236 



MOTIONS OF INSECTS. 



one upon your hand, you will find that they draw a thread 

 as they go ; when they move, their head is extended as far 

 as they can reach with it ; then fastening their thread there, 

 and bringing up the rest of their body, they take another 

 step ; never moving without leaving this clue behind them ; 

 the object of which, however, is neither to measure, nor to 

 mark its path that it may find it again ; but thus, whenever 

 the caterpillar falls or would descend from a leaf, it has a 

 cord always ready to support it in the air, by lengthening 

 which it can with ease reach the ground. Thus it can drop 

 itself without danger from the summit of the most lofty trees, 

 and ascend again by the same road. As the silky matter is 

 fluid when it issues from the spinners, it should seem as if 

 the weight of the insect would be too great, and its descent 

 too rapid, so as to cause it to fall with violence upon the 

 earth. The little animal knows how to prevent such an 

 accident, by descending gradually. It drops itself a foot or 

 half a foot, or even less, at a time ; then making a longer or 

 shorter pause, as best suits it, it reaches the ground at last 

 without a shock. From hence it appears that these larvas 

 have power to contract the orifice of the spinners, so as that 

 no more of the silky gum shall issue from it ; and to relax it 

 again when they intend to resume their motion downwards : 

 consequently there must be a muscular apparatus to enable 

 them to effect this, or at least a kind of sphincter, which, 

 pressing the silk, can prevent its exit. From hence also it 

 appears that the gummy fluid which forms the thread must 

 have gained a degree of consistence even before it leaves the 

 spinner, since as soon as it emerges it can support the weight 

 of the caterpillar. In ascending, the animal seizes the thread 

 with its jaws as high as it can reach it ; and then elevating 

 that part of the back that corresponds with the six perfect 

 legs, till these legs become higher than the head, with one of 

 the last pair it catches the thread ; from this the other 

 receives it, and so a step is gained : and thus it proceeds till 

 it has ascended to the point it wishes to reach. At this time 

 if taken it will be found to have a packet of thread, from 

 which, however, it soon disengages itself, between the two 



