342 SCIENCE FROM AN EASY CHAIR 



about protected by this case. There are not many insects 

 which thus construct portable cases for themselves when in 

 the grub or caterpillar state of life. Such " cases " must 

 not be confused with the very similar " cocoons " by which 

 some moth-grubs surround themselves (as, for instance, 

 the silkworm moth) when their growth is completed, and 

 they become quiescent and hard, and are known as 

 chrysalids. Such " cocoons " are constructed in the same 

 way as the lining of the clothes moth's case, by threads of 

 silk secreted by the caterpillar, but they are made once 

 for all when the grub has ceased activity. The little 

 clothes moth caterpillar, on the other hand, has con- 

 tinually to enlarge its tunic or case as it itself increases 

 in size. There is a hole at the end, from which the head 

 and three legs of the caterpillar emerge, so that it can 

 crawl and feed freely. The outer surface of the case 

 consists of cut lengths of the fibre on which the grub is 

 living, and so is protective in resembling the surrounding 

 material and hiding the minute ravager. It is easy 

 enough for the little grub to add a bit to the case at 

 the end from which its head protrudes, and, being very 

 flexible, it can turn right round in the tube and put its 

 head out at the other end and secrete a bit more there, 

 cementing cut hairs to the outer surface. But in order 

 to increase the breadth of the tube or case, the caterpillar 

 has, from time to time, to undertake a formidable operation. 

 It actually slits up the case lengthwise for about half its 

 extent, and fills in the gaping space with new material ; 

 then it cuts up the opposite face of the same half of the 

 tube, and puts in a new patch there. And after that, it 

 has to treat the remaining half of the tube in the same 

 way, making two more cuts, one opposite the other, and 

 filling in the gap in each case as before. Students of 

 these little creatures have amused themselves by changing 

 the position of the caterpillar and its case, from fur or 



