XVIII METAMORPHOSIS 463 
been observed in which the outer envelope of the egg becomes 
brown and hard, and splits longitudinally, so as to allow the 
thin inner membrane to become visible through the fissure. 
More room is thus obtained for the developing larva, which is, 
moreover, protected, over most of its surface, by a hard shell. 
The deutovum stage may occur either within the body of the 
mother, or after the egg has been laid. 
Tur Larva.—Omitting, for the moment, the very aberrant 
Vermiformia (see p. 464), it is the almost universal rule for the 
egg to hatch out as a hexapod larva. The larvae of the genus 
Pievoptus are said to be eight-legged. Winkler has stated that 
the early embryo of Gamasus possesses eight legs, of which the 
last pair subsequently atrophy, but this observation requires 
confirmation. 
THE Nympu.—The nymph-stage commences on the acquisition 
of eight legs, and lasts until the final ecdysis which produces the 
imago. This is the most important period of Acarine life, and is 
divided into a prolonged active period, during which the animal 
feeds and grows, and an inert period, sometimes prolonged, but 
at others very short, and differing little from the quiescence 
observable at an ordinary moult, during which the imago is 
elaborated. In many species the nymph is strikingly different 
from the imago; in others there is a close resemblance between 
them. It would appear, from the cases which have been most 
thoroughly investigated, that the imago is not developed, part 
for part, from the nymph, but that there is an “ histolysis” and 
“ histogenesis ” similar to that which occurs among certain insects 
(see vol. v. p. 165). There may be more than one nymphal stage. 
THE HYPOPIAL STAGE occurs in the Tyroglyphinae, the “ Cheese- 
mite” sub-family. Here some of the young nymphs assume an 
entirely different form, so different that it was for a long time 
considered to constitute a separate genus, and was named Hypopus. 
The animal acquires a hard dorsal covering. The mouth-parts 
are in the form of a flat blade with two terminal bristles, but 
with no discernible orifice. The legs are single-clawed, and all 
more or less directed forward, and they are articulated near the 
middle line of the ventral surface. Suckers are always present 
under the hind part of the abdomen. 
It appears that these remarkably modified nymphs are 
entrusted with the wider distribution of the species, and that 
