376 



Ohservations on the Turnip Saw- Fly. 



the alarm is over ; but when farther advanced in life they lose 

 this power, and are consequently obliged to crawl up the stem 

 until they reach a leaf; after this period they fall on the slightest 

 touch, and lie curled up as if dead until their apprehensions of 

 danger have subsided. 



They are not long, after assuming the slate colour, before they 

 descend from the leaves and enter the earth, and are sometimes 

 two hours and a half engaged in burying themselves 1 or 2 inches 

 below the surface, where they form an oval silvery cocoon of 

 silken threads and gluten,* more or less brown outside (fig. 3), 

 but of a beautiful smooth silvery texture within : it is impervious 

 to wet, and its glutinous nature v/hen first spun causes it to ad- 

 here to the particles of earth and sand in which it may be im- 

 bedded, so that it appears like a small lump of earth, and is not 

 easily detected ; but when formed in a box it partakes of its 

 colour on the outside, and requires a knife to detach it, and then 

 it cannot be well separated without making a hole in the case. 

 Some of the early broods pass very rapidly into the perfect 

 insects, three weeks being sufficient, and the females were found 

 already to be full of eggs ; but later in the season it is three 

 months before they change to pupse, and, in order to secure a 

 succession the following season, probably one-third remain in the 

 cocoons in the caterpillar state through the winter (fig. 4) : 

 eventually, however, the caterpillar casts its skin in the cocoon, 

 and becomes a whitish-yellow nympha or pupa, the limbs and 

 figure of the adult fly being distinctly visible through the thin 

 skin (fig. 5). 



Having traced their progress and economy from their first 

 appearance as flies to the pupa or chrysalis, the next object for 

 consideration will be the remedies ; but before we enter upon 

 this subject there are some facts that we have collected in our 

 investigations which deserve especial notice ; one of which is, that 

 light soils seem to be most attractive ; for instance, the coast of 

 Norfolk, where the saw-flies were first observed in such multi- 

 tudes, is exceedingly sandy ; Mr. Yarrell says they affect light 

 and chalky soils : in another district the sharpest gravel was in- 

 fested the most ; and where it was a sandy loam upon mountain 

 limestone they proved very destructive, and attacked the crops in 

 patches ;t but the most inexplicable trait in their economy is, 

 that, whilst in some places and in some seasons the caterpillars 

 refused to eat the Swedish turnips, J in other instances they shared 



* This substance is said by some to be an exudation from its body, but I 

 believe it to be spun like the cocoon of the silkworm from its mouth, 

 f Royal Agric. Journ., vol. i. p. 417. 



X I myself so invariably witnessed this preference, that I unhesitatingly 

 Slated in 1836 "they would not touch the swedes" (Curtis's Brit. Ent., 



