168 TENTHREDINIDJE ; TURNIP-FLY. 



change into the pupa state, they spin a cocoon, either in the 

 earth or on the plants on which they have fed ; but they do not 

 become pupse, until they have been inclosed in this for many 

 months, and only a few days before they come forth as perfect 

 Saw-flies. To this group belongs the Athalia centifolice, or 

 Turnip-fly, which occasionally appears in this country in such 

 vast numbers as to produce the greatest devastation. The larva 

 is twenty-two-footed, and of a greenish-black colour ; whence it 

 is commonly known by the appellation of the nigger, or black 

 caterpillar of the Turnip, to which plant it is chiefly detrimen- 

 tal, by devouring the leaves, and thus totally destroying the 

 crop in an incredibly short space of time. It was especially 

 abundant in the south-eastern counties of England, in the years 

 1835, 1836, and 1837. The appearance of the black larvse is 

 preceded by that of the imago, a pretty yellow and black 

 insect, which is first seen hovering over the turnip-fields about 

 the middle of May or the beginning of June ; it deposits its eggs 

 in the soft tissue of the leaf, puncturing the cuticle by its ovipo- 

 sitor ; and these are hatched in five or six days. In a few days 

 more, a whole field has been often devastated by the voracity of 

 the larvsa, which devour the soft tissue of the leaves, leaving only 

 their skeletons and stalks. The most effectual remedy for these 

 attacks was found to be the introduction of Ducks into the fields, 

 by which the plants were cleared of the larvas more effectually 

 than they could be by any other means. Many other species 

 exist, however, almost equally injurious to different tribes of 

 plants. Thus the Gooseberry is subject to the attacks of a 

 Tenthredo, of which the larvse often amounting to as many as 

 one thousand upon a single tree devour its leaves at the begin- 

 ning of summer. The Apple, again, suffers from the deposition 

 of the eggs of another species in its fruit. And the Willow is 

 subject to the attacks of many species, some of which devour its 

 leaves, whilst others cause the production of galls by perforating 

 its branches. The perfect Insects of this group are of moderate 

 size, not exceeding an inch in length ; some of them, however, 

 are among the largest Hymenoptera inhabiting this country. 

 Their flight is usually heavy, and is attended in the larger spe- 

 cies with a humming noise ; they seem, however, to be more 



