Turnip Saw-Fly. 197 



rain drops, and hovered about the turnips in such myriads, that 

 the whole fields were coloured with a rainbowy tinge, when the 

 hot sun shone on the filmy gauzy wings of the flies." Mr. 

 Curtis says, " The fly appears principally in August and Sep- 

 tember ; but I have found them as early as March 29., and as 

 late as the middle of October. I first observed these flies in 

 abundance in a potato field at Battersea, and afterwards in a 

 field near Heron Court : but last year they were distributed over 

 the whole country, after an absence in many places, as I was 

 informed, of upwards of thirty years. They have appeared again 

 this year ; and Mr. R. Taylor and myself, in a botanical excur- 

 sion last August, saw the flies coming out of the ground in 

 myriads, in a ploughed field near Bristol, where potatoes had 

 apparently been grown. The flies do not appear to be attached 

 to any particular plant. Whether the larvae will attack any other 

 than the English turnip, I cannot determine ; but it is a remark- 

 able fact, that they will not destroy the Swedes;" a circumstance 

 also confirmed by Messrs. Yarrell and Saunders, and one which, 

 as it shows the nice distinctions which insects sometimes make 

 in the choice of their food, may be turned to considerable ad- 

 vantage in an agricultural point of view ; the former gentleman 

 {Ent. Trans., vol. i. p. 77.) having observed that the Swedish 

 turnip is not infested, in consequence of containing a greater 

 quantity of oily matter, as well as from the more pungent taste 

 of the leaf, and the stronger taste of the root. 



From the more prevalent appearance of this insect in the 

 eastern and south-eastern parts of England, as well as from its 

 periodical appearance, it has been supposed that it is not an in- 

 digenous species, but that it is brought over from the opposite 

 coast of Europe by wind. " From their more frequently' ap- 

 pearing on the sea coast," observes Mr. Marshall, in the memoir 

 above referred to, " and from the vast quantities which have, I 

 believe, at different times been observed on the beach washed 

 up by the tide, it has been a received opinion among the farmei's, 

 that they are not natives of this country, but come across the 

 ocean ; and observations this year greatly corroborate the idea. 

 Fishermen upon the eastern coast declare that they actually saw 

 them alight in cloud-like flights ; and, from the testimony of many, 

 it seems to be an indisputable fact, that they first made their 

 appearance on the eastern coast; and, moreover, that, on their 

 first being observed, they lay upon and near the cliffs, so thick 

 and so languid, that they might be collected into heaps, lying, 

 it is said, in some places two inches thick. From thence they 

 proceeded into the country ; and, even at the distance of three 

 or four miles from the coast, they were seen in multitudes 

 resembling swarms of bees." 



These facts are, however, so completely analogous to what 



o 3 



