94 Miscellaneous. 



morphoses of the Acarina of this and some other allied families ; and 

 I now communicate the results to the Academy. 



Besides the curious facts of heteromorphism mentioned in the two 

 notes just referred to, and which are only produced under certain 

 determinate conditions, I find that the Sarcoptidae arrive at the 

 adult state by a series of moults, by which the little animal gradu- 

 ally acquires all its organs, but without its general form departing 

 from the normal type ; the hexapod larva becomes the octopod 

 nymph and then the sexual individual, always presenting the 

 specific characters in the form and constitution of the rostrum 

 and legs. 



It has hitherto been supposed that these moults were effected as in 

 the Insects with an incomplete metamorphosis or in the Crustacea — 

 that is to say, that a sort of caducous epidermis was alone detached, 

 retaining the impression of the parts of the skeleton which remained 

 intact, and that the mite issued from it after having successively 

 drawn its rostrum and its legs from the old envelope as from 

 a sheath. This is the opinion expressed by all French and foreign 

 authors who have attended to the embryogeny and development of 

 these microscopic creatures. Claparede, the last of these authors, 

 says* with reference to the moults of the Tyroglyphi : — " The 

 rostrum and the legs are drawn out of their chitinous sheaths, a new 

 segmentation of the thorax into three parts takes place in the soft 

 animal, and the octopod mite is gradually formed under the protec- 

 tion of the integuments of the hexapod larva." 



Nevertheless Claparede had made so complete an investigation of 

 the extraordinary moults of the aquatic Acarina known under the 

 name of Atax, that I am surprised he should have found the moults 

 of the common terrestrial Acarina of the genus Tyroglyphus so 

 simple. 



M. C. Robin f, while admitting that the rostrum and legs are 

 drawn out of the envelope corresponding to them, and folded under 

 the belly between the old and new integuments, has nevertheless 

 recognized that we cannot see the hairs torn out from the interior 

 of the older ones, and that, being more numerous, they originate at 

 the points where they are inserted as soon as the new integument 

 is separated from the old one, and that this is the case with all the 

 organs to which there is nothing to correspond in the old form, such 

 as the fourth pair of legs in the nymphs and the caudal processes of 

 the males of most of the avicolar Sarcoptidae. 



My observations show that in all the Sarcoptidae all the organs, 

 both those which are represented in the older form and those which 

 are not, are completely formed anew, without the assistance of the 

 old organs and completely independent of them. It is a new birth, 

 if we may say so — a production of a new creature in the body of 

 the old one. 



Claparede, in the memoir which I have quoted, proves that in 



* u Studien an Acariden," in Zeitschr. fiir wiss. Zool. xviii. 1868. 

 t " Memoire sur les Sarcoptides avicoles, et sur les metamorphoses des 

 Acariens," in the Comptes Rendus, tome lxvi. p. 770 (18G8). 



