72 NOTE ON LEAF-FOLDING CATERPILLARS.
T a piece of fine thread; the upper end is carried through a
tiny hole pierced in the blade of the leaf close to one of the ribs R
and is attached to a small piece of fine wire at A; the lower
end of the thread similarly passes downwards through the
surface of the leaf and carries a tiny cardboard pan suspended
at W.
Weights are cautiously added at W until the edge of the leaf
E exactly touches the surface of the leaf resting on the block.
The point of attachment' is A then varied to A1 and the
weights adjusted as before.
Proceeding thus, it was found—
Determination of breaking strain of thread.
Advantage was taken of the fact that the larva, when alarmed,
drops from the leaf by means of a thread. The larva is able to
arrest its descent at will by seizing the thread just above the
head by its front legs ; it ascends by rolling up the thread with
its feet.
It was easy to obtain a suitable thread for experiment by
causing the larva to descend for the desired distance and then
attaching to the extended thread a tiny paper pan by means
of seccotine; the larva was then removed and the pan cautiously
weighted until the thread broke.
Proceeding thus, it was found:—