FOLSOM: MOUTH-rARTS OF AXUPJDA MARITIMA. 139 



rings, in each of which, in the more specialized insects, tergum, pleura, 

 and sternum are present. 



In the head region of the Collembola, however, segmentation occurs 

 only on the ventral side of the germ baud. The entire gular region is 

 labial in origin, and there is reason for regarding the clypeus as the 

 tero-ite of the ocular segment. The mouth-folds are undoubtedly ex- 

 panded pleura. Aside from these, however, it is idle to speculate about 

 the location of other sclerites which are differentiated in more spe- 

 cialized insects. Here, in the absence of such differentiation, it may be 

 be said that the head-cylinder represents seven ideal rings, which dorsally 

 and laterally are in no way demarcated from each other. Admitting 

 that the procephalic lobes do extend backward and encroach upon other 

 segments, the lobes may not be regarded as the tergites or pleurites of 

 those segments, for they are simply thickened blastoderm, and increase 

 in area in proportion as the blastoderm thickens ; but the convenience 

 of applying a single term, " procephalic lobe," to either of these thicken- 

 ings should not blind us to the fact that the lobe eventually represents 

 the blastoderm of more than one segment. 



In the finished head (Plate 5, Figure 33) are certain elevated dorsal 

 areas which, however, are not sclerites bounded by sutures, and are not 

 clearly to be homologized with sclerites of other hexapod orders. The 

 elevations referred to are directly correlated with underlying glands and 

 muscles. 



The sides of the face in Apterygota, then, are homologous with the 

 gence of Pterygota. In all insects the skull represents seven somites, 

 but the cephalic sclerites of Pterygota, excepting labrum, clypeus, and 

 labial sclerites, are not differentiated in the Apterygota. 



Tentorium. 



The tentorium of Anurida is essentially like that of Orchesella (Folsom, 

 '99), consisting of a chitiuous plate parallel with the frontal plane (Plate 

 8, Figure 51, int.), from which diverge two pairs of chitinous arms (Plate 

 6, Figure 35) extending to the skull : a dorsal pair (br. d.) and a poste- 

 rior pair {br. p.) embracing respectively the supra- and infra-oesophageal 

 ganglion. A third, or anterior, pair was found in Orchesella, but not in 

 Anurida. 



Regarding the development of the tentorium in insects, most diverse 

 opinions are held. After considerable study, I have come to the con- 



