STRUCTURE OF INSECTS. 283 



let themselves down to the ground. They continue, 

 while walking, to spin a thread as they advance, so 

 that they can always easily retrace their steps, by 

 gathering up the clue they have left, and reascend 

 to the height from which they had allowed them- 

 selves to drop. 



§ 6. Imago, or Perfect Insect. 



The process which nature has followed in the de- 

 velopement of the structure of insects, has for its 

 object the gradual hardening and consolidation of 

 texture, and the union and concentration of organs: 

 for we find that the segments which were at a dis- 

 tance from one another in the larva, are approxi- 

 mated in the perfect insect, and often closely tied 

 together by ligaments; and in other cases, adjoin- 

 ing segments cohere so as to form but a single 

 piece. Thus the number of separately moveable 

 parts composing the solid fabric is considerably 

 diminished. Other segments, again, fold inwardly, 

 forming internal processes, and adding to the extent 

 and complication of the skeleton. 



The integuments of perfect insects, being de- 

 signed to be permanent structures, are thicker and 

 more rigid than those of their larvae, and are formed 

 of several layers, in which the component parts of 

 the integuments of the larger animals may readily 

 be distinguished. Their rigidity does not, like that 

 of shells, arise from the presence of carbonate of 

 lime ; for they contain but a small proportion of 

 this material : and whenever a calcareous ingredient 

 enters into their composition, it is in the form of 



