ANATOMY. 507 



to the internal fluids, yet the larvaD and nymphs al- 

 most always have soft skins, greatly resembling those 

 of the TyrocjliipliidcB and Sarco})fid(E in many species. It 

 may also be suggested that if the aeration of the blood 

 of the nymphs relied upon the action of a small number 

 of trachea?, havino- their stio^mata in the acetabula of the 

 legs, as in the adults, then, from the softer and more 

 yielding nature of the parts in the immature creatures 

 the stigmata might be very frequently closed, and 

 consequently not be sufficient to admit the requisite 

 quantity of air. It was, inter alia, this combination of 

 the soft integument, and the absence of tracheae, which 

 caused Claparede to say that the Oribatidce passed 

 through an ""Acarus-likeform,"* — a remark which was 

 not surprising looking at the few species which he 

 studied, but which he would probably have considerably 

 modified if he had lived to extend his researches to a 

 larger number. It is also the same characteristic, 

 which is found in some other Acarina besides the Orl- 

 hatidce, which forced Dr. Kramer, in his classification 

 (referred to in vol. i, p. 43), to base his main division of 

 the Acarina into Tracheata and Atracheata entirely 

 upon the Imagines. 



It may be as well to repeat here that tracheae are 

 absent or almost absent in some genera of adults, 

 principally those, such as NotJirus, which resemble the 

 nymphs in some other respects. 



It is interesting to observe that, although all trace of 

 tracheae or other internal respiratory organs is entirely 

 absent from the larvse and nymphs of the Oribatidce, 

 yet in every instance the organs which writers pre- 

 vious to myself have considered to be stigmata and 

 which I call " pseudo-stigmata," are quite as strongly, 

 or even more strongly, developed in these immature 

 stages than in the adult, and are usually similar in 

 character. This presence of these organs in the 

 nymphs and larvse, which is quite indisputable, being 



* " Studien an Acariden," ' Zeit. wiss. Zool.,' Bd. xviii (1868), 

 pp. 515, 516. 



