18 forming a scraping tool. The distal, tooth-bearing part of the mandible is 

 usually separated from the other part of the mandible by a transverse scar. 

 The mandibles are always asymmetrical, and their blades are situated one 

 above the other. The basal outer part bears 2 primary setae. In the 

 concavity of the dorsal surface of the left, or of the left and right mandible 

 is a tuft or row of hairlike spinules (peiiicillus) which is sometimes displaced 

 toward the margin of the upper blade; the rows of small setae are sometimes 

 present also on the outer-distal part of the blades. 



FIGURES 5-7. Labrus, mandibles, and maxillolabium: 



5 — labrus of Limnophilus rhombicus; 6 — mandibles of Plectroc ne mia latissima, dorsal: 

 left (A) and right (B); 7 — maxillolabium: Rhy a co ph il a nubila, general view, ventral (A), Rh. sep - 

 tentrionis, end of galea (B — after Nielsen). 



19 



Maxillae and labium are so tightly fused that they represent a maxillo- 

 labium (Figure 7) which encloses the submentum, the mentum and the 

 hypopharynx. The maxillolabium is contiguous with the ventral side of the 

 gnathal foramen and forms the wall of the preoral cavity opposite to the 

 labrum. The middle of the basal part of the maxillolabium forms the 

 submentum which is contiguous with the pregula. The submentum is 

 situated dorsally, inside the preoral cavity; it is membranous or contains 

 thin, rodlike, lateral or transverse sclerites; ventrally, near the base or 

 the middle of the submentum there is a typical, usually single sclerite; 

 there is a seta on each side of the anterior margin of this sclerite; this 

 sclerite may be single, paired or divided into three small plates. Deep, 

 oblique or longitudinal grooves separate cardo and stipes from the sub- 

 mentum. The cardo, which is contiguous to the gnathal margin of the 

 cranium, is almost completely sclerotized on the ventral and lateral sides, 

 and also partly on the dorsal side; in the larvae of Annulipalpia, this sclerite 



15 



