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370 0. GORDON HEWITT. 



the dorsal side was convex and the ventral side flat. Linnaeus 

 (1758) called this mite Acarus muscarum from de Geer's 

 description, and Geoffroy (1764) found what appears to be 

 the same, or an allied species of, mite, which he called the 

 "brown fly-mite." Murray (1877) describes a form, Tro ru- 

 bidium parasiticum, 1 which is a minute blood-red mite 

 parasitic on the house-fly. He says : " In this country they 

 do not seem so prevalent, but Mr. Eiley mentions that in 

 North America, in some seasons, scarcely a fly can be caught 

 that is not infested with a number of them clinging tenaciously 

 round the base of the wings. " As it only possessed six legs 

 it was doubtless a larval form. 



Anyone who has collected Diptera as they have emerged 

 from such breeding-places as hot-beds, rubbish and manure 

 heaps will have noticed the frequently large number of these 

 insects which are to be found carrying immature forms of the 

 Acari. These are being transported merely by the flies in 

 the ma.jority of cases. Mr. Michael tells me that he used to 

 call such flies "the emigrant waggons" a very descriptive 

 term. Many of these mites belong to the group Gamasidaa 

 the super-family Gamasoidea of Banks (1905). These mites 

 have usually a hard coriaceous integument. In shape they are 

 flat and broad and have rather stout legs. Sometimes imma- 

 ture forms of these mites swarm on flies emerging from rubbish 

 heaps. Banks holds the opinion that they are not parasitic, 

 but that the insect is only used as a means of transportation. 

 It is difficult to decide whether this is so in all cases. I have 

 illustrated, (fig. 14) a specimen of the small house-fly, H. 

 canicularis, caught in a room; on the under-side of the fly's 

 abdomen a number of immature Gamasids 2 are attached, 



1 This species was named Atoma parasiticum and later Astoma 

 parasiticum by Latreille (' Magazin Encyclopedique/ vol. iv, p. 15, 

 1795). Mr. A. D. Michael tells me that the genus was founded on 

 Trombidium parasiticum of de Geer. They were really larval 

 Trornbidiidse and Atoma was founded on larval characters ; probably 

 any larval Trombidium came under the specific name. 



2 Being unable to identify these immature specimens I submitted 

 them to Mr. Michael, who kindly informs me that it is extremely dim- 



