FOLSOM: MOUTH-PARTS OF ORCHESELLA CINCTA. 29 



4. Middle elevator. This muscle (Figs. 3 aud 25, 4- ^^i' '^•)i which is 

 longer than any of the preceding, arises from the posterior arm of the 

 tentorium, bends downward, follows the inner wall of the labium directly 

 forward, and terminates at the distal extremity of the labium. The func- 

 tion of the muscle is like that of the mcsal elevator. 



5. Lateral elevator. This is the longest and most lateral of the labial 

 muscles (Fig. 25, 5. Ivt. /.). As to msertion and course it is similar to 

 the middle elevator, but its origin is more posterior and dorsal, being 

 upon the dorsal chitinous surface of the salivary duct. The principal 

 difference between the three elevators described is that, by reason of 

 their different insertions, they may raise different portions of each 

 terminal movable lobe of the labium. 



The labium is supplied by the third pair of nerves from the infra- 

 oesophageal ganglion ; the nerves originate a little to one side of the 

 median plane from the ventral part of the ganglion, and pass forward, 

 downward, and outward into the dorsal portion of either half of the 

 lower lip. The labium is lined with a single layer of confluent, pig- 

 mented, hypodermal cells, with moderately large, round nuclei. Be- 

 neath the bristles of either half of the labium is a cluster of large oval 

 nuclei, which belong to filiform cells exactly like those of the labrum ; 

 ganglionic cells may also be distinguished, which bear the same relations 

 as those described for the upper lip. 



The Cephalic Glands. 



The glands in the head of Orchesella comprise two pairs, the principal 

 pair lying in the base of the head and occupying most of the region be- 

 hind the maxillae (Plate 1, Fig. 2, gl.). Each of the larger glands consists 

 of a single tube, which is thrown into several longitudinal convolutions. 

 The deeper half of the tube is of larger caliber than the remainder, and 

 constitutes the secretory part (Plate 4, Fig. 32, gl.), while the more 

 superficial half is distinctly smaller in caliber and simply conductive in 

 function (Fig. 32, dt). The chitinous evacuating duct of either side 

 finally runs forward and downward on the mesal side of the gland, pass- 

 ing just beneath the maxillar evagination and trending toward tne median 

 plane. The ducts open separately on either side of the extreme proximal 

 portion of the labial cleft (Plate 3, Fig. 25, dt.). As the mesal sur- 

 faces of the two halves of the labium are fused together along their 

 ventral edges only, a trough or gutter (sul.) is left above the line of 

 fusion, through which the secretion may run forward to the mouth 

 opening. 



