14 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



protruding, and retracting the chitinous plate. The homologous muscles 

 of Orchesella, however, and probably of other Collembola, I believe 

 serve severally, not to move the tentorium, but to dilate the oesophagus 

 (Fig. 6, dil. (£.), to move the antennae, and to effect certain movements 

 of the entire head (Fig. 6, mu.). The tentorium appears to be immov- 

 ably fixed in place by means of the chitinous arms and ligaments 

 already described. As I shall show, contrary to the views of other 

 authors, the protrusion and retraction of the mandibles and maxillae are 

 accomplished, not by corresponding movements of either the tentorium 

 or the " Sttitzapparat," but in both cases by special muscles. 



I shall now describe the mouth-parts in the order of their position, 

 passing from the dorsal toward the ventral side of the head. 



Labrum. 



The labrum is trapezoidal in external aspect (Plate 1, Fig. 1, Ihr.) and 

 wedge-shaped in sagittal section (Plate 1, Fig. 3). The external surface 

 bears three transverse rows of stout bristles. Between the labrum and 

 clypeus is a deep transverse suture, formed by the infolding of the chiti- 

 nous cuticula, which becomes thin to form a hinge (Fig. 3). At either 

 end of the hinge, however, the cuticula is swollen into a conspicuous 

 chitinous lobe, which projects into the pharynx to fit against a cor- 

 responding prominence of the mandible. This relation between labrum 

 and mandibles was expressed by de Olfers ('62, p. 12) in the following 

 passage, which has been overlooked by subsequent writers : " Margo 

 posterior [labri] incrassatus et formam literse C imitans deorsum inflexus, 

 qua re fit, ut duo apices in cavum oris promineant, qui mandibulas sus- 

 tinent." The mandibles, when at rest, are held in place by these pro- 

 tuberances, and the surfaces against which the mandibles are applied 

 show stout parallel ridges, which perhaps hold the mandibles effectively. 

 Immediately behind the distal margin of the labrum are minute teeth 

 projecting into the mouth in a transverse row, which becomes inter- 

 rupted in the middle (Plate 2, Fig. 9, de.). Between these submarginal 

 teeth and the margin itself is a transverse groove, in which I have found 

 the apex of the glossa locked by means of a corresponding transverse 

 ridge (Plate 1, Fig. 3). 



Tullberg ('72, p. 20) discovered an epipharynx in Tomocerus vulgaris, 

 and briefly described it in a sentence which I translate : "The pharynx 

 is bounded above [anteriorly] by a palate, or epipharynx, which consists 

 of a chitinous membrane furnished with several toothed elevations." The 

 same structure also occurs in Orchesella (Plate 2, Fig. 9, e'phy.). The 



