382 ME. A. D. MICHAEL ON HYPOPI, 



At this time Hypopi were numerous in moss in the neighbour- 

 hood. In April (1882) I took two similar sj)ecimens from moss, 

 and placed them alone in a small glass cell with blotting-paper. 

 In a few days they became inert, and one changed into a Tyro- 

 glyphus nymph, leaving its exuvium on the blotting-paper. The 

 other soon afterwards did the same. Early in May one of the 

 nymphs underwent ecdysis, and became an adult TyroglypJius ; 

 about the 13th of May the second became inert, and shortly 

 afterwards changed to an adult Tyroglyphus. 



One thing that became evident in breeding the Tyroglyphi and 

 Hypopi was that the full-grown nymphs of the former did not 

 change into the latter, but that it was the young nymphs that 

 changed, and that the change occurred always at bhe same stage 

 of the life-history. It appeared to me, with those species which I 

 have observed, that the change occurred at the second nymphal 

 ecdysis, and the hy]Dopial form continued until the following 

 ecdysis : this probably accounts for the small size of the Hypopus 

 compared with the adult Tyroglyplms. 



I found, by carefully watching individual specimens, fcliat they 

 do not all turn into Hypopi during the course of their life-history, 

 but, on the contrary, most proceed from larva to nymph, and 

 through all the nymphal ecdyses, and become adult, without 

 assuming a hypopial condition ; but that, whatever be the con- 

 ditions under which they are kept, Hypopi will keep appearing 

 if the species be one where the Bypopus is easy to breed, and if 

 young nymphs be present. 



Another matter which I was careful to watch, was whether 

 both sexes or only one assumed the hypopial form, as it might 

 be possible that, although Claparede was wrong about their being- 

 adult males, they still might be a form confined to one sex. I 

 therefore watched this in cells into which I had put several 

 Hypopi, but not any other Acarina nor ova. I found that the 

 Tyroglyphi which emerged from the hypopial skins were of both 

 sexes ; the female predominated, but not more than it usually 

 does in Acarina. 



What, then, is the reason of this hypopial stage ? It seems to 

 me that it is simply to facilitate the distribution of the species, 

 so that the Acarus may lay hold of any small living object that 

 comes within its reach, and be carried to " fresh fields and pas- 

 tures new," which it could not otherwise reach. It is manifest 

 that, being carried by such creatures as bees, flies, &c., which 



