54 



REPOET 108, U. S. DEPARTMENT OF AGRICULTURE. 



In Oxus the body is about t^vice as long as broad, and the venter is mostly occupied 

 by the great epimeral plate, which is emarginate in the middle behind for the genital 

 opening. The legs are all crowded up near the front of the body. Frontijpoda is 

 similar to Oxus. 



A number of water mites were described by Haldeman in 1842 in a new genus 

 Unionicola. The characters were all in the colors, so it is possible that all or most of 

 them are variations of the common Atax ypsilophorus Bonz. He also described several 



^— ^ 



Fig. 97.- 

 Atax. 



-Tarsal claw of 

 (Original.) 



Fig. 98.— Palpus of Atax. 

 inal.) 



(Orig- 



Hydrachna, but these also have not been placed in modern genera. Many genera have 

 been made for exotic species. The African fauna has been explored more thoroughly 

 than the others, but most of the forms are similar to the European. Bargena is remark- 

 able on account of its large, median ventral furrow. 



Family HALACARID.E. 



This is a small family of marine mites. They have a leathery skin, frequently granu- 

 late or striate, but commonly destitute of bristles. Sometimes there are coriaceous 

 plates or shields. The body usually shows the division into cephalothorax and abdo- 

 men, both above and below. The rostrum is often quite prominent, sometimes as 

 large as in the Bdellidse. The cephalothorax usually has three eye-spots, one being 

 located on the middle in front. The palpi are three or four jointed, the last article 

 sharp-pointed at tip. The mandibles are rather prominent and end in a single straight 

 or recurved claw. The legs are moderately long, rather widely separate at base, lateral 



or sublateral in origin, and end in two claws (fig. 99), 

 They bear a few scattered bristles and sometimes 

 dense plumes of fine hair. Some species have lam- 

 ellae on their femora, similar to those in certain Ori- 

 batidse, and the joints are swollen near tip as in' 

 many Oribatidai. The genital opening is quite large 

 and far back; the anus is small and at the tip of the 

 abdomen. These mites have no trachea?, but do not 

 appear to be related to other atracheate acarians, 

 but rather to the Bdellidae and Oribatidae. It is 

 perhaps not a natural family, but derived from several groups. The llalacaridae are 

 found crawling slowly over algae, fi-equently in shallow water, but some have been 

 dredged at considerable depths. The adults are free and feed on diatoms and other 

 minute vegetation. The young of some forms feed on the eggs of copepods, which 

 often are attached to various animals. One (Ilalixodes) is known to occur on a Chiton. 

 They are from less than 0.5 to 2 mm. in length and their colors depend largely on the 

 nature of their food. The y6ung have the general appearance of the adults, and 

 nymphs sometitnes possess rudimentary genital organs. The legs of the nymphs have 

 often one le.'^s joint than in the adults. The ovipositor is a fleshy exsertile organ t ipped 

 with several pairs of bristles. But little is known of their life history. The eggs 

 appear to be few in number, one species having from 8 to 12. Some species apparently 

 deposit eggs in spring, and during the summer one finds only immature specimens, the 

 adults appearing in the fall or early winter; in other species the adults and young may 

 be taken at any time. The same species may occur in deep water and near the shore, 



Fig. 



99.— Claws of Halacarus. 

 thor's illustration.) 



(Au- 



