MECHANISM OF SUCTION IN SCHIZONEUKA LANIGEEA. 323 



of the proximal extremity of each stylet *, and passing backwards through 

 the head they become attached to the antero- ventral arms of the tentorium, 

 at the base of those structures where they join the transverse plate. 



The other group of muscles is smaller, and is attached to the projecting 

 portion of the maxillary lever. The muscles from each lever extend laterally 

 through the head to become inserted on the antero-ventral arms of the 

 tentorium, at the place where that structure bends into the fore-head, vide 

 figs. 2 and 18, mx'.re. 



The large protractor muscles of the maxillary stylets are inserted along the 

 external face of the prolonged base of each stylet, and extend in an anterior 

 direction to become inserted on the walls of the maxillary sclerites (fig. 23, 

 mx.pr.^. 



K. Musculature of the Head. 



I have shown that the fore-head freely articulates with the proximal 

 portion of the head by means of a raenibranous chitinous fold. This allows 

 the fore-head to be raised from the deflexed position it occupies in repose, 

 to a position more or less at an angle to the body, which it occupies when 

 the aphis is feeding. These movements are brought about by two sets of 

 muscles, the elevator muscles of the fore-head and the d.epressor muscles. 



Extending from the transverse plate of chitin are two bands of muscles 

 which pass through the proximal portion of the head, in a postero-lateral 

 direction, and become inserted on the post-lateral border of the head at its 

 junction with the prothorax. These muscles are not shown in fig, 1, as they 

 are attached at the ends of the transverse plate, and are not seen in median 

 section. I have indicated them in the schematic text-figure 2, m.d.p. When 

 these muscles contract they pull down the transverse plate of the tentorium 

 and as this structure, by means of the tentorial arms, is firmly attached to the 

 fore-head, this latter structure is elevated from its deflexed position. I call 

 these muscles therefore, the elevator muscles of the fore-head. 



The depressor muscles of the fore-head consist of two strono- muscles which 

 are attached, in the median line, to the floor of that structure (fios, 18 & 19 

 m.d.f). They diverge laterally, in a slightly posterior direction, and become 

 inserted on the antero-ventral arms of the tentorium. When the aphis has 



* Muir and Kershaw (1912) consider that the part of the stylets projecthio- into the head 

 and supporting the retractor and protractor muscles are chitinized tendons orioinatiuo- " by 

 the invagination of the ectoderm at the base of the setae, which becomes solid or partly 

 solid, by deposits of chitin." As may he seen in fig. 23, the maxillary stylets are apparently 

 continued for some little distance into the head, thus affording attachment for the muscles. 

 This part is chitinous, and in the case of the mandibles the chitin possesses small chitinous 

 papillaj, which afford firm attachment for the retractor muscles. The base of the stylets 

 seems to be continued into the head for a short distance beyond the continuation of the 

 integument of the buccal cavit)', thus affording an attachment for the muscles. 



LINN. JOURN. ZOOLOGY. VOL, XXXII, 2^ 



