154 PROC. ENT. SOC. WASH., VOL. 24, NO. 6, JUNE, 1922 



head capsule of Scolopendrella, and was apparently inherited 

 from a similar source, for insects seem to have inherited certain 

 traits from the "symphyloid" side of their ancestry, as well as 

 more directly from their crustaceoid forebears although those 

 traits which the Insecta have in common with the Symphyla 

 might possibly have been inherited from the common crustace- 

 oid ancestors of both Symphyla and insects, rather than through 

 the insertion of a purely symphyloid stage in the development of 

 one phase of insectan evolution. The pearshaped head capsule 

 of the Protura is also a very primitive type, and is very sugges- 

 tive of that occurring in certain diplopods, so that this type and 

 the symphyloid type in insects may have been inherited from 

 the myriopodan side of insects' ancestry. The most important 

 of the head types found in the Apterygota, and the types ot 

 head found in the great assemblage of winged insects, however, 

 are clearly traceable directly to crustaceoid prototypes, as I 

 shall attempt to show in the fo lowing discussion. Of the insec- 

 tan types of head capsule derived from crustacean prototypes, 

 the most important are the aselloid type, such as that found in 

 Lepisma and its allies, and the oniscoid type occurring in most 

 pterygotan insects, and it is with these two types that we are 

 largely concerned in the following discussion. 



In the isopod crustacean Asellus (Fig. 8), the head is flattened 

 above, and is somewhat broader than long. The eyes, e, are 

 widely separated, and the mandibles (labelled nid in Fig. 8) 

 extend backward to, or behind, the eyes. Since this type of 

 head is typical of Asellus and its immediate relatives, I have 

 referred to it as the aselloid type of head capsule. The head 

 capsule of Lepisma (Fig. 7) and Nicoletia (Fig. 6) is markedly 

 of the aselloid type. Thus in Lepisma (Fig. 7), as in Asellus 

 (Fig. 8), the head is flattened above, is broader than long, and 

 the huge mandibles labeled md in Fig. 7, extend backward to 

 the eyes, <?, in the peculiar fashion characteristic of Asellus. 

 The eyes are wanting in Nicoletia (Fig. 6), but the mandibles, 

 md, are huge, and extend backward in the aselloid fashion, 

 although the head capsule of Nicoletia is somewhat intermediate 

 between the true asselloid type of head capsule and the symphy- 

 loid type characteristic of the insects nearer the myriopodan 

 side of the group. The head capsule of certain myriopods, 

 such as the chilopods, is also rather suggestive of the aselloid 

 type; but in all of the chilopods I have examined, the mandibles 

 are reduced, and I have been unable to find any in which the 

 mandibles are of the type found in Asellus and Lepisma. 



In the insect Machilis (Fig. 5), the head capsule is longer 

 than broad, and the eyes, e, have become approximated along 

 the median line of the head. The mandibles, md (which are 

 overlapped by the maxillae, nix) are large, and extend backward 

 for some distance, but the space between the base of the man- 



