14 ILLINOIS BIOLOGICAL MONOGRAPHS [256 



The relative length of the epicranial stem presents more conspicuous 

 variation than any other character in these larvae, except, of course, the 

 number of uropods. In the majority of nocluid larvae the length of this 

 suture is not strikingly different from that of the front. In certain Agro- 

 tinae, however, it is reduced to its adfrontal portion and in Chamyris 

 cerinlha, Erastrinae, it is markedly longer than the front, all gradations 

 (Figs. 2, 15-17) between these extremes being found. The shortening of 

 this suture, where it occurs, has been brought about apparently by a 

 splitting apart of its two sides at the caudal end, the area between these 

 separated sides being taken up by the coria continuous with the cervano- 

 tum. The triangular area thus formed is known as the vertical triangle, 

 although morphologically it is composed of cervacoria and a part of the 

 epicranial suture greatly widened. The apex of this triangle is usually 

 heavily chitinized. The shortening of the epicranial stem is correlated 

 with a general shortening of the cephalic aspect of the head, which has 

 probably been induced by a change in the position of the head from the 

 typical vertical one with the mouth-parts directed ventrad to a some- 

 what horizontal one with the mandibles directed cephalo-ventrad or cep- 

 halad in extreme cases. 



The shape of the clypeus (Fig. 2) presents some variation, the fronto- 

 clypeal suture being either straight or curved upward in the middle more 

 or less prominently. The relative widths of the preclypeus and postclypeus 

 also vary to some extent. These characters appear to be of generic value. 

 The width of the labrum relative to that of the clypeus and the depth of 

 the labral cleft present characters applying to smaller groups. This 

 sclerite is rarely nearly divided into two parts as in an undetermined species 

 of Catocala. 



The position of the ocellarae, although presenting no striking difiPerences 

 within the family, offers some convenient characters evidently applying 

 chiefly to groups of species, although constant specific differences have 

 been noticed in certain genera. In the majority of cases the distance 

 between ocellarae 1 and 2 is distinctly less than that between 2 and 3, the 

 line 1-2 often equalling one-half of the line 2-3. Sometimes 1 and 2 are 

 nearly contiguous. 



The coloration of the head is principally cuticular and, therefore, 

 practically permanent in alcohol. Such markings offer much variation 

 within the family, often providing easily recognizable specific characters. 

 The general scheme of coloration is usually constant within a genus. In 

 some species it differs markedly according to the instar. Individual varia- 

 tion is sometimes considerable. The entire head capsule is uniformly col- 

 ored in but relatively few species. In these it ranges from light brown to 

 nearly black according to the species. Often the vertex is darker than the 

 rest of the head, the preclypeus especially being lightly pigmented. This 



