54 SIXTH ANNUAL REPORT 



glyphs. This happened more especially in the fall of the year, and I 

 could not help suspecting that the former might prove to be a winter 

 or hibernating form of the latter. There is so much yet to learn of the 

 polymorphism of mites that the suspicion may yet prove justifiable. 

 But with our present knowledge it is safest to explain the facts above 

 stated on the ground that the Hoplophoras were at first buried, and 

 consequently invisible, within the roots examined, and that the 

 decrease in the number of Tyroglyphs was owing to death and other 

 causes — an explanation which is all the more plausible from the fact 

 that I subsequently found the same narrow-bodied Hoplophora swarm- 

 ing in decaying cottonwood logs. 



Iloplop/wra (meaning armed or weapon-carrier) is an anomalous 

 genus belonging to a curious family of mites {Orihatidoc)^ distin- 

 guished, as Nicolet well sets forth, from all other families by having a 

 hard covering which is analogous to that of many hexapods, but less 

 elastic or yielding, so that, while resisting a great degree of pressure, 

 when it once gives way it cracks and fractures with t^e brittleness of 

 glass. These coverings are differently formed and sculptured and fre- 

 quently ornamented with protecting spines. The mites of this family 

 are also distinguished from other mites by their organs of respiration 

 which are at the superior base of the cephalothorax and look like 

 eyes, being rounded elevations surmounted by a hair. The family 

 comprises two great divisions : First, those which in the larva state 

 have a form and appearance widely different from those of the adult, 

 which are assumed only at the last molt. Second, those which are 

 born more in the image of the adult form, to which they gradually 

 approach by each successive molt. The genus Iloylophora^ accord- 

 ing to Nicolet, is further anomalous by being born with eight legs, 

 whereas all other mites, even those of its own family, are born with 

 but six; Claparede, however, shows that at least one species {R. con- 

 ■tractilis) is hexapod before the first molt. Another peculiarity 

 belonging to it is the mobility of the cephalothoracic shield which 

 closes over the abdomen and fits tight like the lid of a box, when- 

 ever the animal withdraws its head and limbs, which it does on the 

 slightest disturbance. Indeed, the hardened body-covering is admira- 

 bly constructed for the purpose of protection. It is composed of three 

 principal parts, the thoracic lid, a superior piece covering the back, 

 the sides and the posterior portion of the body, and a ventral piece, 

 more narrow, rounded behind, and divided in four by slits which form 

 openings for the protrusion of the anal and genital organs. 



The species under consideration diff"ers from all others described 

 in the form of the horny covering, which is so narrow that the animal 

 topples over on its side the moment the limbs are withdrawn. 



