Observations on the Oribatidse, By A. D. Michael. 17 



I incline to think that it varies. I have also found that instead of 

 being similar in all species the system differs materially. 



To commence with the more ordinary arrangement, such as 

 that fomid in Orihata glolula, a very good species for seeing the 

 tracheae, Damseus geniculatus, &c. The main tracheae are simple 

 tubes, never anastomosing, and usually without any branches or 

 dichotomous or other furcations or divisions ; indeed I doubt if such 

 ever exist. These tubes are much convoluted, or serpentined, and 

 interlaced amongst the other organs. One large trachea on each 

 side, which I will call the great dorsal, winds above the ahmentary 

 canal, very close to the notogaster, until near the posterior margin, 

 when it takes a deeper course, following the ahmentary canal, and 

 becomes very difficult to trace. Another large trachea on each 

 side, which I will call the great ventral, passes in a serpentine line 

 along the ventral surface and is in connection with the sexual 

 organs, winding about the oviduct and ovary in the female. Either 

 one or two large tracheae on each side proceed more along the 

 lateral edges of the abdomen, between the two before described ; 

 there are also two or more shorter and finer unbranched tracheae 

 allotted to the cephalothorax on each side, one pair being distributed 

 to the great muscles of the mandibles. The tracheae are of almost 

 even diameter throughout the principal part of their length, 

 to\'.ards their ends they usually diminish and end in a point ; this 

 is not, however, invariable, as the large tracheae, on the contrary, 

 sometimes enlarge at their distal ends so as to form small bulbs. 



The large tracheae above described approach very near to each 

 other at their origin, and if one does not actually follow them to 

 their commencement (a matter of no slight difficulty), but is 

 satisfied with their appearance after the removal of the notogaster, 

 they unquestionably do look as if they were proceeding to the old 

 so-called stigma (my pseudo-stigma) which seems to be the natural 

 place for them to go to ; if, however, the adipose tissue and muscle 

 be removed, and the tracheae followed to their origin, it will be 

 found that they turn away from the pseudo- stigma, and end 

 separately in the acetabula of the legs. The tracheae are often 

 enlarged at the commencement so as to form a small air-sac, which 

 either wraps round or is attached to the inner side of the acetabulum : 

 this is seen in plate I. fig. 5 and plate I. fig. 9. 



One main trachea only usually proceeds from the acetabulum 

 of each leg, but in some cases two proceed from that of the second 

 leg, and more than one of the small cephalothoracic tracheae from 

 that of the first leg. In these cases they often join before reaching 

 the stigma and form a very short joint trunk (plate I. fig. 6). 



In consequence of the interlacing of the tracheae among the 

 organs it is far from easy to ascertain which trachea proceeds from 

 each acetabulum, but it appears to me, as well as I could trace 



Ser. 2.— Vol. III. G 



