PROC. ENT. SOC. WASH., VOL. 24, NO. 6, JUNE, 1922 155 



dibles and the eyes is much greater than in Lepisma (Fig. 7) 

 and other forms of the true aselloid type. In some respects, 

 Machi/is' head capsule approaches the gammaroid type such 

 as that shown in Fig. 4 of the amphipod crustacean Talor- 

 chestia. In Talorchestia (Fig. 4) the head is longer than broad, 

 and in certain related forms there is a tendency for the eyes to 

 approach one another along the median line of the head, thus 

 suggesting the operation of a tendency which resulted in the 

 approximation of the eyes in this region of the head in Machilis 

 (Fig. 5). The mandibles, md, however, are not huge in Talor- 

 chestia (Fig. 4) and other gammaroids, and there is a consider- 

 able space between the base of the mandibles and the eyes in 

 these forms. 



The most important type of head capsule from the standpoint 

 of the derivation of the types found in winged insects is the 

 oniscoid type, exemplified by the isopod crustacean Ligyda (Fig. 

 1). The head of Ligyda (Fig. 1) is somewhat triangular in out- 

 line, and in this type, the head is usually slightly longer than 

 broad, with the eyes, e, situated at the sides of the head. There 

 is considerable space, g, between the bases of the mandibles, 

 md, and the eyes, and the mandibles are not exceptionally 

 large. A well defined labral suture demarks the labrum, /, 

 from the clypeus, cl, behind it, and the posterior limits of the 

 clypeal region, d, are demarked by a clypeal suture extending 

 from the base of one mandible to the other as in pterygotan 

 insects (see Annals Ent. Soc. America, Vol. 14, p. 65). The 

 head capsule of this type is usually more hypognathus, though 

 this is not always the case. By comparing Fig. 3 of a typical 

 winged insect (a psocid of the genus Thyrsophorus) with Fig. 1, 

 it is at once apparent that the oniscid (or better ligydoid) type 

 of head is the precursor of the type occurring in most winged 

 insects, since the labrum, /, of Fig. 3 clearly corresponds to the 

 labrum, /, of Fig. 1, and the clypeus, cl, of Fig. 3, clearly corres- 

 ponds to the clypeus, cl, of Fig. 1. There is an anterior frontal 

 region labeled of in both forms, the head is hypognathous in 

 both, and the nature of the mandibles (which are not huge, and 

 are separated from the eyes by a considerable space) is much the 

 same in both forms. The location of the eyes in the insect 

 shown in Fig. 3 is suggestive of that in the isopod shown in Fig. 

 1, and if a mantid (which usually has a triangular head) had 

 been used instead of the insect shown in Fig. 3, the resemblance 

 between the head of the isopod shown in Fig. 1, and a winged 

 insect, would have been even more pronounced. The resem- 

 blance of the oniscoid type of head (Fig. 1) to the insect shown 

 in Fig. 3, however, is sufficiently evident, and the resemblance is 

 all the more striking when one takes into consideration the fact 

 that (so far as I am aware) no other known arthropods than the 

 isopods and insects have exactly this type of head capsule. 



