1898.] 157 



Var. a. — mauritaniea. 

 Pubescence of tlie head and thorax, from certain standpoints, concolorous with 

 the humeral tuft, fringe of the alula and the other hair that usually matches these 

 at the base of the wing, and with the dorsal hair of the Ist abdominal segment, the 

 colour being very light brown-ochre, approaching flaxen ; shifted and viewed from 

 above, the pubescence of the metanotum, and of most of the anterior part of the 

 thorax becomes tinged with light bistre-brown, the humeral tuft, &c., keeping their 

 original colour. In the wings, from the radius to the pobrachial, and sometimes to 

 a much smaller extent on the axillar nervure, the bristling hairs tend to constitute 

 a dark spot at the end of each rank. The legs (correspondingly posed) display a 

 more silvery gloss than those of specimens of the normal British form above de- 

 scribed ; but the difference is probably attributable to the employment of a minimum 

 charge of fluid ammonia (instead of a small lump of potassium cyanide), in the 

 collecting-bottle, and to the timely expulsion of the vapour directly its work was 

 done. Length of wing, 1"75 to 2"5 mm. 



Var. /3. 

 Wings, from many standpoints, similar to those of Ps. phalcenoides (2), 

 especially when the humeral tuft and fringes are posed so as to exhibit a uniform 

 whitish or a dark grey tint ; but, from certain positions, the humeral tuft is whitish, 

 and the fringes light grey, or the former whitey brown-ochreous, and the latter con- 

 colorous with the disc, or partly dark grey and partly light glossed, the tints readily 

 shifting and interchangeable. But, by suitable posing, the colour of the wings can 

 be made to approach that of the typical form in most particulars, although (owing 

 to the fineness of the hair) the blue reflection of the membrane is more distinct, and 

 the bristling hair less apparent. In the colouring of the thoracic pubescence, var a 

 is approached, but rather as if it were faded ; and the darker tints induced, by 

 change of pose, are more greyish than bistrous, nowhere approaching light ferrugin- 

 ous-brown. Length of wing, 1'25 to 1"5 mm. 



Haliday's description, and that in the volume o£ this Magazine 

 for 1893, loc. c^Y., apply to what is here assumed to be the typical form 

 in prime condition. This has been bred from putrid snails* by Mr. 

 Yerrall ; his specimens (12 $ , Lewes, 1885) have the light ferrugi- 

 nous-brown colour well preserved in the parts above specified. Gim- 

 merthal, Arb. d. Naturf. Yer. zu Eiga, i, 326 (1848), recorded the 

 breeding of the fly from larvae found in rotten potatoes — a fact 

 referred to by Zetterstedt, Dipt. Scand., xii, 4888 (1855), who, like 

 other continental authors, applies the term albus or its equivalent to 

 the humeral tuft. The form described as var. a is the Algerian race ; 

 localities will be recorded in a separate supplementary article. Var. /3 

 is found by streams in Somersetshire ; the specimens described, 

 captured with others that have been distributed, are 1 ? (Compton 

 Pauncefoot, 1, viii, 1892) and 1 ,^ (Beer's Plantation, Eedlynch Park, 



* Refer also to Wostwood (1840), cited above in the bibliography of this species. 



