THE ACARINA OR MITES. 



93 



that bore in decaying wood are usually a milky or yellowish white. WTiile in the 

 nymphal stage the mite undergoes three molts; the last molt bringing it to the adult 

 condition. In some cases the nymph during the latter part of its life approaches 

 the appearance of the adult; in other species the nymph at each molt becomes more 

 and more specialized, and in no wise approaching the adult in appearance. The 

 hairs are often very long, frequently serrate, and sometimes scalelike and even fan- 

 shaped. In several cases of allied species the nymphs are similar in appearance, while 

 the adults are very dissimilar. 



'WTien a nymph is about to become an imago it seeks some sheltered spot and, fixing 

 its legs firmly in the substance upon which it rests, it gradually becomes inert and 

 apparently dead. It remains in this condition about 10 days. "\Mien about to trans- 

 form to the adult the skin splits behind and shows the imago beneath; this split 

 increases without any perceptible movement of the 

 mite until it is quite large, when the mite begins 

 to back out of its old shell. It may be noticed 

 then, according to Michael, that the legs of the 

 adult are not withdrawn from the legs of the nj-mph, 

 but are folded beneath the adult. It may be, 

 however, that they had been withdrawn from the 

 nymphal legs before the skin begins to split. 

 Once out, the mite walks off, leaving its old skin 

 with the legs outstretched in the position they 

 had during the resting stage. Nymphs can live 

 over the winter, and while they are most abun- 

 dant in spring, they are also found at other times. 

 Michael, who has reared many oribatids, thinks that 

 the winter is usually passed in the egg or adult 

 stage, and that there may be several but no regular 

 number of broods each year. 



Some species (Ameronothus) , it is claimed, are 

 viviparous, Bostock having observed the birth of 

 living larvae. These forms occur in water, and the 

 aquatic habit has, perhaps, something to do with 

 their viviparity. 



Many of the oribatids have a general resem- 

 blance to tiny beetles, and are often taken by 

 coleopterists in the field for them. One species, Notaspis castaneus Herm., deceived 

 Robineau-Desvoidy, who described it as a new genus and species of beetle — Xenillus 

 dypeator. 



The internal anatomy of the Oribatidae is quite well known, having been investi- 

 gated by Nicolet, and later and more thoroughly by Michael. The alimentary canal 

 is composed of a pharynx, oesophagus, stomach or ventriculus, intestine, colon, and 

 rectum. The oesophagus has, near its posterior part, an enlargement or ingluvies of 

 varj'ing size, according to the species. The stomach is a large sac in the upper part 

 of the abdomen, provided with two large caeca, one each side, reaching back to near 

 the tip of the abdomen. The small intestine is very short and enlarges to the colon, 

 which is separated from the rectum by a constriction. In most, if not all, forms there 

 axe two large glands, the proventricular glands, wliich open into the ventriculus near 

 the caeca. They are supposed to secrete some fluid useful in digestion. The male 

 organs of generation consist of one central testis, usually large, two rasa deferentia, 

 uniting into a ductus ejaculatorius, which opens through an extensile penis. In the 

 female there is a median ovary (sometimes showing traces of division) opening into 

 two oviducts which unite in a vagina ; the latter opens tlirougli a protrusible oN-ipositor. 

 It is possible that the ovary is connected by two fine tubes to an aperture near the anus, 



Fig. 184.— Oribatella valida. (Original.) 



