PROCEEDINGS OF THE NA TIONAL MUSEUM. 



VOL. XXVIII. 



INTRODUCTION. 



The Acai'ians form lui order in the gw^nt ehiss Arachiiidu. Thev are 

 thus related to spider.s, daddy-lono-deo-s, and scorpions. A few writers 

 at various times have claimed that the mites were a separate class, l)ut 

 tiie best sense of modern autliors is that they are g'enuine Arachnids, 

 and in many ways closely related to Solpugids and Phalang-ids. 

 Although quite easily recog-nized at sight, it is not so easy to give 

 definite charactei's whereby to distinguish a mite from other Arach- 

 nids. The abdomen and cepbalothorax are broadly united to each 

 other, and often there is no distinction l)etween these parts. Usuallv 

 there is no trace of segmentation, but in some forms it is (piite dis- 

 tinct. Eyes ar(^ often present, but rarely only a median pair as we 



find in Phalangids and Solpugids. The 

 "^^ • mouth segments have Ijecome united to 



b form a l^eak, rostrum, or capitulum. How- 

 '^ ever this is not easily made out in some 

 '^ forms. Commonly tiie larva at birth has 

 but three pairs of legs, and ol)tains the 

 •^ fourth pair only after a molt and meta- 



^ -:ig morphosis. In the Eriophyidte, how- 



/ f / \ ,\ cvei-, there are l)ut two pairs of legs in 



I I /y\ botli adult and young, and in l*frr<)j>ti(s 



the young have eight legs at l)irth. The 

 adult mite has (except in the Eriophyid;e) 

 four pairs of legs; often arranged in two 

 groups, the hinder pairs apparentl v arising 

 from the abdomen. However it is not 

 prc^bable that such is the case; rather the 

 coalescence of the abdomen and cephalo- 



Fiij 1. — Dorsal view of a mite, (i, . , ^ 



mandibles; 6, palpi; r, leg I; d, thorax has effaced the true outlines of these 

 ^^i5^;":'iI^r?^t™^t^ ^ Portions.^ It is probable that the a},domen 



of mites is more than the abdomen of other 

 Arachnids. If one examines a So/j)u(/a, he sees that the cephalo- 

 thorax is divided into several portions, and it may be that the ceph- 

 alothorax of mites represents only the anterior of these, while the 

 al)domen of mites represents the abdomen of S()lj)\uja phis the two 

 posterior divisions of the cephalothorax. In T((r-'«>)icnius^ and some 

 other genera, the a))domen shows on the dorsum distinct traces of 

 segmentation. On the venter there is still less distinction between 

 parts, and in several groups, as Ixodidty and some Gamasidt\i, 

 the genital segment is pushed forward between the legs so far that 

 the genital aperture is close to the mouth. In other forms the geni- 

 tal opening is at the extreme tip of the body, and the anus is upon 

 the dorsum. 



