374 British Hydrachnidce : The Getius Fiona. [Sess. 



easy to mistake one for another, so that however careful one 

 may be in making a drawing of a specimen, the result only 

 depicts that particular individual. When a number are taken 

 from one pond they are generally found to be all very much 

 alike, but may be quite different to those obtained from 

 another pond. It is as well to draw attention to this, so that 

 any one using this paper and its keys of identification may 

 not be disappointed because their particular specimens may 

 have a different number of acetabula, or the colour and contour 

 of the body may be quite different to those here described. 



External structure. 



The body is elliptical or oval in outline, sometimes slightly 

 compressed on either side of the posterior region and some- 

 times, particularly in the male, slightly flattened or bowed 

 in on the anterior. The body is more or less arched and 

 thick. The skin is thin and transparent, with a fine surface 

 very often finely striated, but never papillose. The eyes, 

 composed of one or more lenses, are sessile, wide apart and 

 usually very dark in colour. In front of the eyes are two 

 hairs, known as antenniform bristles. These vary much in 

 length and thickness in different species. On and across the 

 median line of the dorsal surface is situated the Malpighian 

 vessel. This belongs to the internal structure, but it is so 

 conspicuous, exhibiting, as it does, through the thin skin 

 of the dorsal surface such a variety of, sometimes brilliant, 

 colouring that it adds much to the beauty of these inter- 

 esting creatures, and we must take it into our considera- 

 tion. The Malpighian vessel varies in form very much, par- 

 ticularly in the females, but it is generally T-shaped. The 

 dermal glands are found on various parts of the body on both 

 dorsal and ventral surfaces. There is usually a hair near the 

 edge of each gland. 



The maxillary shield, mouth-organs and the palpi are 

 attached to the anterior portion of the ventral surface.' 

 Frequently, the mandibles can be seen projecting forward 

 from the anterior part of the shield. The epimeral plates 

 of the female are arranged in four groups on the ventral 

 surface, the first two pairs being close to the maxillary 



