Carpknter — Relationships hetioeen Classes of Arthropoda. 325 



A full description of these organs in the Collembolan Orchesella 

 was three years ago published by Folsom ('99) ; and, although he 

 did not at that time recognise them as a pair of jaws, his figures 

 show that they correspond closely with the maxillulse of Machilis, 

 though less highly developed than the latter. In Isotoma — perhaps 

 the least specialized genus of the Collembola — the maxillulae are 

 more strongly developed than in Orchesella. As observed in 

 Isotoma palustris (fig. 2), the lacinia is distinctly toothed at the tip ; 

 and the series of spines along the inner edge of the basal region are 

 stronger than in Orchesella. The association of the maxillulse with 

 the tongue is closer in the Collembola than in the Thysanura, as 

 might be expected from the greater specialization of the former 

 group. 



A final proof that the maxillulae (or " superlinguse") are indeed 

 a distinct pair of appendages has been afforded by Folsom ('00), who 

 has studied their development in the Collembolan Anurida maritima. 

 He finds that they arise from paired rudiments like those of the 

 mandibles and maxillae, and that their association with the central 

 rudiment of the tongue is secondary — exactly as would be anticipated 

 from a comparative study of the adult insects. Although the 

 maxillular rudiments arise between rather than behind the rudiments 

 of the mandibles, a special post-mandibular ganglion and a pair of 

 coelomic spaces are associated with them. It is evident from the 

 figui-es given by Eaton ('83) and Heymons ('96) that these appendages 

 are present in the Ephemerid larva, though in a reduced state. 

 According to Hansen, their vestiges can be clearly made out in the 

 Earwigs and Hemimerus, and in a still more reduced condition in the 

 Cockroach and other Orthoptera. 



The two pairs of maxillae ("maxillae" and "labium" of 

 entomologists) are the appendages of the two hinder postoral somites 

 of the head. A point of considerable interest to be noted is the fact 

 that, in. the more generalised Insects at least, the labial segment is 

 incompletely fused with the head-capsule, part of its skeleton forming 

 the cervical sclerites or so-called " microthorax," very evident in the 

 Cockroach. This interpretation of the cervical sclerites, suggested 

 by Huxley ('78), has been established by Comstock and Kochi ('02), 

 "We conclude, therefore, that in the Insectan head are six limb- 

 bearing segments, whereof the hindermost, at least in the more 

 generalised orders, is incompletely fused with the rest. It is likely, 

 as will be seen later, that an extra, primitively limb-bearing, ocular 

 segment in front of the feelers must also be reckoned. 



Behind the head, the segmentation is comparatively simple. The 



