§ 320. 



THE ARACHNOIDAE. 



39T 



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 Emydmm/'^ 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 Troynbidia 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 Kroi/er, Naturhist. Tids. III. 1840, p. 

 299, or Isis, 1841, p. 713, Taf. III. Tab. III. or 

 Ann. d. Sc. Nat. XVII. p. 288, PI. XIII. B. 



6 See Doyire, loc. cit. p. 358. The embryos of 

 the other Tardigrada have four pairs of legs. 



T See Dus:es, Ann. d. Sc. Nat. I. p. 166, PI. XI. 

 fig. 47. It follows clearly from the position of the 

 eyes, which are situated not upon this snout, but 

 upon the ceplialothorax, that tliis snout is only a 

 support of the parts of the mouth, and not a head. 



8See AudouiHy Mem.surl'Achlysie iutheM^mde 

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



found these red Achlysiae with monstrous bodies on 

 the dorsal surface of the abdomen oi 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 Dug-es (Ann. d. Sc. Nat. I. 

 1834, p. 166, PI. XI. fig. 49-55). 



9 See Gervais, in IValckenaer, Apt^res, &c., III. 

 p. 17S. 



* [ § 320, 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 



the Acarus muscarum of Deg'eer, and the Ilypo- 

 pus of Duges. 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 } — Ed. 



