THE HYDRACHNA GLODULA. 439 



long as their palpi, their having the third joint of the palpi the 

 longest, and by the presence of sharp blades for mandibles. A 

 full-grown female of HydracJuia globnla measures two lines and 

 a quarter in diameter ; and her shape, although apparently 

 globular, is really ovoid. The body is smooth, red, and madder- 

 coloured, and is marked by some muscular impressions on the 

 back. When it is moderately grown, the skin has a . satin-like 

 look, on account of the ramifications beneath it of the myriads 

 of trachea, the white and shining appearance of which is seen 

 through the transparent cuticle. The same transparency of the 

 skin allows the intestinal canal to be distinguished, as well as 

 the granules and cells of the tissue itself. 



There is a double sub-triangular rough and thick plate on the 

 skin, in the front of the body, which supports the muscles of the 

 palpi and of the other parts of the mouth. The eyes are external 

 to these plates, and they are red and kidney-shaped ; and in front 

 of each of them there is a small hole, out of which water comes 

 when the insect is allowed to become slightly dry. These are 

 spiracles, and there are others behind the hind legs. M. Duges 

 saw tracheae springing from these spiracles, and forming a dense 

 layer under the skin ; and he noticed that the tubes did not 

 contain a spiral fibre like those of insects, and that they were 

 excessively delicate and silky. It is an interesting question 

 whether or not the water in which the HydrachncE live is absorbed 

 or taken into the tracheae ; some may be, but there are evidences 

 that the air which is contained in the water is the essential prin- 

 ciple for respiration for these insects, as it really is for all other 

 aquatic beings. When the Hydrachncs are quiet under the shade 

 of the leaves of water plants, they constantly move their hind legs 

 about, in order to maintain a current of water around the body. 

 If the water becomes stagnant, they come up to the surface or the 

 edge, and permit the greater part of the body to remain in the 

 air, moistening it from time to time, and washing the back with 

 the long hind legs. They take care not to become too dry, for 

 three quarters of an hour's drought kills them. M. Duges found, 

 moreover, that if one of these spiders was placed in spirits of wine 

 it swam about, and continued to do so for half an hour ; then, on 

 being replaced in water, it became as lively as ever in a ^&\v 



