26o ARACHNOIDEA. 



CASE potatoes shut up in a barn of the annex to the imperial farm at 

 ^^^' Vincennes. What was most remarkable was the immense 

 quantity of them that were developed in less than eight days. 

 The soil of the ground floor, where the potatoes had been placed, 

 was covered with a bed of this little Tyroglyphus, which looked 

 like an animated dust of a grey colour. A sample of this living 

 dust, which M. Guerin-Meneville had collected, was composed 

 of myriads of millions of these little animals, and was swept to- 

 gether in some minutes and simply by the help of the feather of 

 a pen. This mass of beings was composed of a reunion of indi- 

 viduals of all different ages and degrees of development 



This innumerable assembly had attracted, as is always the case, 

 many other little hunting insects, who had found there an 

 abundant and easy prey. There were larvae and perfect insects 

 belonging to several genera, Coleoptera, Hemiptera, Hymenoptera 

 Diptera, etc., on which these mites had attached themselves in 

 commensurable quantities, literally covering them and giving them 

 a most singular appearance. 



The potatoes, which looked quite sound and healthy, were all 

 covered with these Acarids. As these latter could no longer all 

 hold on to the surface, they accumulated between the paving- 

 stones, thence even on to the pavement, where they formed a 

 bed of several millimetres in thickness on a surface of about four 

 square metres, 



Rhizoglyphus rostro-serratus (Tyroglyphus rostro-serratus, Megnin, 

 Tourn. Anat. Phys. 1873; Acarus fungoram (?) jS'm^. Ent. Hort. 1867, 

 p. 90^. 



This is the species on which M. Megnin made the observations 

 on the development of Hypopus, which we have above recorded. 

 We have already mentioned that around Paris the cultivated 

 mushrooms, more especially the common Agaricus campestris, are 

 often attacked by a moist black rot, which, until lately, had been 

 regarded as spontaneous. M. Megnin has shown that it is caused. 



