322 PHYLUM ARTHROPOD A. 



from the neuromeres of the antennal and premandibular seg- 

 ments respectively, the procerebrum representing the proto- 

 cerebrum together with the complex syncerebrum of Scolopendra 

 (Heymons). 



With regard to the postoral segments of the head of Insects 

 the evidence is somewhat conflicting. In the development of the 

 Orthoptera investigated by him, Heymons finds three consecutive 

 segments, each with a well developed neuromere and pair of 

 coelomic sacs, which he identifies as belonging to the mandibles 

 and two pairs of maxillae, and which would thus correspond 

 with the appendages so named in Scolopendra. On the other 

 hand several observers (Hansen, Folsom, Carpenter and others) 

 have shown that paired structures are present in the mouth of the 

 lower insects, between mandibles and first maxillae, sometimes 

 uniting with the median ligula or hypopharynx but sometimes 

 distinct from it, and presenting (especially in the Thysanura) 

 all the characters of appendages. Thus in Machilis maritima 

 Carpenter shows that each ends in two lobes, comparable with 

 the galea and lacinia of the succeeding segment, and bears exter- 

 nally a palp (unsegmented in Machilis, Fig. 372, but 3-segmented 

 in Japyx). These appendages are named by Hansen maxillulae 

 (superlinguae by Folsom). It seems impossible to resist the 

 evidence that they represent a head somite, and Folsom does in- 

 deed find, in a later stage of development of the Collembolan 

 Anurida, a neuromere corresponding to them. The only diffi- 

 culty in the acceptance of this view is that Folsom himself 

 finds in an earlier stage of development of Anurida no trace of 

 this segment and figures the mandibular and first maxillar 

 segment in juxtaposition. Moreover it is remarkable that this 

 segment should be apparently unrepresented in the develop- 

 ment of the Orthoptera so carefully investigated by Heymons. 

 Nevertheless the balance seems to incline in the direction of the 

 true segmental nature of the maxillulae. 



Accepting this view we conclude that the maxillulae, first 

 maxillae and labium of insects correspond respectively with the 

 two pairs of maxillae and the poison claws of Scolopendra. The 

 fact that the labial (2nd maxillary) segment of Insects is less 

 completely fused with the segments in front of it than they are 

 with one another, affords some confirmation of this view. 



In the Crustacea the evidence for segmentation afforded 



