DEVELOPMENT AND IMMATURE STAGES. 143 



THE HYPOPIAL NYMPH. 



The hypopial stage is biologically far the most 

 interesting portion of the life-history of the Tyro- 

 glyphidse ; it is also of considerable commercial 

 importance, because it is the existence of this remark- 

 able provision that enables many of the most destructive 

 species to spread themselves almost all over the world 

 as they do. As far as I know there is not anything 

 really similar existing in any group of living creatures 

 outside the Acarina ; even amongst them the stage is 

 entirely, or almost entirely, confined to the Tyro- 

 glyphidaB. It is true that Prof. Berlese suggests that 

 the creature described by me as an adult female under 

 the name of Disparipes bombi, which is one of the 

 Tarsonemidse, is a hypopus, and that the adult female 

 differs slightly from it in having smaller claws on the 

 first leg, etc. This observation of Berlese's I think 

 certainly requires confirmation, because the learned 

 Italian professor does not appear to have based his 

 statement upon specimens bred in confinement and 

 carefully .observed, but upon captured specimens sup- 

 posed to be identical, which is not in my opinion a 

 reliable method; whereas I have carefully bred the 

 creature in confinement through several generations 

 without detecting any such form as Berlese describes ; 

 although such forms may easily be captured in the 

 open, and certainly some of them belong to different 

 species, for the males are different. I hope some day 

 to find time to repeat my experiments specially with a 

 view to this point; in the meantime I do not think 

 that it can be assumed that this is a hypopial form. 

 The only other case which I am aware of where it has 

 been suggested that a hypopial stage exists outside the 

 Tyroglyphidae is in certain of the Sarcoptidae which in 

 the mature stage are found on the feathers and exterior 

 of birds, but which in the immature stages live inside 

 the shafts of the quills, or in the interior of the birds. 



