NO. lO FEEDING ORGANS (»F AKACHNIDA- SNODGRASS Si 



Specialization of the feeding organs for piercing is carried farthest 

 in the Tetranychidac, the nicnihers of which family, known as spider 

 mites, are plant feeders. The movable digits of the chelicerae of 

 the tetranychid mites are drawn out into long, slender stylets with 

 recurved bases attached on the proximal segments of the chelicerae 

 in such a manner that they are individually protractile and retractile. 

 The basal segments of the chelicerae, however, are united with each 

 other to form for the stylets a common support, which is itself pro- 

 tractile and retractile beneath an anterior fold of the dorsum of the 

 body. The epistome and the hypostome compose a conical rostrum 

 containing the mouth and the pharynx. The dorsal surface of the 

 epistome is deeply grooved to form a channel in which the chcliceral 

 stylets slide back and forth. The pharynx is cup-shaped, with the 

 dorsal wall invaginated in the form of a plunger, activated by 

 muscles arising on the epistome. Two pairs of silk glands open by a 

 common duct into the distal end of the epistomal groove, and the 

 duct of an unimpaired salivary gland transverses the united cheliceral 

 segments to open beneath them anteriorly. Closely associated with 

 the chelicerae are the two spiracular apertures of the tracheal system, 

 which lie medially in the infolded membrane just behind the united 

 parts of the cheliceral bases. From each spiracle a long, finely ribbed, 

 external groove, known as a "peritreme." extends posteriorly and 

 laterally in the dorsal integument ; inwardly the spiracles open into a 

 pair of vertical respiratory tubes with thick sclerotic walls supported 

 below on an apodeme of the epistome. From the lower ends of these 

 tubes a large tracheal trunk is given olT on each side, from which 

 issue bundles of finer tracheae distributerl anteriorly and posteriorly 

 throughout the body. 



The feeding organs of Tcfranychus tclarius (L.) have been de- 

 scribed in detail by Becker (1935) and by Blauvelt (1945), but the 

 terminology used by these writers is likely to give the uninformed 

 student the impression that the feeding organs of the tetranychids 

 have little relation to those of other Acarina. as indeed tlie unusual 

 features of these mites themselves would seem at first sight to suggest. 

 However, it is not difficult to fit the descriptions of Becker and 

 Blauvelt into an interpretation entirely in accord with that given here 

 of the acarine feeding organs in general. 



A dorsal view of the forward part of the body of Tctranychus 

 (fig. 29 A) shows anteriorly, projecting from beneath a flexible 

 fold {(if) of the back, what appears to be a broad, heart-shaped plate 

 iStphr) with a rounded outline in front and a deeply notched margin 

 behind. This structure is rommonly calle<l the "mandibtilar plate.'" 



