Ohservaticnis on the Turnip Saw-Fly. 



379 



It is well known to all practical entomologists that caterpillars 

 are attacked with purging when fed upon wet leaveS;, and this 

 disease makes sad ravages amongst the " silkworms/' if proper 

 precautions be not taken in rearing them. Rains are conse- 

 quently singularly destructive to the black caterpillars, by render- 

 ing the turnip-leaves very watery, which speedily kills them ; the 

 caterpillars can undoubtedly resist a shower, yet, if they be 

 brushed off in wet, cold weather, after casting their first skins, I 

 doubt if they would recover ; and on the contrary, if the earth be 

 very dry, I expect they would not regain their position without 

 difficulty, especially on sandy soils, over which the}' travel very 

 indifferently, and for this reason probably, they migrate in troops 

 at night when the earth is moistened with dew ; but these are 

 merely hints thrown out for the more mature consideration of 

 those most interested in the subject. There are, however, many 

 other causes in operation to decrease this formidable enemy, which 

 probably might be taken advantage of; for instance, if a cater- 

 pillar be removed after it has fixed its fleshy feet to any substance 

 in order to cast its skin, it has not the power to attach itself a 

 second time, consequently it cannot disengage itself from the old 

 skin, and it dies : this operation takes place every six or seven 

 days, as already shown, and such would be the best periods for 

 disturbing them : when the caterpillars are preparing for this 

 moult they become unsettled and will not eat, but as soon as they 

 have cast off their skins all their vigour returns with redoubled 

 force, and they are more ravenous than before. When they have 

 overcome the exertions of their final moult, if the earth be very 

 dry it is greatly against them, for the larvae do not like to enter it, 

 I have repeatedly observed, as they do when it is moist. Pro- 

 bably they are obliged to bury them^selves deeper under such a 

 condition of the soil, until they arrive at damp earth, which will 

 allow them to form their cells ; they are also longer burying them- 

 selves when the earth is dry, and I believe descend close to the 

 bulb of the turnip as the most protected from drought, and this 

 operation requires several hours when they are in any way en- 

 feebled^ but is otherwise speedily accomplished. 



Rooks as usual are eminently serviceable in diminishing the 

 caterpillars, and Mr. Marshall observed that ''a large piece of 

 turnips lying in an open field had escaped in a remarkable 

 manner 3 it lay near a rookery, which was a general rendezvous 

 for these birds ; and I recollect " (he adds) to have seen this 

 piece more than once covered with them." The swallows also 

 soon become attracted by the flies, and are constantly skimming 

 over the fields in pursuit of them ; and when it is remembered 

 that the capture of one female may prove the destruction of a very 

 extensive brood of the caterpillars, the benefit conferred by a 



