OR LTrE-HISTOBT OP CERTAIN AC4RIlsrA. 383 



deliglit in hot sunshine, a thing which kills most Acari, the 

 hypopial condition, which enables this creature to endure greater 

 heat and absence of moisture than the ordinary Tyroglyphus-ioYva. 

 can survive, must be of advantage to it. 



It may be worth consideration whether the travelling of 

 Hypopi may not be the explanation of Tyroglyphidse appearing 

 suddenly in places where they have not appeared before, and 

 where their presence is not desired or its cause understood, a 

 matter very troublesome occasionally in a household. A fly or 

 a bee would not be suspected, and may convey a few small, 

 almost invisible Hypopi, which would soon become adult, and 

 then multiply with great rapidity. Another instance may be 

 the frequent appearance of mites {Tyroglyphus entomologicus) in 

 collections of insects ; two or three specimens, if they became 

 adult, would soon produce a quantity of Tyroglyphi, which 

 would greatly injure the entomological preparations, and these 

 two or three specimens might often be adhering to the insects in 

 the collection when introduced, and pass unnoticed, being con- 

 cealed by hairs &c. Hypo^pi often remain a long time in that 

 condition ; I have had them over three months. 



It now remains to consider Dufour's Trichodactylus. It has 

 been mentioned above that, n 1839, this naturalist instituted a 

 genus, thus named, for a creature resembling IIypop)us, which he 

 found on bees of the genus Osmia. Dufour's figure and descrip- 

 tion show the regular Sypopus rostrum and the first three pairs 

 of legs, similar to each other, somewhat long, of nearly equal 

 thickness throughout, and terminated by a long, slightly curved, 

 double claw. The fourth pair of legs are clawless, and terminate 

 in long setse in the true Sypopus fashion. Dufour's figure and 

 description are rather slight. 



In 1875 Prof. A. L. Donuadieu published * an elaborate and 

 careful treatise upon this species (as he considered) and on an 

 Acarid found by him upon Xylocopa violacea, which he names 

 Trichodactylus Xylocopice. These creatures have the fourth leg 

 terminated by one or three very long setse, according to species, 

 in the Hypopus manner, like Dufour's Trichodactylus. In Donna- 

 dieu's specimens, however, the first leg is much thicker than the 

 others, and is terminated by a very large and remarkable single 

 claw, apparently formed for holding hairs, and greatly resembling 



* " Recherches anatomique et zoologique sur le genre Trichodactyle," 

 Ann. Sci. Nat. 5^ ser. Zool. t. x. pp. 69-85. 



