1911] Muir and Kershaw — Embryology of Head of Pristhesancus I'll 



has fused with the elypeus, the false sutures on the head-capsule 

 are distinct. The retractor muscles of the setae can be seen pass- 

 ing to the back of the head-capsule. 



17th day. The head has become firmer; otherwise there is 

 little change. 



18th day. (Fig. 11) The pseudo-sutures across the head from 

 eye to eye are distinct. The tips of the mandibles are barbed, 

 and the maxillary setse have their lateral edges turned up, and 

 the outer sides of their tips corrugated. There are bristles on 

 the labrum. 



19tli day. The maxillary setae are three-ridged longitudinally 

 (a central ridge, with the lateral edges turned up to form two 

 lateral ridges). 



20th day. (Fig. 12) A slight alteration in the pseudo-sutures 

 of the head-capsule. The maxillary plates are firmer and more 

 chitinous. Tip of mandible reaches to tip of epipharynx. Tip 

 of maxillary seta reaches somewhat beyond tip of epipharynx. 

 The seta-ends are still outside the labium, i. e., not in the trough 

 of the labium. The lateral edges of the labium meet along dorsal 

 central line from tip to epipharynx. The basal joint of the labium 

 has become fused with the head capsule, the gular region and the 

 ventral part of the maxillary plates, the only free part being a 

 small ventral sclerite to which the retractor muscle is attached. 



21st day, 9 a. m. (Fig. 13) The tips of the setae reach a 

 little beyond the tip of the epipharynx. The epipharynx has 

 slightly lengthened. 



21st day, noon. (Fig. 14) There are slight changes in the 

 pseudo-sutures of the head. The seta-tips remain as before. 

 The second joint of labium is well defined. The labrum and 

 epipharynx are well defined. 



21st day, half-hatched nymph. The occipital region, hitherto 

 partly invaginated into the prothorax, has greatly lengthened, 

 owing to the straightening out of the head, and become chitinous. 

 The 2nd joint of the labium has lengthened. The junction 

 of the gular region with the basal joint of labium is complete. 

 There is no line of demarcation between elypeus and labrum, 

 but the epipharynx is distinct from the latter. The only pseudo- 

 sutures remaining are those bounding the labral-clypeal region, 

 and the deep transverse constriction between the eyes. In fact. 



