134 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



arise on the frons, are never attached to the triangular plate of the 

 caterpillar face, but take their origin from the median ridge dorsal 

 to it (fig. 50 B, E, /). In many caterpillars the lower part of the clyp- 

 eus is strengthened by an internal sul)marginal thickening (E, I, h) 

 forming a bracing ridge between the articulations of the mandibles 

 (c, c). 



The frontal area of the head, as has been shown, is to be identified 

 by the origin of the labral retractor muscles upon its inner surface 

 (fig. 47 B, C). In the caterpillar the labral muscles arise either upon 

 the median internal ridge of the cranium that extends between the apex 

 of the posterior emargination of the vertex and the apex of the clyp- 

 eus, or upon the dorsal bifurcations of this ridge that are continued 

 into the margins of the vertical emargination (fig. 50 B, E, 53 E, /). 

 This ridge, then, is at least a part of the frons. It is formed by a deep 

 inflection of the median line of the cranium dorsal to the apex of the 

 clypeus, which appears externally as a median suture (fig. 50 A, B, C, 

 H, Fr). In a softened specimen this frontal invagination can often be 

 widely opened, when it is seen that its inflected surfaces are continuous 

 with the so-called " adfrontal " strips lying laterad of the clypeus and 

 extending ventrally to the bases of the mandibles. The sutures, 01 

 membranous lines, along the outer margins of the " adfrontals " thus 

 become the true frontal sutures (fig. 50 A, H, I, fs). 



The frontal region of the caterpillar, therefore, includes the invag- 

 inated frontal groove (fig. 50 A, E, Fr), the " adfrontals " (fr), and 

 perhaps the apical margins of the vertical emargination. When the 

 mature caterpillar sheds its skin at the pupal molt, the head cuticula 

 splits along two lines, which, beginning at the notch of the vertex, 

 follow the external lips of the median frontal invagination and then 

 diverge along the " adfrontal " sutures to the bases of the mandibles. 

 An elongate piece is thus cut out which includes the median frontal 

 inflection, the " adfrontals " and the clypeus. In some caterpillars the 

 molting cleft follows only one of the adfrontal sutures, the other re- 

 maining closed. 



The median part of the vertex in the caterpillar's head is obliterated 

 •by the dorsal emargination, and the angle of the emargination usually 

 extends into the frontal invagination (fig. 50 I) ; in some cases the 

 notch is so deep that the latter is reduced to a very small area dorsal 

 to the apex of the clypeus (F). 



The labrum of the caterpillar (fig. 50 A, B, Lin) is commonly sep- 

 arated from the lower edge of the clypeus by a wide, flexible membran- 

 ous area. Some writers, having mistakenly identified the true clypeus 

 as the frons, have regarded this membranous area as the clypeus, 



