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 Hciniptcra 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 NcpcB 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 DytiscidcB demolish the spiders. 



The HydracJina 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 Hydrachncs 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 



