16 THE CATERPILLAR. 



so as rather to merit the name of tertiary jaws. 

 The opposing parts, however, never have any 

 movement toward each other, but all are consoli- 

 dated into a single piece. In caterpillars, the 

 basal portion resembles that of one of the maxil- 

 lae, and bears on either side at the tip a very mi- 

 nute jointed appendage similar in structure to the 

 maxillary palpi, and to be compared with the 

 outer of the two appendages of the maxillae ; they 

 are termed the labial palpi ; between them it bears 

 another appendage, the equivalent of united inner 

 labial palpi, but having a special structure ; it is 

 a conical slender horny tube, serving the purpose 

 of the spinneret of the spider ; for, wherever the 

 caterpillar walks, at least during its early stages, 

 it sways its head constantly from side to side in a 

 manner almost painful to behold, spinning a zig- 

 zag ladder of silk upon the surface on which it 

 treads, by means of which its hold of the swing- 

 ing leaf or twig is made more secure. 



I am thus particular in describing the parts of 

 the head in order to show how vast is the change 

 they undergo in the growth of the insect. The 

 pieces I have described are called in general the 

 mouth-parts, and it is interesting to see with what 

 astonishing compactness they are all massed to- 

 gether in one little spot. In the largest of our 

 butterfly caterpillars, they would not together be 

 larger than a pin' s head. The only other movable 



