184 INSECT TRANSFORMATIONS. 



unavailing in raising the loaded glass, except in con- 

 nexion with two favourable circumstances under which 

 the experiment was performed, and which are neces- 

 sary to be borne in mind to render the operation per- 

 fectly credible : 1st, that the wedge-like form of the 

 caterpillar- s head, in connexion with the peculiar shape 

 of the glass, enable it to lift it ; and 2d, that, one 

 side of the glass resting on the table, the insect only 

 bore half the weight of the glass and book. 



Caterpillar of Cossus escaping from under a loaded glass. 



A peculiar toughness of external covering some- 

 times supplies the place of this muscular power in 

 caterpillars. A singular instance occurs in the 

 history of a common downy two- winged fly, with 

 gray shoulders and a brown abdomen, (Eristalis 

 tenaXy FABE,.). The grub, which is rat-tailed, lives 

 in muddy pools, with the water of which it has some- 

 times been taken up by paper-makers, and, though 

 subjected to the immense pressure of their ma- 



