94 



EEPORT 108, U. S. DEPARTMENT OF AGRICULTURE. 



Fig. lS5.—0ribatella armata. (Author's illustration.) 



and that this is the bursa copulatrix. Coition, however, has not been observed, so it 

 is not certain that the male does not use the vagina. 



The tracheae when present 

 vary much in shape and size. 

 They open at the acetabula 

 of the legs; one or two tra- 

 cheae proceed from each ace- 

 tabulum ; sometimes they 

 are verylongand wind about 

 in the body; sometimes 

 short, and again they may 

 be enlarged to form air sacs. 

 In HopJoderma there are no 

 tracheae and in Nothrus they 

 are rudimentan.', and they 

 are lacking in the larvae 

 and nymphs of all forms. 

 There are various excre- 

 tory organs; one pair, the supercoxal glands, open near the acetabula of the second 

 legs; others, the expulsory v'esicles, open on the sides of the abdomen. The Oriba- 

 tidae have a delicate sense of touch, which re- 

 sides apparently in the long hairs or setae upon 

 the legs, particularly a very long hair on the 

 tibia. They have no eyes, yet have a quick ap- 

 preciation of light and darkness, and prefer the 

 latter. It is quite possible that the pseudo- 

 stigmata are organs of hearing. 



The food of the Oribatidae is usually of a vege- 

 table nature, but a few species affect decaying 

 animal matter; one of our common species is usu- 

 ally found on bones. Many feed on lichens and 

 fungi, and some bore into decaying wood. Sev- 

 eral kinds are found on the bark of liA-ing trees, 

 and others under dead bark. Many species oc- 

 cur in moss, but do not necessarily feed upon it. 

 None of them is parasitic in any stage. Most 

 species are slow in traveling, and often, when 

 disturbed, feign death . Some of the adult mites, 

 as stated, carry their molted skins and other rub- 

 bish on their backs. 



Practically none of the Oribatidae is of eco- 

 nomic importance. A few have been recorded as damaging grass, and one {Lia~ 

 carus cnpitatvs Banks) occurs in immense numbers in wheat fields of Kansas and 

 Oklahoma; but as a whole they probably ar^ slightly beneficial. There are doubt- 

 less 400 species of this family in the United States, for about 250, mainly from 

 the Eastern States, have been described. On account of their minute size, 

 obscure habits, and small economic va,lue they are not favorites with collectors, 

 but their complexity of structure furnishes more characters for classification than in 

 the case of other acari. 



The superfamily Oribatoidea contains three distinct families; some writers Innc 

 split it up into six or more, but on characters of minor importance. 



1. Mandibles very large and prominent; no division between cephalothorax 



and abdomen; two claws to front tarsi, three claws to other tarsi 



Labidostommatiu.e 



Fig. 1S6. —Oribatdla sp. (From Marlatt.) 



