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 suleus 
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. 1, b). 
This homogeneous substance first appears as a narrow band between the epi- 
ee ee ate Per wgy reat 
thelial 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 i 
stratification parallel to the basal epithelium. 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 epithelium 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 thinning 
away as the region of the anterior margin of the mandible is approached. Thus" 
