'J^ SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 10 



at the base of the labrum, composed of two strong lateral arms 

 blended medially to form a long horizontal plate projecting freely 

 into the body cavity. The subcheliceral plate of Argas persicus is 

 said by Robinson and Davidson to be thin medially, but strengthened 

 along the sides by marginal thickenings (fig. 28 L, Epst). The plate 

 serves the dual purpose of furnishing a smooth surface on which the 

 chelicerae slide, and of giving attachment ventrally to the dorsal di- 

 lator muscles of the pharynx (did). The descriptions of Ixodes rici- 

 nus by Samson (1909), and of Dermaccntor andersoni by Douglas 

 (1943) are not specific concerning the nature of the epistome. 

 Douglas makes the obscure statement that the buccal cavity "is 

 formed in the subcheliceral plate," but in his figure A on plate 15 

 he shows dorsal muscles of the pharynx attached on subcheliceral 

 sclerites, which must be the thickened lateral parts of an epistomal 

 plate. True (1932) represents the subcheHceral plate of Ornhhodoros 

 coriaccus as given by Robinson and Davidson for Argas persicus. 



From the end of the subcheliceral plate, or epistome, there projects 

 over the mouth a small lobe (fig. 27, Lni), which is inserted into the 

 widened proximal end of the gutter of the hypostome. Samson ( 1909) 

 refers to this structure in Ixodes ricinus as a thin plate, shown in 

 sectional view as a short flap extending over the mouth ; Christophers 

 (1906) observed it in Ornithodoros savignyi, but mentions it only 

 as a small tongue protecting the pharyngeal orifice ; Robinson and 

 Davidson (1913, '14) describe the same thing in Argas persicus as a 

 "tongue-like process," and show it in section as a tapering lobe (fig. 

 28 A, Lm) projecting over the mouth. Later writers have noted what 

 appears to be a slender rod extending forward from the apex of the 

 lobe, and have termed the whole structure the "styletHke process," or 

 "tonguelike process" (fig. 28 E, Lui). Much has been written con- 

 cerning the nature of this organ. Sen (1935) contended that the 

 "stylet" is an open tube, the lumen of which is continuous into that 

 of the pharynx, and that the mouth of the ticks is therefore a minute 

 orifice at the apex of the stylet. Douglas (1943) accepts this inter- 

 pretation, and represents the pharynx of Dermaccntor as opening 

 through the narrow tip of the stylet. Bertram (1939) ^^^ Arthur 

 (1946), however, have shown that the stylet is an imperforate process 

 projecting above the mouth, and normally lying over the gutter of 

 the hypostome (fig. 28 J, Lm). 



In Ixodes, Dermacentor, and Amhlyomina the so-called "stylet" is 

 long and slender ; when pulled away from the hypostome there is 

 usually to be seen attached to each side of it a narrow, very delicate 

 membrane with an irregular and apparently broken margin (fig. 28 B, 



