LABIUM OF AXAX J UNITS. 289 



The abductors of the lateral lobes lie wholly within the mentum 

 (Figs. I, 10 and n). They are strong flattened muscles lying 

 laterally, broad at the origin upon the dorsal surface of the skele- 

 ton slightly distad from the hinge and tapering to a point at the 

 insertion of the tendon upon the outer side of the lateral lobe, 

 where a slight prominence gives leverage. The mesial side of each 

 abductor overlaps the corresponding adductor muscle. A tra- 

 cheal branch lies ventral to it laterad of the adductor. 



The adductors of the lateral lobes are the largest muscles of 

 the labium (Figs. I, 8 to 13). The origin is upon the central area 

 of the rounded extension of the submentum where it gains excel- 

 lent support from the heavily chitinized and concave surface. The 

 muscle tapers to its insertion upon the long tendon attached to the 

 mesial side of the lateral lobe. As shown in Fig. 8 this muscle 

 seems to be comprised of a dorsal and a ventral layer. In Fig. 

 9 most of the muscle had been dissected off. In the region of the 

 hinge and about their origins the adductors are flanked by the sec- 

 ondary (accessory) flexors (Fig. 13). Fig. 9 shows how the ad- 

 ductors may also aid greatly in flexing the labium. The first effect 

 of their contraction being the adduction of the lateral lobes, that of 

 continued contraction across the hinge, flexion. 



Muscles Activating the Mentum and the Submentum. Four 

 pairs of muscles within the labium are concerned with its extension 

 and flexion, the primary flexors, the secondary (accessory) flexors, 

 the primary extensors, and the secondary (accessory) extensors. 



The primary flexor muscles lie in the ventral portion of the sub- 

 mentum (when the labium is folded). They have their origins 

 side by side upon a V-shaped tongue of chitin on the median 

 ventral region of the base of the submentum (Fig. i). They run 

 distally, accompanied by a pair of tracheal trunks, taper gradually, 

 pass over the hinge lateral to the adductors, and insert upon the 

 ventral surface of the mentum at its proximal end. Each is at- 

 tached to a short, transverse, chitinized, thickening projecting 

 mesiad from the lateral carina (Figs. I, 9, 10 and 12). In pass- 

 ing around the adductor muscle it narrows to nearly tendon-like 

 proportions. A comparison of Figs. 8, 9, and 10 will make clear 

 its action as a flexor. 



The secondary flexor muscles are located in the region of the 

 21 



