<§,320. 



TUB ahaciinoidae. 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 Hydradina. 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 Achhjsia, 

 until it was discovered that they possessed eight legs after their first moult- 

 ing.*"^' Similar metamorphoses occur with the Tromhidia 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, Lepius, and Ocypeta/-''' '^ 



5 See Krtii/er, Naturhist. Tids. III. 1840, p. found these red Achlysiae with monstrous bodies on 

 299, or Isis, 1841, p. 713, Taf. III. Tab. III. or tlie dorsal surface of the abdomen of Dytiscns and 

 Ann. d. So. Nat. XVII. p. 288, PI. XIII. B. Hydrophilus. Others, smaller, were observed even 



6 See Doi/ere, loc. cit. p. 358. The embryos of more frequently upon the segments of the body and 

 the other Tiirdigrada have four pairs of legs. the articles of the extremities, with Nepa, and Ra- 



7 See DugSs, Ann. d. Sc. Nat. I. p. 166, PI. XI. natra. The true nature of these epizoa has been 

 fig. 47. It follows clearly from the position of the cleared up by Burmeister (Isis, 1834, p. 138, Taf. 

 eyes, which are situated not upon this snout, but I. fig. 1-6), and by Dus:is (Aim. d. Sc. Nat. I. 

 upon the ceplialothorax, that this snout is only a 1834, p. 166, PI. XI. fig." 49-55). 



support of the parts of the mouth, and not a head. S* See Gervais, in fVaickenaur, Apt6res, &c., III. 



8See ^udoMjn, Mem.surl'AchlysieintheM6mde p. 178. 

 laSoc.d'Hist.Nat. deParis,I.p.98,Pl.V.No.2). He 



* [ § 320, end.] There is some ground for the the Acarus muscarum of Deireer, and the Ilypo- 



opinion that alternation of generation, so called, pus of Duges. It has neither mouth nor digestive 



occurs with some of the Arachnoidae. Dujardin apparatus, but simply adheres to the animal on 



(Ann. d. Sc. Nat. 1849, XI. p. 243) has examined which it Uves, by a sucker. It may be the nurse 



the wall-mite found on the house and other flies — of a Gamasus ? — Ed. 



34 



