CH. XVI. J THE CATERPILLAR. 273 



scarcely be attributed to the degree of contraction 

 produced in these little threads by the process of 

 drying ; for when fresh spun and moist they are so 

 extremely short, that such an effect could scarcely 

 arise from this cause. 



If one of these cords be attentively examined, 

 instead of being made up of parallel rows of threads, 

 it will be found to be composed of two sets which 

 cross each other, or decussate. Various reasons 



have been assigned for this method of proceeding ; 

 the following appears the most satisfactory. The 

 second set of threads can only be carried across the 

 first by the head of the insect: to effect this its 

 body must rest upon the first fasciculus ; and con- 

 sequently the leaf will be drawn forward by its 

 whole weight. By this means the second set of 

 threads alone act upon the leaf, the first set being 

 visibly relaxed. Each of these sets of threads is 

 singly equal to resist the elasticity of the curved 

 leaf, being indeed strong enough to bear the whole 

 weight of the caterpillar. De Geer, attending to 

 the operations of a species of this kind of caterpillar, 

 observed, that at each new thread it spun, the edges 

 of the leaf gradually approached each other, and 

 were bent more and more as the caterpillar spun 

 new threads : when the last spun thread became 

 tight, that which preceded it appeared loose and 

 floating in the air. To effect this, the caterpillar, 

 after it has fixed a thread to the two edges of the 

 leaf, and before it spins another, draws it towards 

 itself by the hooks of its feet, and by these means 

 bends the leaf: it then spins another thread, to main- 



