DESCBIPTIONS OF GENERA AND SPECIES. 197 



predominates, and some specimens are almost wholly 

 of that tint. The colour apparently arises from the 

 diffusion of food-material or products ; it is not pigment 

 in the cuticle. This can be demonstrated by placing a 

 dark specimen in a drop of water on a glass slip under the 

 microscope, and placing a cover-glass over it; as the water 

 evaporates the cover will be slowly drawn down, produc- 

 ing pressure upon the creature ; the result of this will 

 be that what appears like the whole contents of the 

 body are gradually discharged through the anus, and 

 the opaque dark creature becomes yellowish white and 

 transparent. During life, however, the brown colour 

 does not look as if it were caused by food-contents ; it 

 has every appearance of being the true colour of the 

 greater part of the body. The rostrum and legs are 

 always pale pinkish yellow. 



Texture polished. 



Shape. This also depends considerably upon whether 

 the creature is fully fed ; when it is so the distinctive 

 form is lost, and the Acarus becomes almost a roll with 

 little shape in it ; but when not quite so fully-fed the 

 shape is rather striking. The cephalothorax is slightly 

 broader than the abdomen, but much thinner dorso- 

 ventrally ; so that where the two join, the dor sum of 

 the abdomen stands high above the cephalothorax. 

 There is a sharp indentation in the lateral edge of 

 the creature where the cephalothorax and abdomen 

 join; behind this the abdomen of the female is 

 almost sack-shaped; that of the male narrows a 

 little more posteriorly. In both sexes the hind 

 margin is indented in the middle, so that each side 

 forms a rounded lobe. 



Cephalothorax. The rostrum is a smooth tube or 

 collar, long for the family ; the strong chelate man- 

 dibles (fig. 7) project considerably; each arm of the 

 chela is tridentate. The five-jointed palpus (fig. 8) 

 is otherwise of the ordinary type; and is adherent to 

 the membranous maxillary lip, below which is a tri- 

 angular chitinous sclerite which might possibly be 



