§ 320. THE ARACHNOIDAE. 397 
changes should be most prominent, for their embryos have a short unartic- 
ulated body, and, beside the cheliceres, are provided with only four bi- or 
tri-articulated feet. There is, however, a very long lash, attached, some- 
times to. the two cheliceres, sometimes to each of the four legs. It is not 
until after successive moultings, that the other legs, the divisions of the body, 
‘and the extremities, appear.” 
With Emydium,® and most of the Acarina, the embryos have only six 
legs, when hatched; but as they otherwise resemble the adults, their 
metamorphosis consists only in the appearance of another pair of legs. A 
true metamorphosis is observed only with Hydrachna. Here the embryos 
have a very long and large snout which might easily be taken for a head 
distinct from the trunk.” This disproportion between these two parts is 
subsequently reversed, when the young pierce with their snout the bodies 
of insects, while their own bodies, gorged with food, become of a monstrous 
size. These young have six legs, and, during their parasitic life, were for a 
long time described as distinct species under the generic name Achlysia, 
until it was discovered that they possessed eight legs after their first moult- 
ing.® Similar metamorphoses occur with the Trombidia which, as red, 
hexapod larvae, are attached to flies, grasshoppers, plant-lice and various 
other terrestrial insects. These, also, have been formed into proper genera 
under the names of Astoma, Leptus, and Ocypeta.© * 
5 See Kreyer, Naturhist. Tids. III. 1840, p. 
299, or Isis, 1841, p. 713, Taf. ILI. Tab. III. or 
Ann, d. Sc. Nat. XVII. p. 288, Pl. XIII. B. 
6 See Doyére, loc. cit. p. 358. The embryos of 
the other Tardigrada have four pairs of legs. 
7 See Dugeés, Ann. d. Sc. Nat. I. p. 166, Pl. XI. 
fig. 47. It follows clearly from the position of the 
eyes, which are situated not upon this snout, but 
upon the cephalothorax, that this snout is only a 
support of the parts of the mouth, and not a head. 
8See Audouin, Mém.sur l’Achlysie in the Mém de 
la Soc. d’Hist. Nat. de Paris, I. p.98, Pl. V.No. 2). He 
*[§ 820, end.] There is some ground for the 
opinion that alternation of generation, so called, 
occurs with some of the Arachnoidae. Dujardin 
(Ann, d. Sc. Nat. 1849, XI. p. 243) has examined 
the wall-mite found on the house and other flies ——- 
34 
found these red Achlysiae with monstrous bodies on 
the dorsal surface of the abdomen of Dytiscus and 
Hydrophilus. Others, smaller, were observed even 
more frequently upon the segments of the body and 
the articles of the extremities, with Nepa, and Ra- 
natra. The true nature of these epizoa has been 
cleared up by Burmeister (Isis, 1834, p. 138, Taf. 
I. fig. 1-6), and by Dusés (Ann. d. Sc. Nat. L. 
1834, p. 166, Pl. XI. fig. 49-55). 
ry Gervais, in Walckenaér, Aptéres, &c., III. 
p. 178. 
the Acarus muscarum of Degeer, and the Hypo- 
pus of Dugés. It has neither mouth nor digestive 
apparatus, but simply adheres to the animal on 
which it lives,by a sucker. It may be the nurse 
of a Gamasus } — Ep. 
