THE HYDRACHNA GLOBULA. 
44I 
like a great head, more or less angular in shape, and stuck on 
in front of the eyes, and it terminates in a narrow mouth. How 
long these larvae swim freely in the water is not known, but they 
cannot go out of the element without perishing by becoming 
dried up ; but when they become nymphs they are carried 
about in the water and in the air also, by the insects they are 
attached to as parasites. Towards the end of the summer and 
during the autumn the nymphs are to be found attached to 
the bodies or limbs and elytra of such Hemiptera as Nepa , and 
they are slung on to their hosts by tail-like threads. When the 
nymph is found attached to the hard parts of Nepce minute per- 
forations in them will be noticed with a magnifying glass. The 
water beetles are preyed upon by these nymphs, and in return 
some of the Dytiscidoe demolish the spiders. 
The Hydrachna larva, immediately after it is hatched, has a 
globular body, but the hinder part soon begins to elongate, and 
becomes more or less pointed after a while, and the little spider larva 
is noticed to be flat and spindle-shaped. In a short time the shape 
alters to that of an elongated ellipse, and when it has attained its 
full growth the larva becomes pear-shaped, and is curved beneath 
and tinted of a red violet colour. The young Hydrachnce only 
attain this size and shape during the winter time, and then they 
are about a line in length. It is remarkable that although the 
the body increases so much in bulk, the sucker, legs, and scale 
do not grow larger,- but are always of the same size, and are 
situated invariably at the front of the animal. When the larva 
has attained the greatest possible size the sucker appears to con- 
stitute the head, the scale is present with the remains of eyes, and 
six thighs; but it frequently happens that the palpi and even the 
legs have disappeared partly or entirely, especially if the larva 
living as parasite has been attached to a vigorous and actively 
swimming water beetle. In this case the membranous space which 
unites the sucker to the body is really elongated like a neck. This 
is most remarkable, and the manner in which the retrogade de- 
velopment is accomplished is well worthy of study. Early in the 
larval life, and before the body begins to elongate, the legs and 
palpi appear to retreat within the animal and to leave their out- 
side skin attached to their prey. This skin continues to act as 
