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. 



THE 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 

 Pteroptus 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 NYMPH. 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 



