Observations on the Turnip Saw- Fly. 



367 



instinct and favourable circumstances. Indeed in some instances 

 there cannot be a doubt of it ;* and probably the stocks of our 

 innumerable common species are occasionally augmented by the 

 arrival of their cousins-german. The sudden disappearance of 

 certain insects is only a proof that our climate is not suited to 

 their habits and constitution for any long period. But to return. 



Like the Cicada septendecim of America^ the appearance of the 

 turnip saw-fly has been supposed to occur about every seventeenth 

 year ; but this is not correct, for their visits have been so irregular 

 that nothing can be determined from the data before us. The 

 earliest record of their appearance was in 1756. Then we have 

 notices of their being observed in 1760, and perhaps two years 

 after, and again in 1782, 1806, 1818, 1833, 1835, 1836, 1837, 

 and 1838, leaving intervals of three, twenty-one, twenty-three, 

 eleven, fourteen, and one year. Probably they escaped notice in 

 1834 ; and, if such were the case, they were ravaging our turnip- 

 crops for five or six successive years ; and it is far from improbable 

 that the fly may be found every year in small quantities, and that 

 the recorded dates are merely the periods when their ravages 

 called the attention of the country to the subject. 



I believe its effects were severely felt in 1760, and in 1782 

 many thousands of acres of turnips were entirely destroyed in 

 Norfolk, and Mr. Marshall thought it probable that two-thirds 

 of the turnip-grounds had to be ploughed up and re-sown ; and, 

 from the farmers not being thoroughly acquainted with the eco- 

 nomy of the insects, they allowed those plants to remain in the 

 fields which had escaped by being under the hedges and trees, by 

 which means the second crop was not unfrequently lost, as the tur- 

 nips left, supported the caterpillars until the fresh crop came up., 



I do not find any account of the extent of the mischief in 1806, 

 but in 1818, which was a very dry summer, they were in great 

 numbers. 



In 1833, Mr. Newport says, the fly appeared in very large 

 flights on the turnips at Meon Stoke, Hants, and nearly through- 

 out that part of the country ;t possibly they received some check 



* Numbers of a large and beautiful moth, called Daphnis Nerii, figured 

 in Curtis's Brit. Ent, pi. 626, were several years migrating from Africa to 

 the north of France, and at last reached England, where that insect had pro- 

 bably never been seen alive before. Moths unknown as inhabitants of these 

 islands have been caught at night at the North Lowestoft lighthouse, to 

 which common species are attracted in such multitudes that the attendant, 

 I have been informed by Captain Chawner and Mr. C. J. Paget, is obliged 

 to take a broom in humid summer nights and brush them off, on account of 

 their obscuring the revolving light. 



t Observations, &c., on the Saw-fly of the Turnip, by George New- 

 port, Esq. 



