertion of each is on a 

 fairly large circular area of the ventral surface of the mentum at its 

 proximo-lateral angle (exclusive of the squame), and marked on 

 the surface by a convexity lateral to the thickened central portion 

 of the hinge. 



The secondary flexor muscles are thin and band-like, and twisted 

 at an angle of about 90 about the lateral aspects of the primary 

 flexors. They are crossed midway and ventrally, as indicated 

 above, by the secondary extensors. The origin is upon a trans- 

 verse chitinous semi-circle located ventro-laterally about midway 

 of the length of the submentum. The insertion is upon a similar 

 semi-circle on the latero-dorsal aspect of the squame. 



SUMMARY. 



Studies carried out upon the larval, transitional, and adult stages 

 of development show that : 



