1893.] ACAEXJS rOTJND IHf CORNWAIiL. 265 



LEH^TTTIfG-CXA ALGITOEAIS-S. (Plate XVIII.) 



Average length without mandible about '38 mm. 



Greatest breadth about '20 mm. 



Length of legs 1st pair about "13 mm. 

 „ „ 2nd „ „ •14 mm. 



„ „ 3rd „ „ -15 mm. 



„ „ 4th „ „ "15 mm. 



Colour. — The actual colour of the creature, if it had been fasting 

 for a long time, would probably be almost entirely light yellowish, 

 but as ordinarily seen it is dark olive-brown with very numerous 

 light yellowish spots and marl-iings. The yellowish colour is chiefly 

 in spots and spaces surrounded with the olive, but the spots are not 

 arranged in any definite pattern ; although a few spots on the 

 cephalothorax have a tendency to be permanent, the whole of the 

 markings are most irregular and varynig. The olive-brown colour 

 greatly predominates, and some specimens are almost wholly of that 

 tint. Thecolour apparently arises from the diffusion of food material 

 or products, it is not pigment in the cuticle ; this can be demon- 

 strated 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, producing 

 pressure upon the creature ; the result of this will be that what 

 appears like the whole contents of the body are gradually dis- 

 charged from the anus, and the opaque dark creature becomes 

 yellowish white and transparent. During life the brown colour 

 does not, however, look hke 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. 



Tea-ture 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 

 qmte so fully fed the form is rather striking. The cephalothorax 

 is slightly broader than the abdomen, but much thinner dorso- 

 ventrally, so that where the two join the dorsum of tbe abdomen 

 stands high above the cephalothorax. There is a sharp indentation 

 in the lateral edge of the creature, where 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 mandibles project considerably ; 

 each arm of the chela is tridentate (fig. 7). The five-jointed palpi 

 (fig. 8), of the ordinary type, are adherent to the membranous 

 maxillary lip, in the centre of and below which is a chitmous 

 triangular sclerite which might possibly be considered to represent 

 a labium. The central portion of the cephalothorax, behind the 

 rostral tube, forms a large, rounded, fleshy lobe which overhangs the 



