THE ACAEINA OR MITES. 



Ill 



the scarcity of food were necessary causes. But Michael has shown that hopopi are 

 developed in the absence of these conditions, and that the hypopus is the natural 

 and normal means of distributing the species. There are two t\'pes of hypopus: most 

 of them are provided with a plate of sucking disks fitting them to cHng to the smooth 

 surface of an insect. A few, however, called the " homopus " type, do not have the 



fWm\^ 



Fig. 230.— Beak of Histiostoma 

 americana. (Author's illus- 

 tration.) 



Fig. 231.— Leg I, and tip of mandible of Histio- 

 stoma americana. (Author's illustration.) 



plate of sucking disks. They are fitted for clinging to the hairs of small mammals, and 

 for this piu-pose there is a longitudinal groove on the ventral plate, ajid each side of the 

 groove a broad, raised, striated surface that may be pressed against a hair lying in the 

 groove. In the early days of acarology the relation of the hypopus was unknown, 

 and Hypopus stood for a separate genus, allied more to Gamasus than to Tyroglyphus. 

 The history of the discovery of its relationship is replete with interesting details. 

 Duges in 1834 wondered if Hypopus was not 

 a larva. In 1844 Gerv^ais placed Hypopus 

 as a section or subgenus of Tyroglyphus, but 

 not doubting they were mature creatures. 

 Dujardin in 1847, after a close examina- 

 tion of the mite, decided that Hypopus 

 was an immature migratorial form of Gama- 

 sus. In 1868 Claparede bred various tyro- 

 glyphids and noticed that certain nymphs 

 when they transformed did not produce 

 the adult Tyroglyphus, but a Hypopus. 

 His observations were correct, but his con- 

 clusion that Hypopus was the male of 

 Tyroglyphus was erroneous. About the 

 same time Robin and Fumose published 

 a paper' in which they described the true 

 male of Tyroglyphus, thus showing that 

 Hypopus was not the male Tyroglyphus. 

 In 1873 Megnin published a famous work 

 on the life history of Tyroglyphus rostro- 



(Author's 



Fig. 232. — Ghjciphagus obesus. 

 illustration.) 



serratus Megnin, in which he proved that Hypopus was a nymphal form oi Tyro- 

 glyphus, due, he supposed, to the dryness of the atmosphere or the lack of food. 

 Murray in 1876, in investigating the subject, came to the conclusion that Hypopus 

 was a ferocious parasite which devoured the entire internal anatomy of its victim 

 and then left the cast skin in search of fresh prey. Haller in 1880 accepted Megnin's 

 interpretation, in considering Hypopus the "traveling dress" of Tyroglyphus. In 

 1884 Michael went carefully over the whole ground, confirming the facts of Megnin, 



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