314 THE DOLABELLINAE. 



compressed prismatic form, their distal ends appearing usually as somewhat 

 elongated polygons when seen in surface view, (Plate 8, fig. 7). Here a repre- 

 sents the younger region of the mandible, toward the bottom of the sulcus, 

 while b is toward the outer or older border. The rodlets are longest near the 

 middle of the mandible and are shortest at the bottom of the groove or sulcus 

 in which they are produced (Plate 7, fig. 1). The distal end of each rodlet 

 is terminated by a strong oblique cap, the anterior edge of which tends to over- 

 lap the next one in front of it. Each rodlet tapers somewhat toward its proxi- 

 mal or basal end and rests upon a single epithelial cell in the posterior half of 

 the mandible, of which it is plainly a cuticular differentiation. The develop- 

 ment of these rodlets may be followed in fig. 1 and 2 of Plate 7. The whole 

 pharyngeal cavity is lined by a single layer of columnar epithelial cells, bearing 

 a cuticle of varying thickness. At the posterior margin of the mandible this 

 epithelium is invaginated into a deep sulcus, as before described, the innermost 

 portion of which is slightly inrolled forward (Plate 7, fig. 1 c, fig. 2). The cells 

 forming the floor of this groove secrete the mandibular rodlets, a single one 

 being produced by each cell. At the bottom of the sulcus the rodlets are short- 

 est and youngest, appearing as a thin light yellow cuticular layer. New layers 

 are added beneath this outermost deposit successively, and various stages in 

 the growth of the rodlet may be met with in series toward the front. About 

 midway of the width of the whole of the mandible the rod-forming cells cease 

 the production of these layers, and take on the formation of a common basal 

 cuticular secretion, upon which the anterior rodlets are borne (Plate 7, fig. l,b). 

 This homogeneous substance first appears as a narrow band between the epi- 

 theUal cell and its rodlet, and increases progressively in thickness toward the 

 anterior border. Beyond the mandible it continues as the common thick 

 cuticular lining of the mouth-tube, homogeneous save for slight indications of 

 stratification parallel to the basal eiiithelium. The maximum length of the rod- 

 lets is thus reached about midway of the width of the mandible, and the 

 progressive shortening seen from that region forward is evidently due to the 

 wearing away of their distal ends, until the rodlet itself finally breaks away 

 and disappears, as at a, Plate 7, fig. 1. While this is taking place along the 

 floor of the mandibular sulcus the epithehum forming its upper wall or roof is 

 secreting a continuous mass of chitin (Plate 7, figs. 1, 2, d) which fills up the 

 remainder of the space above the rodlets. This upper layer is faintly striated 

 and extends well forward beyond the edge of the sulcus, gradually tliinning 

 away as the region of the anterior margin of the mandible is approached. Thus 



