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SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I39 



cranium and the greatly reduced labium is concealed above a median 

 postgenal lobe (Hstm) between the maxillae (Mx). 



The darkly sclerotized dentate lobe between the maxillae has com- 

 monly been regarded as a part of the labium, "mentum" or "submen- 



Fig. 2. — Structure of the posterior or ventral wall of the head in different 

 insects, mostly diagrammatic. 



A, Generalized structure of the posterior head wall and the mouth part at- 

 tachments. B, The labium enclosed between the postgenae. C, The labium 

 separated from the occipital foramen by intruding hypostomal lobes of the post- 

 genae. D, The labium entirely separated from the foramen by union of postgenal 

 lobes forming a postgenal bridge. E, Postgenal bridge lengthened. F, Larval 

 head of Chironomus, postgenal bridge produced in a toothed lobe, labium dis- 

 placed dorsally. 



For, occipital foramen ; Gu, gula ; hs, hypostomal sulcus ; Hstm, hypostomium, 

 hypochilum ; Lb, labium ; Md, mandible ; ms, median postgenal suture ; Mt, 

 mentum ; Mx, maxilla ; Pge, postgena ; pos, postoccipital sulcus ; Prmt, pre- 

 mentum ; pt, posterior tentorial pit ; Smt, submentum. 



turn." To cut a long argument short, however, we have only to look 

 at a tipulid larva (fig. 3 B) to see that the lobe is formed by the union 

 of two processes extended forward from the anterior median angles 

 of the postgenae, which themselves are not united in the tipulid. Above 



