THE HYDRACHNA GLOBULA. 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 Hydrachna globula 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 trachez 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 Hydrachne@ live is absorbed 
or taken into the trachez ; 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 Hydrachne 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 few 
