18 Transactions of the Society. 



tliem, that tlie great ventral trachea proceeds from the acetabulum 

 of the fourth leg, the great dorsal from that of the third leg, the 

 lateral trachea or tracheae, which wind among the organs and cross 

 right over the body, from the acetabulum of the second leg and the 

 small cephalothoracic tracheae from that of the first. 



I think it possible that the two small tracheae belonging to the 

 cephalothoracic system find their stigmatic opening at the base of 

 the maxillae. I should be extremely doubtful about this last point 

 were I speaking only of the species whose tracheae are described 

 above, as in them these tracheae, wherever they open, are extremely 

 fine and delicate ; but in Nothrus theJeproctus, &c., referred to 

 below, the air-vessels in this situation are more powerful, and seem 

 as though they were proceeding to the mouth, and it must be 

 remembered that this is the position of the principal stigma, or 

 stigmata, in Cheyletus* Tromhidium,'\ Mijohia,X the Hydraeh- 

 nidse, &c. 



In other members of the family, such as Nothrus theleproctus, 

 the respiratory system above described is greatly modified : the 

 origins of the tracheae are placed in the same situations as in the 

 species above described, but the nature of the air-vessels is very 

 difierent ; we no longer have long winding tracheae of small 

 diameter and very delicate walls, we have instead much shorter 

 organs, thicker and less regular in form, and of a stouter and 

 different texture, by which they may be easily recognized when the 

 body is opened, as they have a thick, silvery appearance not easy 

 to describe. Under a low amplification they assume that slightly 

 iridescent appearance characteristic of a lined object under a power 

 insufficient to resolve it ; using a higher power regular cross lines 

 are strongly developed, and a still greater amplification will show 

 an object thickly and regularly covered with small circular bosses 

 which will remind the observer of such a diatom as Pleurosigma 

 formosum : this may, however, possibly be a deceptive appearance. 



The tracheae or air-vessels proceeding from the acetabula in the 

 species now being described, are very much shorter and thicker 

 than in the form before referred to, and are not convoluted or 

 interlaced between the organs to the extent which we find in the 

 longer and finer tracheae of Orihata, &c. ; at their extremity furthest 

 from the stigma (the blind end) they are usually suddenly 

 diminished in diameter and carried on for a very short distance in 

 a blunt point. 



In this form the air-vessels in the cephalothorax, which may 



* Fumose et Robin, " Me'moire aiiatomicjue et zoologiqne sur les acariens des 

 genres C'heyletus, &c.," Jouru. Anat. et Pliya. (Robin) 18G7, p. ^63. 



t Piigenstecher, ' Beitrage zur Anatomie iler Milben,' Leipzig, 1860, Heft 1, 

 p. 19. 



X Claparede, " Studien an Acariden," Zeitschr. f. Wiss. ZooL, 1868, pi. 37. 



