folsom: mouth-parts of orchesella cincta. 17 



Immediately behind the stirrup is a gland (to be described later) which 

 may, as a secondary function, lubricate the pivot of the mandible. 



The mandibles are situated in finger-like evaginationa of the pharynx, 

 and, except for muscular and nervous attachments, are unconnected 

 with the pockets in which they lie, as is easily demonstrated in trans- 

 verse sections of the mouth -parts. As von Stummer-Traunfels ('91, 

 p. 220) observes, " Die mandibeln sind ganz frei in der Kopfkapsel 

 gelegen, mit dem Stiitzapparate nur durch jener starken Muskel ver- 

 bunden, und verdanken die Stellung, die sie einnehmen, nur noch dem 

 Zuge der Kaumuskeln und einem Chitinvorsprunge an der Innenseite 

 der Kopfkapsel, auf dem sie mit ihren hinteren Enden pivotiren und 

 der diesem entgegenwirkt." It is evident that this arrangement facili- 

 tates the protrusion of the mandibles, which lie obliquely in the head 

 (Plate 1, Fig. 2, Plate 2, Fig. 10), their bases close beside the skull 

 on either side, while their apices converge, so that the opposing incisive 

 teeth and molar surfaces meet in the sagittal plane. On account of the 

 oblique position of the mandibles (Fig. 2), which in this agree with the 

 maxillae and tongue, it happens that microtome sections frontal or trans- 

 verse in relation to the oesophagus are oblique in relation to these organs, 

 and vice versa. Throughout this paper, when I refer to frontal and 

 transverse sections, I shall use those terms with reference to the internal 

 mouth-parts, conceiving the axial line between the mouth-parts to be 

 their long axis, unless otherwise specified. 



From the abundance of muscles in the head I have studied out the 

 surprising number of ten distinct pairs which are concerned in moving 

 the mandibles alone. This has been done by the study of serial sections 

 in different directions and of dissections. The diverse directions taken 

 by the muscles render them difficult to follow on sections in any single 

 direction. On the whole, however, most may be learned from sections 

 which are frontal, i. e. parallel with the plane in which the mandibles 

 lie, and I shall describe the muscles as studied in successive frontal 

 planes, beginning on the dorsal side. The order in which the muscles 

 are numbered is necessarily somewhat arbitrary, but is chosen as being 

 that in which they may with least difficulty be identified by any one who 

 may wish to study the subject hereafter. 



1. Lateral Rotator. This muscle (Plate 2, Fig. 14, /. rot. /.) arises on 

 the skull at the side of the head, passes forward and downward, crossing 

 obliquely the dorsal surface of the mandible, and is inserted on the 

 conical, medio-dorsal projection of the mandible. The same muscle is 

 also represented as it appears in a transverse section of the mouth-parts 



