68 The Scottish Naturalist. 



of lines, prettily marked with rows of hair-bearing points ; ab- 

 domen narrowish ovate, the tip finely tapered, shining tes- 

 taceous, with some duskier tints, and the base sub-cinereous ; 

 legs as in the male ; wings more ample than in the male ; the 

 second cross nervure slighly crooked ; length 2^; expanse of 

 the wings 5^ — 6 lines. 



A small variety of the male, which came out during the 

 spring, differs in having the antennae and palpi entirely black ; 

 the thorax darker, the abdomen shorter, of a faint testaceous 

 hue and less shining ; the fore thighs dusky, not so bright slaty ; 

 the wings shorter, the second cross nervure straight, and not 

 so far apart from the first ; length 1 % ; expanse of the wings 

 4 lines. This dwarf condition may have arisen from a de- 

 ficiency of nourishment during the maggot state. •. 



Meigen ascribes to A. mitis a black frontal band, ferruginous 

 palpi and black antennae ; but out of the large number that I 

 have reared, not one corresponds to this character, excepting 

 the last variety in one particular. An English specimen, how- 

 ever, taken in the open air, agrees with Meigen's characters, 

 the reared specimens appear thus to constitute a distinct variety, 

 which may be named A. mitis, var. rumicis. 



Mr Walker in his " List of specimens of Dipterous Insects in 

 the British Museum" (1849) p. 922, has doubtingly united A. 

 mitis with A. bicolor, but having reared both species from 

 different plants, and with evident tokens of dissimilarity, I cannot 

 subscribe to his opinion. 



A. mitis is infested with an Ichneumonideous parasite, while 

 in the pupa state ; and the same species attacks A. bicolor. 



This species also, at least judging from the maggot and pupa 

 case, occasionally mines in the leaves of the Coltsfoot ( Tussila- 

 go farfard). I have also found the maggot of a Trypeta feeding 

 upon them, but did not succeed in rearing the fly of either. 



POLARITY IN THE GEOLOGICAL DISTRIBUTION OF GENERA 



AND ITS CAUSE. 



By the Rev. J. WAR DROP. 



A CCORDING to this generalisation, the greatest numbers 

 of organic forms generically different are found at the 



