Observations on the TurnijJ Saic-Fly. 



the same fate as the Enpflish varieties, or were even exclusively 

 attacked. At first this led to a belief that the leaves of this 

 variety of Brassica campestris, containing a greater portion of 

 oily matter^ and being more pungent to the taste, than B. Rapa, 

 they were altogether secure from the black caterpillar ; this how- 

 ever in the sequel proved unfortunately a decided fallacy : yet 

 how interesting, and perhaps beneficial, would it be to reconcile 

 these anomalies, if we had sufficient recorded facts to assist in 

 such an undertaking ! There seem to be fair grounds for believ- 

 ing that the saw-fly does give a preference to the English turnip, 

 but rather than be disappointed, she will deposit her eggs in the 

 leaves of the swedes, and it is possible that they afford the larvae 

 more wholesome food in wet w^eather : it would be easy to enlarge 

 upon like inquiries, but, as we have no means of answering them, 

 it seems idle to do so. The caterpillars appear to be naturally 

 fastidious, for when feeding on an old leaf they do not relish a 

 younger one, being clearly partial to that which gave them birth, 

 the juices I conceive being more congenial to their constitutions, 

 especially in their early stages, which may account for the silken 

 thread with which they are then provided to guide them back to 

 their native spot ; when half grown they spare none of the older 

 leaves oi full-grown turnips ; indeed their instinct shows that the 

 outer rough leaves are best suited to their wants, for the eggs 

 being laid in them they first fall a sacrifice, whilst the central 

 ones nourish the plant, daily becoming more developed, and con- 

 sequently better adapted to their increasing and inordinate 

 appetites. The charlock (Sinajns arvensis) is decidedly their 

 favourite food, for they always attack that first, and will feed upon 

 the flowers as well as the leaves. The period of the caterpillar 

 and eating state is about three weeks, during which an indi- 

 vidual will consume a very great quantity of food, but how many 

 times its own weighty when fully grown, has not been calculated 

 at this time they are exceedingly voracious, and of course most to 

 be dreaded. The caterpillars are generally discovered under the 

 leaves when the plants are about three weeks old, and they daily 

 increase in numbers from the successive hatching of the eggs, so 

 that they vary greatly in size and colour in a short space of 

 time, and, as the swarms of flies pass in a body from one spot 

 to another, the larvae do not appear simultaneously; indeed they 

 may be seen full grown in one field, whilst in a distant locality 

 the saw^-fly may be depositing its egg, but this arises from another 

 cause — the numerous broods that hatch in one year, for the larvoe 

 arrive so soon at maturity, that a second brood of flies is produced 

 in July and August, whose eggs are deposited forthwith, and thus 



vol. xiii. p. 617) ; and in the Entom. Mag., vol. iii. p. 340, Mr. Newman 

 says, "It was remarkable that at Godalmmg the swedes were untouched." 



