OE LIFE-HISTOKY OF CERTAIN ACAEINA. 379 



and preserve microscopical slides, mounted during the progress 

 of the change, and showing the Hypopus forming, or formed, inside 

 the Tyroglyphus nymph, which I still possess. This appears to 

 be sufficient to answer Andrew Murray's view that Hypopus is 

 an internal parasite ; because, firstly, the Hypopios is very nearly 

 as large as the Tyroglyphus from which it emerges, filling up the 

 whole interior, which seems bighly improbable with a creature 

 ft^iich cannot grow inside; for ive never see young Rypopi 

 smaller than their fellows either withia the Tyroglyphus or living- 

 free. All the Hypopi of the same sort which are found are about 

 the same size. Secondly, we never, by any chance, see a Hypopus 

 within the larva or within the adult, only within the nymph ; 

 although the difi'erence between larva, nymph, and adult in Tyro- 

 glyphus is very slight. Thirdly, we never see two Hypopi within 

 the same Tyroglyphus, although, when the Hypopus has emerged, 

 we often find numbers ectoparasitic upon the same Gamasus, 

 insect, &c. Fourthly, the emerging of the Hypopi is preceded 

 by an inert period, just as the ecdysis is in most Acarina. 

 Fifthly, the Hypopus, when it emerges after the ecdysis, leaves 

 behind it the cast skin quite clean, and without any torn par- 

 ticles of internal organs adhering to it ; in fact, in the ordinary 

 state of the exuvia of Acarina and insects. Sixthly, the mouth- 

 organs of Hypopus are not in any way fitted to consume the 

 solid tissues of its host. Seventhly, the Hypopus returns to the 

 Tyroglyphus-ioTm after the next ecdysis. 



If these considerations dispose of Andrew Murray's sugges- 

 tion, as I think they do, they also dispose of the view that 

 Hypopus is a distinct creature ; for that could now only be sus-, 

 tained if Murray's view were received ; otherwise the evidence 

 of Claparede, Megnin, Berlese, Macintyre, and myself, who have 

 all actually seen the change, would probably be accepted as 

 sufficient to prove that the form is a stage in the life-history of 

 Tyroglyphus. 



Claparede's view, that Hypopus is the adult male, was practically 

 answered by Robin and Fumose ; and it will, I think, be found 

 below that it is even more effectually disposed of by my own 

 observations in 1882, which would also answer Gervais's idea. 

 There therefore only remain Megnin's and Haller's explanations. 

 My observations decidedly confirmed Megnin's view, that the 

 true Hypopus is a heteromorphous nymphal form of Tyroglyphus, 

 and possibly of some allied, or other, genera. 



LrNN. JOURIf. ZOOLOGY, VOL. XVII. 29 



