folsom: mouth-parts of orchesella cincta. 19 



of the skull, being behind the preceding muscle and crossing the median 

 plane (Fig. 14, 8. rot.). It also ends (Fig. 15, 8. rot.) in a tapering pig- 

 mented tendon, which is inserted close in front of the tendon of the last 

 described muscle. Both muscles must act as rotators, twisting the man- 

 dible so that its molar surface moves upward and outward. 



9. Adductor. This muscle, the only one hitherto mentioned by writ- 

 ers, is the most powerful muscle in the head. It originates principally 

 on the tentorium (Fig. 14, 9. add.), passes directly outward, penetrates 

 the large triangular orifice of the base of the fulcrum (Fig. 15, 9. add.), 

 and is inserted on the inside of the lateral wall of the mandible. In 

 addition, several of its fibres pass under the tentorium (Plate 1, Fig. 7, 

 9. add. md.) and become continuous with similar fibres from the oppo- 

 site mandible. Thus, these fibres must pull against each other with the 

 effect of closing the jaws, but with this exception the adductors are at- 

 tached to the tentorium. These strong muscles are counteracted by 

 muscle No. 2. 



10. Rotator. A long, slender muscle, beginning at the median dorsal 

 line, passing forward, outward, and downward, and inserted by a pig- 

 mented tendon along with Nos. 7 and 8. To avoid confusion I have 

 omitted this muscle from Figure 14, but the figures of No. 7 (Figs. 14 

 and 15, 7. rot.) will serve perfectly well for No. 10, if it be remembered 

 that the latter muscle lies under the former. It will be observed that 

 I have described as many as four muscles (Nos. 1, 7, 8, and 10) which 

 appear to rotate the mandible in the same direction ; I see no other 

 function for these muscles, however. The rotation in the opposite direc- 

 tion seems to be comparatively unimportant, being accomplished by a 

 single slender muscle (No. 3), the primary function of which is perhaps 

 retraction. Among the mass of antennal muscles originating on the 

 tentorium there is one which might easily be mistaken for a rotator of 

 the mandible, in function similar to No. 3. This muscle (Fig. 16, mu. at) 

 in some sections is moulded against the mesal surface of the conical 

 projection to which is inserted rotator No. 1. Iu other preparations, 

 however, in which the mandible happens to have been rotated so as to 

 remove the projection from its proximity to the antennal muscles, it 

 may be seen that the apparent rotator is really unattached to the 

 chitinous projection. 



The nerves to the mandibles are the first pair of the infra-oesophageal 

 ganglion ; they arise from either side of the anterior part of the ganglion, 

 pass directly outwards, enter the mandible at the anterior angle of its 

 large lumen and extend the length of the mandible and into fine canals 



