NOTE ON LEAF-FOLDING CATERPILLARS. 71
follow in rapid succession, until a narrow web is formed. The
larva then slightly changes its position and commences to make
a second web in the same way. This in turn is abandoned and
a third is started. In the meantime the edge of the leaf is seen
to have begin to move over, slowly but quite perceptibly. The
larva continues to lay out threads, first in one place, then in an-
other, until the edge of the leaf is firmly secured. The whole
operation takes about two hours; during the whole of this time
there is no evidence of any pulling strain exerted by the larva ;
it spins the threads and occasionally rests; that is all.
The mechanism of the folding process appears to be extrenely
simple: the thread at the moment of formation is liquid, on
exposure to air it at once solidifies and then contracts somewhat,
multiplication of threads at a given point ensures sufficient
strength in the web to overcome the resistance of the leaf, which,
related to the breaking strain of a single thread, is enormously
great—at least 20 to 1; change of position affords opportunity
to one set of threads to dry and contract while a new web is
being formed.
It seems to be of interest to determine the actual force re-
quired to deform a full sized birch leaf expressed in terms of
grammes. The following method was employed—see fig. 1.
B is a block of soft wood on which the leaf L is fastened by
means of pins P P passed through the stalk S. R R are leaf
ribs.