NO. 10 MiliDING OKi.ANS OF AKACHNIUA — SNODGRASS 63 



.spending section of any other arachnid it will be seen that the anterior 

 cephalic wall in the Acarina, instead of going direct from the edge 

 of the carapace to the base of the epistome, as in the spider (fig. 2 D, 

 O'C), makes a complex folding (fig. 22 B, a-b-c-d-c) between the same 

 two points. The mouth {Mth) between the base of the labrum and 

 tiie base of the hypostome leads directly into the pharynx (Phy), 

 which, as in other arachnids (fig. 2D), has dorsal dilator muscles 

 arising on the epistome (Ef^st). 



To this simple basic structure of the capitulum there are added in 

 the different groups of Acarina various secondary modifications, 

 which may include the following: (i) a retraction of the capitulum 

 into the anterior end of the lx)dy; (2) elongation, invagination, or 

 other modifications of the chelicerae ; (3) formation of membranous 

 cheliceral sheaths; (4) invagination of the epistome; (5) reduction 

 of the labrum; (6) elongation of the hypostome; (7) development 

 of appendicular lobes on the pedipalp coxae associated with the hypo- 

 stome ; (8) reduction of the palps. 



The exact composition of the acarine capitulum is perhaps more 

 complex than that indicated above. However, Wagner (1894), in 

 describing the embryonic development of Ixodes calcaratiLs Bir., 

 ascribes the major part of the capitulum to the pedipalp coxae. The 

 coxae, he says, are at first simple, but later a lobe grows out from 

 each ; then the coxae take a longitudinal position and their basal parts 

 gradually grow upward around the bases of the chelicerae to form the 

 capitular walls, while the lobes unite in an unpaired under-lip process 

 (the hypostome). Renter (1909) is more complete in his account of 

 the development of the capitulum in Pcdiculopsis (jraminum (Rent.). 

 The rudiments of the chelicerae and pedipalps, he says, undergo very 

 great changes in the transformations by which these appendages, to- 

 gether with the cephalic lobe, are converted into the gnathosoma. The 

 chelicerae undergo a considerable reduction and are transposed to a 

 preoral position. The pedipalps are also reduced in their distal parts, 

 but their basal parts embrace the chelicerae laterally, while ventrally 

 they unite medially with each other, thus forming the lateral and 

 ventral walls of the gnathosoma. The upper wall of this hcadlike 

 structure, however, is derived from an unpaired projecting anterior 

 part of the cephalic lobe, which unites laterally with the dorsal parts 

 of the pedipalp coxae. The primary head lobe (Koptlappen), Renter 

 says, grows out between the proximal upper jxirts of the pedipalp 

 coxae, and thus covers the cheliceral rudiments dorsally. "Dann 

 verschmelzen die proximalen Teile der Pedipalpcn unten median mit 

 einander, oben mit den primaren Kopflappen, wodurch ein vorn die 



