CH. XXII.] METAMORPHOSES OF INSECTS. 297 



closed in a mummy-like case, which gives them a 

 swathed appearance. 



The last kind of metamorphosis which we have 

 to notice is that undergone by the majority of two- 

 winged flies {Diptera), and which is known by the 

 name of the coarctate species of transformation. 

 Here we find the pupa enclosed, not in a cocoon 

 spun by the larva, as in the moths, but in a harden- 

 ed case formed of the outer skin of the larva itself, 

 which in these insects is not cast off as in the re- 

 mainder, and within which the real pupa, having its 

 limbs enclosed in distinct cases, is to be found. 

 Here, however, two variations occur, in the first of 

 which the skin of the larva retains its old form, as 

 in the StratiomidcB and some other families ; while 

 in the second, of which the blowfly is an example, 

 the larva by degrees loses its elongated form, and 

 assumes the appearance of a small oval, barrel- 

 shaped mass,* the skin being hard and brittle, on 



Barrel-shaped pupa-case of the blowfly, formed of the skin'of 

 the larva, 



breaking which the enclosed pupa is found to "occu- 

 py the whole space; whereas, in the former, the 

 pupa is much smaller than the larva, several seg- 



* In our former volume, page 185, we have figured the foot- 

 less larva of the syrphus, which feeds in the former state upon 

 the aphides, and which contracts itself by degrees into a co- 

 coon, in which the real pupa is enclosed : and m page 205, we 

 have represented the pupa of the botfly ( CEstms), which also 

 belongs to the same class of transformations. This pupa, or 

 rather pupa-case, is, however, capable of a shght motion by 

 means of numerous recurved hooks upon the rings of the body. 



