378 



Observations on the Turnip Saw- Fly. 



a third brood is feeding in congenial seasons until the middle or 

 end of October, which is frequently a warm and dry month, but, 

 should hoar-frosts set in at this declining period, multitudes will 

 be seen perishing on the leaves and ground. A low temperature 

 generally arrests their progress, but as soon as it becomes again 

 mild, all flattering hopes vanish with the suspended growth of the 

 bulb ; it pines away from the loss of the leaves, which are the 

 lungs of the plant, and, even should it survive their attacks, it never 

 can arrive at its full size. 



Mr. Marshall states that in about ten days after the arrival of 

 the saw-flies the young caterpillars were visible beneath the leaves, 

 and in about ten days more the plants were entirely eaten up, ex- 

 cepting a small patch or two towards the centre of the field, and a 

 space round it by the side of the hedge, proportioned to its 

 height, and varying in this respect where trees occurred : this was 

 accurately ascertained and is a very curious fact, for in small 

 pieces called pightles, set round with high trees, the plants had 

 almost entirely escaped; and, as might be expected from this 

 evidence, large open fields and smaller enclosures lying exposed 

 to the sea suffered most, and lands dipping from the sea were 

 less affected. It seems probable that the shade produced by the 

 trees and hedges, or the moisture under them, would not prove 

 favourable to the hatching of the eggs, for, as soon as the other 

 parts of the field w^ere consumed by the caterpillars, they pro- 

 ceeded to devour the space on one side, and then ''travelled with 

 wonderful instinct in bodies towards the other." The whole field 

 being finished, the gateway and the adjoining roads had, it was 

 said with great confidence, been seen black with them. 



In Mr. Newport's Prize Essay we find that, when he inspected 

 a field in Hants, of healthy white turnips of 15 or 20 acres, 

 which had been sown about a month, the saw-flies swarmed over 

 about an acre at one end. They seemed to have arrived very 

 recently in a swarm or cloud, for they had not been observed 

 there before, and were hourly increasing. But the remarkable 

 fact was, that the great mass of the flies was confined almost en- 

 tirely to the eastern end of the field at first, while there was 

 scarcely a fly to be met with in the middle or at the v/estern end ; 

 now it appears there was a light westerly wind at the time, 

 and the saw-flies had come in an opposite direction, confirming 

 the opinion, given in the Report of the Turnip-fly, that insects, 

 being directed by scent, frequently fly against the wind. In four 

 days they had passed over in a body to the western end, depositing 

 eggs in their progress, from whence they would proceed to other 

 fields hitherto free, if the ovaries of the females were not ex- 

 hausted : Mr. Newport had observed in another instance that 

 they came from the east. 



