290 ARTHUR D. WHEDON. 



hinge. They have not been described previously. In the con- 

 tracted condition they are somewhat fan-shaped (Figs. 9 and n). 

 The origins of the muscles lie at their broad ends, the attachments 

 to the submentum nearly circling those of the adductors (Fig. 13). 

 From here they pass ventrally next to the skeleton to insert for a 

 very short distance on the ventro-lateral floor of the mentum just 

 proxinad to the origins of the abductors of the lateral lobes. When 

 the labium is extended they assume so different a form that they 

 have probably been mistaken by earlier workers for the proximal 

 ends of the abductors. The dissection shown in Fig. 10 indicates 

 the true relations. It will be noted that each passes laterad to the 

 chitinous process, which serves for the insertion of the primary 

 extensor muscle. 



The primary extensor muscles occupy the dorsal portion, when 

 the labium is folded, of the submentum (Figs, i and 8 to 13). 

 Their origins are quite far apart upon the ends of the posterior 

 branches of the tentorium. They are heavy at the bases but each 

 soon tapers to attach to a small cord-like tendon which passes be- 

 tween the adductor of the lateral lobe and the secondary flexor to 

 a triangular chitinous process projecting from the mentum into the 

 cavity of the rounded elbow of the submentum. This mechanism 

 is a very nice adaptation for increasing the leverage of this strong 

 extensor muscle. A comparison of Figs. 9 and 10 leaves no doubt 

 of the mode by which the extensor functions. 



The secondary (accessory) extensor muscles have also escaped 

 description in the past. They are small band-like muscles at- 

 tached to the meso-lateral regions of the elbow of the submentum 

 (Figs. 5, 6, and 8 to 10). Their origins are at the extreme distal 

 end of the submentum ; their insertions latero-ventral to those of 

 the secondary flexors. Their function seems to be that of produc- 

 ing in the skeleton of the elbow of the submentum the necessary 

 increased curvature during extension of the labium. 



THE LABIUM DURING TRANSFORMATION. 



As indicated earlier, a period precedes transformation during 

 which the larva ceases to feed and wanders restlessly in and out 

 of the water, finally remaining quietly on a twig or other support 



