NO. lO 



I--KEDING OKGAN.^ OF AKACUNIDA — SN0D<;RAS- 



/O 



The cheliceral sheaths are double-walled tubular folds of the 

 capitular integument extended individually around the shafts of the 

 ohelicerae. In figure 2"] the sheaths are represented diagrammatically 

 as they are shown by Dcniglas (1943) in Dcrmaccntor andcrsovi. 

 'Ihe outer wall of each fold, or outer sheath (oChS), is produced 

 forward from the dorsal wall of the capitulum above (Tect), and 

 from the base of the labrum {Lin) below to the end of the cheliceral 

 shaft ; it is then invaginated into itself to form an imicr sheath (iChS ) 

 closely investing the chclicera. In Dennaccntor variabilis the che- 

 liceral sheaths appear to be the same as in P. andcrsoux, but in Arcjas 



oChS iChS 



»\ I \ 



Tect 



Chi 



Hst 



Epstdnv* 

 ^dld 



51Dct 



IG. 27. 



-Acarina, diagram of the structure of tlic capitulum in longitudiiiai 



section. 



Chi, chelicera; did, dorsal dilator muscles of pharynx; dlv, ventral dilators 01 

 pharj-nx ; Epst, epistome, invaginated ; jc. food canal ; Hst, hypostome ; lICx. 

 pedipalp coxa; iChS, inner cheliceral sheath; Lui, labrum ("styletlike process") ; 

 Mill, mouth; oChS, outer cheliceral sheath; Oe. oesophagus; Phy, phar>-nx : 

 SlDct, salivary duct; Sh', salivarium; Tec, tectum (dorsal wall of capitulum). 



persicus, described by Robinson and Davidson (1913. 14), and in 

 Ornithodoros coriaccus, described by True (1932), the lower wall of 

 each inner sheath tube is said to be united for a part of its length with 

 the upper surface of the subcheliceral plate (fig. 28 A, Epst). 



The sulKheJiceral plate of the ticks is clearly the epistome (fig. 2j, 

 lipst), since it supports the labrum (^"t) at its distal end, and gives 

 attachment on its ventral surface to the dorsal dilator muscles of the 

 pharynx (did). Inasmuch, however, as the ventral folds of the che- 

 liceral sheaths arise at the base of the labrum in the .\rgasidae (fig. 

 j8 A) it is evident that in these ticks the epistome is entirely in^'agi- 

 nated. Christophers (1906), in fact, describes the epistome of 

 Ornithodoros as an endoskelctal plate arising from a transverse bar 



