NO. lO IIILIDJNG OKt.ANS (Jl AKACUNIDA SNOLKjKASS 6I 



(jf Iletcropoda vcnatoria is laterally compressed (fig. 2i E, Oe) ; its 

 dorsal wall is sclerotizcd and its ventral wall membranous (D). The 

 proventriculus is Y-shaped in section (E, PvP), with the membranous 

 lower wall of the stem inflected. A section of the provcntricular 

 j)ump of Agelcna nacvia (G), as figured by Brown (1939), shows 

 strong sets of dilator muscles arising dorsally {did) on an apodeme 

 {Ap) of the carapace, and latcrovcntrally (<///) on the endosternum 

 (Endst) ; compressor muscles icpr) unite the dorsal and ventral 

 folds of the proventricular wall. 



X. THE ACARINA 



The distinctive feature relating to the feeding apparatus of the 

 Acarina is the presence of a discrete head structure carrying the 

 mouth parts, known as the capitidum, capitellHm, or gtiathosoma. 

 The first term is adopted here as being more generally used than the 

 others. The capitulum projects in front of the part of the animal 

 that bears the eyes, when eyes are present, and hence does not include 

 the entire head region derived from the cephalic lobe of the embryo 

 (fig. I E, IfL). The acarine capitulum is simply a special development 

 of the part of the cephalon that lies before the carapace in other arach- 

 nids, together with the cheliccrae and the pedipalp coxae. The capitu- 

 lum is more or less retracted into a recession of the body behind 

 it, within which it is attached by a flexible conjunctiva that allows of 

 retraction and protraction. The socketlike cavity that receives the 

 base of the capitulum is commonly called the "camerostome," but 

 the etymological significance of the term in this connection is not dear. 



The essential thing that differentiates the capitulum of the Acarina 

 (fig. 22 A) from the head of a spider or other arachnid (fig. 2 A) 

 is the sclerotization of the cephalic wall above the bases of the che- 

 licerae to form a dorsal fold or plate (fig. 22 A, Tcct) projecting 

 from beneath the anterior edge of the dorsum of the body (D). 

 This plate, termed "rostrum" and "epistome," may more appropriately 

 be named the tcctuw (tectum capituli), since it forms the dor.'^al wall, 

 or "roof," of the capitulum. Laterally the tectum is fused with the 

 dorsally extended basal angles of the pedipalp coxae (IICx). which 

 are united with each other ventrally, so that there is thus formed a 

 continuously sclerotized ring, the basis capituli. The coxae bear the 

 palps (Pip), and their dorsal surfaces, as in other arachnids, are 

 united mesally with a subcheliceral epistomal plate (Epst), which 

 bears the labrum (/->») overhanging the mouth. The ventral wall of 

 the capitulum is produced beneath the mouth and the labrvun as a 



