rOLSOM: MOUTH-PARTS OF AXURIDA MAPJTIMA. 135 



pus is formed and resorbed, — an indication of degeneracy. The entire 

 giilar region of Apterygota is labial in origin ; but fewer sclerites are 

 dififerentiated than in Pterygote insects. The labium of insects is homo- 

 dynamous with the " first maxillte " and homologous in detail with the 

 first maxillipeds of decapod Crustacea. The labium of Campodea is 

 equivalent to the " second maxillse " of Symphyla, and is represented in 

 the Diplopod gnathochilarium. 



SkuU. 



The principal mouth-parts of Collembola, unlike those of all other 

 insects, except certain Thysanura, are internal ; the way in which they 

 become so will now be described. 



The beginning of the process is seen at Stage 3 (Plate 1, Figure 3), 

 when the ventral surface of the germ band is almost flat. In lateral 

 aspect (Plate 2, Figures 9, 10) the edge of the germ band is produced 

 downward as a small crescentic lobe {pli. or.) outside the fundaments 

 of the mouth-parts. This lobe usually originates on the mandibular 

 segment, as represented in Figure 9, but may arise more anteriorly, as 

 in Figure 10. These figures represent, respectively, the left and right 

 sides of the same individual. Earely, the lobe begins behind the man- 

 dible. A transection of the germ band near the middle of the lobe 

 (Plate. 3, Figure 16) proves the lobe (^j^li. or.) to be an evagination of 

 the ectoderm enclosing mesoderm. In ventral aspect at this stage 

 (Figure 11) the mouth-fold is clearly distinguishable at its widest part, 

 or place of origin, but gradually disappears anteriorly and posteriorly on 

 account of its confluence with the rest of the germ band. 



At Stage 4 (Plate 1, Figure 4) and a little later, while involution of 

 the germ band is occurring, the mouth-fold is considerably larger (Fig- 

 ures 12, 19, pli. or.) and forms a crescentic flap, now extending from the 

 second maxilla almost to the labrura. In the next stage (Plate 1, Fig- 

 ure 5) the fold is conspicuous ; in lateral aspect (Plate 3, Figure 20) its 

 ventral margin is well rounded and conforms posteriorly to the con- 

 tiguous anterior surface of the front leg ; the mandibular and maxillary 

 fundaments still project slightly below the fold. In ventral aspect (Plate 

 3. Figure 21) of the same individual, the fold is seen to be of nearly 

 uniform width except anteriorly and posteriorly, where it is expanded 

 against the labrum and second maxillae respectively. Transections of 

 the germ band (Plate 4, Figure 23), when compared with similar sec- 

 tions at Stage 3 (Plate 3, Figure 16), show the folds to have exceeded 



