WINGS OP INSECTS. 



89 



they are soft and flexible ; but they soon dry up, and remain 

 stiff and elastic. In general there are two pairs ; we never find 

 a greater number, but sometimes one or other of these pairs is 

 wanting ; and it is always from the last two rings of the thorax 

 that they arise. Their form varies ; when they really serve for 

 flight, they are thin and transparent, except when lightly covered 

 with a kind of coloured dust, formed by scales of a microscopic 

 minuteness, such as we see amongst the Butterflies ; but often 

 those of the first pair become thick, hard, and opaque, and con- 

 stitute a kind of sheath or casing, named elytra (a, Fig. 303), 



which, in a state of rest, covers 

 over the membranous wings (i), 

 and serves to protect them ; at 

 other times these same wings, 

 still membranous at their extre- 

 mity, become hard and opaque 

 towards their base, and are then 

 designated under the name of 

 demi-casings or hemelytra. We 

 are acquainted, also, with some 

 insects, amongst which the wings, 

 instead of having a leaf-like 

 structure, are divided into a 

 number of membranes, barbed 



along the edges, so as to resemble feathers arranged in a fan- 

 like manner ; this is seen in a genus allied to the Butterflies, 

 and known under the name of Pterophorus, or Plumed-Moth. 



FIG. 303. 



FIR. 304. PTKROPHORUS. 



Fio. 305 CONOPS. 



Again, when the posterior wings arc wanting, they are generally 



