NOTE ON LEAF-FOLDING CATERPILLARS. 
7 1 
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 begun 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 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 extremely 
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 i ; 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. 
