248 Royal Institulion. 
nips. It was remembered by some farmers that this was the fly . 
which prevailed in the year 1818, and which was followed by the 
caterpillars known by the name of the blacks. The eggs being depo- 
sited by the perfect insect in the leaf of the plant, the black cater- 
pillar or turnip-pest speedily makes its appearance, feeding on the 
soft portions of the leaves of the turnips and leaving the fibres un- 
touched; and finally, casting its black skin and assuming one of a 
more slaty or grey colour, it buries itself in the earth. Lodged there, 
it forms for itself, from the soil, a strong oval cocoon; from which 
some of the earlier broods pass almost immediately into the perfect 
state, filled with ova, and ready quickly to supply another generation 
of destroyers. So compiete and so rapid was the destruction in some 
instances, that a whole field was found, in two or three days, to pre- 
sent only an assemblage of skeletonized leaves; and this too when 
the turnips had attained a considerable size. 
The insect whose proceedings have heen thus briefly noticed, be- 
longs to the Hymenopterous family Tenthredinide ; it is the Athalia 
Centifolie, a species first noticed by Panzer. Mr. Yarrell describes 
the perfect insect and the caterpillar; and then recurs to the damage 
effected by the latter. By their repeated broods the devastation was 
continued for so long a time that even the third sowing did not in all 
cases escape destruction ; and it was not until the occurrence of the 
heavy rains in September, terminating the unusually dry summer, 
that the mischief ceased. The destruction of the leaves caused, in 
most instances, the loss of the root also; and where the leaves suf- 
fered from the attacks of the black caterpillar, but not sufficiently to 
occasion the death of the plant, the turnip itself became pithy and 
of little value. It has become necessary, Mr. Yarrell states, to im- 
port the root largely from the Continent to supply the deficiency of 
the home crop. 
The remedial measures adopted on a former visitation were the 
turning into the infested fields of a large number of ducks, who 
greedily devoured the caterpillars as they were brushed from the 
leaves by a boy with a long pole; the passing of a heavy roller over 
the ground at night, when the caterpillars were at their feed; and 
the strewing of quick lime by broad cast over the fields, renewing 
it as often as it was dispersed by the wind. The latter mode was 
generally considered as the most effectual preservative. 
PROCEEDINGS AT THE YFRIDAY-EVENING MEETINGS OF THE 
ROYAL INSTITUTION. 
Jan. 22, 1836.—Mr. Faraday on silicified plants and fossils, and 
the proposed theories of silicification. 
Jan. 29.—Dr. Ritchie. A view of the differential and integral cal- 
culus. 
Feb. 5.—Mr. Brande on the manufacture of paper-hangings. 
Feb. 12.-Dr. Grant on the structure of fishes, considered with 
reference to their aqueous element. 
Feb. 19.—Mr. Faraday on the magnetism of metals as a general 
character (see page 177). 
