NO. 10 ri:i:L)iNG okgans uf akac iimda — snodcjhass 83 



a pair of small thin processes ifd) that possihly represent the ini- 

 movahlc cheliccral digits. 



The chchccral stylets (fig. J9 15, Sty) arc attached apparently in 

 deep anterior inflections or jxjckets of the under surface of the 

 stylophore (hut the structure of the under surface of the stylophore 

 as drawn in the diagram is somewhat conjectural). ICach stylet has 

 a flat pear-shaped base (StB) from which the strong hut .slender 

 shaft extends first posteriorly and then loops downward anrl forward. 

 ( )n a projection of the stylet base, apparently dorsal to the articulation 

 on the stylophore, is attached a group of muscle fibers (nicl), which, 

 Hlauvelt says, produce an up-and-down movement of the tip of the 

 stylet. However, inasmuch as the stylets arc closely held in the 

 groove of the epistomc (D. Sty), a downward rotation of their bases 

 should cause a protraction of the shafts. Muscles antagonistic to 

 the stylet protractors have not been observed, and it is possible that 

 retraction results automatically from the elasticity of the stylets or of 

 their basal connections. On the other hand, the principal movements 

 of the stylets must be brought about by the protraction and retraction 

 of the stylophore. The movements of the latter, according to Rlauvelt. 

 are produced by muscles from the dorsum of the body attached on 

 the posterior lobes, and muscles attached distally in the stylophore that 

 arise posteriorly on the vertical respiratory tubes. Both of these sets of 

 nniscles, however, would appear to be retractors, and it may therefore 

 be supposed that, as in some other Acarina, protraction of the cheli- 

 cerae is effected by a bulblike compression of the body. 



The stylophore is capable of complete retraction beneath the 

 marginal fold of the dorsum that ordinarily covers its basal half. 

 This fold (fig. 29 A, B, df), projecting as it does over the cheliceral 

 bases, possibly represents the tectum, or dor.sal wall of the capitulum 

 in other Acarina; if not, the tetranychid capitulum is incomplete 

 dorsally, and is composed only of the coxal elements and the epi- 

 stomal plate that unites their dorsal surfaces below the chelicerac. 

 and forms the upper wall of the rostrum (B, Ef>st). 



The rostrum (termed "hypostome" by P>ecker) projects as a short 

 wide cone from between the bases of the palps (fig. 29 .\, Rst). Its 

 dorsal, or epistomal, wall is shown by Becker and by Blauvelt to 

 form a trough (C, Epst) in which the stylophore ("mandibular 

 plate") slides backward and forward. Along the bottom of the trough 

 is a deep, thick-walled canal (C, D. StGr) that contains the shafts 

 of the cheliceral stylets (D, Sty) and evidently serves to hold them 

 in place during their functional activity. The common duct e»f the 

 two pairs of silk glands runs beneath the stylet canal ( C. D, SlkPct) 



