70 
J. HOPKIHSON-CHEESE-MITES 
preceding each ecdysis, by the last of which the nymph becomes 
the imago. It is in the nymphal stage that the hypopial condition 
comes in, and a brief summary of Mr. Michael’s history of the 
literature relative to the hypopus, as given in his latest work, the 
‘ British Tyroglyphidse,’ may he of interest. 
De Geer, in 1735, noticed on the house-fly some small reddish 
mites which entirely covered the neck and hack of the fly. They 
were usually quite still, hut ran about actively when touched. 
De Geer considered the creature to he an adult; and Linnaeus, 
in 1758, called this supposed species Acarus muscarum, after the 
fly ( Musca domestica ) upon which it was found. 
Passing by the observations of Geoffroy, Hermann, and Schrank, 
which do not materially increase our knowledge of this stage, we 
come to Huges, who in 1834 created the genus Hypopus , at the 
same time throwing out the first hint that the minute mite to 
which he gave this name might he immature, putting a ? as to 
whether the creature was a larva. Dufour in 1839 added two 
supposed species of Hypopus which he found upon beetles and 
flies, and also added a new genus for an allied mite which he found 
upon bees, regarding them as adult creatures parasitic upon the 
insects on which he found them. 
Again passing by the observations of authors who did not add 
materially to our knowledge of the hypopial stage, Koch and 
Gervais, we come to Dujardin, who in 1849 published a memoir 
which, Mr. Michael says, “ was far the most careful study of 
Hypopus made up to that time,” with most accurate drawings. 
Although he advanced an erroneous hypothesis, he was the first 
who “ proved that Hypopus was an immature form of some Acarus 
different in appearance from itself.” He observed the absence of 
mouth-organs, and also of reproductive organs. 
Piirstenberg in 1861 and Clarapede in 1868 published papers on 
the Acari, giving the results of very careful observations on the 
hypopial nymph, hut neither of them recognised the creature as 
such, and it was not until 1873 that the first suggestion was made 
that this was the case. Megnin then took the precaution to breed 
his mites, a species of Tyroglyphus feeding on mushrooms, in 
captivity, and he saw the “hypopus” inside an inert nymph of 
Tyroglyphus , and found that it “turned into a nymph of Tyro- 
glyphusT This, he perceived, occurred when moisture was added, 
and he came to the conclusion that “ Hypopus was a form into 
which the nymphs of Tyroglyphus changed, when, through dryness 
of the atmosphere or other causes, there was a difficulty in their 
continuing to live as Tyroglyphi , and that it was a provision of 
nature to insure the preservation of the species by carrying it over 
periods of drought, etc.” 
Memoirs on Hypopus were published by J. G. Tatem in 1872, 
Andrew Murray in 1876, and Hr. P. Kramer in 1877, hut they did 
not contribute towards the solution of the mystery. In 1880, 
however, G. Haller got even nearer to the truth than Megnin, 
suggesting that “ the hypopial form is a ‘ travelling dress ’ for 
