DEVELOPMENT AND IMMATURE STAGES. 153 



nourishment during the whole period of their existence 

 in this stage ; this view, which as far as is at present 

 known is a correct one, he supported in a special 

 paper on the subject.* Megnin stated that the nymph 

 which changed into a Hypopus showed rudiments of 

 the sexual organs, but that the nymph which emerged 

 from the hypopial skin did not, and was one third 

 smaller than the other; he concluded therefore that 

 the change into a Hypopus was a retrogression. 

 Megnin summarises his conclusion by saying that 

 Hypopus is the " cuirassed heteromorphous, adventitious 

 nymph of Tijroglyphus entrusted with the preservation 

 and distribution of the species (under adverse circum- 

 stances)." In one of his later works Megnin stated 

 that Homopus elephantis, Furstenberg, was the same as 

 Acarus spinitarsus, Hermann, and was the hypopial 

 nymph of the common cheese-mite (Tyroglyphus siro). 

 Megnin did not say how the last-named fact, or sup- 

 posed fact, was ascertained ; moreover in his memoir 

 upon Hypopus above quoted he stated that Hermann's 

 A. spinitarsus was the hypopial nymph of his Tyro- 

 glyphus mycophagus. I am not aware that anyone, up 

 to the present time, has succeeded in tracing the 

 hypopial nymph of T. siro in any manner upon which 

 any reliance can be placed. 



In 1872 J. Gr. Tatemf figured and described under 

 the names of Acarellus muscae and A . pulicis two crea- 

 tures which are Hypopi, and the latter of which he 

 thought he took from the abdominal cavity of a dead 

 flea. I have seen one of Tatem's specimens of the flea, 

 but the Hypopi, which are numerous, are not within 

 the abdominal cavity, although they look as if they 

 were ; they are between two segments of the abdomen 

 in the place where the anterior end of the more distal 

 segment passes within the posterior end of the more 



* " Note sur la faculte qu'ont certains Acariens avec ou sans bouche 

 de vivre sans nourriture pendant des phases entieres de leur existence 

 et meme pendant toute leur vie," in ' J. Anat. Physiol.,' t. xii (1876), 

 pp. 603606. 



f In Monthly Micros. J.,' 1872, p. 263, pi. xl. 



