TREATMENT OF LARViE. 225 



tinous secretion, which, on heing deposited on glass, adheres 

 firmly to it, and very soon acquires consistence and hard- 

 ness by the action of the air. 



Other caterpillars which feed on trees, and have often oc- 

 casion to descend from one branch to another, send out a 

 rope made with the same material, which they can prolong 

 indefinitely; and thus either suspend themselves at pleasure 

 in the air, or 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 clew they have left, and reascend to the height from 

 which they have allowed themselves to drop. 



§ 6. Imago, or Perfect Insect, 



The process which nature has followed in the develop- 

 ment of the structure of insects, has for its object the gra- 

 dual hardening and consolidation of texture, and the union 

 and concentration of organs; for we find that the segments 

 which were at a distance from one another in the larva, are 

 approximated in the perfect insect, and often closely tied to- 

 gether by ligaments: and in other cases, adjoining segments 

 cohere so as to form but a single piece. Thus, the number of 

 separate parts composing the solid fabric is considerably di- 

 minished. Other segments, again, fold inwardly, forming 

 internal processes, and adding to the extent and complica- 

 tion of the skeleton. 



The integuments of perfect insects, being designed 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 ani- 

 mals may readily be distinguished. Their rigidity does 

 not, like that of shells, arise from the presence of carbo- 

 nate of lime; for they contain but a small proportion of this 

 material: and whatever calcareous ingredient enters into 

 their composition is in the form of phosphate of lime. In 

 external appearance their texture approaches nearer to that 



Vol. I. 29 



