84 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. IIO 



and, as already noted, opens into the distal end of the latter. If this 

 duct represents the paired ducts of the salivary glands of the ticks 

 that discharge above the base of the labrum (fig. 27, SlDct), the 

 short apical part of the dorsal wall of the tetranychid rostrum pro- 

 jecting over the mouth may be referred to the labrum (fig. 29 B, Lm). 



The hypostomal wall of the rostrum (Hst) is a simple ventral 

 prolongation of the coxal region of the capitulum and has no appen- 

 dicular accessories. 



The mouth lies within the tip of the rostrum (fig. 29 B, Mth) and 

 opens directly into the pharynx (Phy). The form of the tetranychid 

 pharynx is unusual for a sucking apparatus, but is one characteristic 

 of the salivary ejection pump of insects. By comparison with the 

 tubular pharynx of other arachnids the cup-shaped sucking organ 

 of Tetranychus is so short that the inflected dorsal wall takes the 

 form of a thick conical plug with the dilator muscles {did) from the 

 epistome convergent upon its center. The oesophagus {Oe) proceeds 

 in the usual manner from the posterior end of the pump chamber. 



The respiratory system is an interesting feature of the tetranychid 

 organization. Just why the spiracles should be in a place so incon- 

 venient as the infolded membrane at the base of the united chelicerae 

 is not clear (fig. 29 A, B, 6'/>), except that they are here by inheritance 

 from prostigmatic ancestors. The so-called "peritremes" (A, Ptr), 

 as above noted, are open channels of the integument that extend 

 posteriorly and outward on the dorsal surface of the body from the 

 spiracles. Their closely ribbed walls (E) give these channels a re- 

 semblance to tracheae, and, in fact, they might with better reason be 

 termed pseudotracheae than are the similar canals on the mouth lobes 

 of phalangiids and the labella of some Diptera that have to do with 

 feeding and not with respiration. Blauvelt observes that the long, 

 slitlike peritremes of Tetranychus give the spiracles access to the 

 outer air at all usual positions of the chelicerae. It is evident that as 

 they are pulled into the fold of integument over the cheliceral bases 

 by the retraction of the latter, the indrawn parts of the canals are 

 converted into closed tubes while the outer parts are still open to 

 the air. When the cheliceral bases (stylophore) are fully retracted, 

 however, Blauvelt says, the peritremes are completely shut ofif from 

 the air, and this fact he points out "may explain in part the high 

 degree of resistance of this mite to certain toxic gases such as hydro- 

 cyanic acid gas and nicotine vapor." The vertical respiratory tubes 

 into which the spiracles open (fig. 29 B, Afr) are said to be en- 

 closed in a common, strongly sclerotic wall, their ventral ends are 

 supported on a median apodeme of the epistome (eAp), and the 



