FLIGHT OF INSECTS. 349 



by far the largest number of genera of insects, 

 the lower pair of wings (w, Fig. 150, p. 321) 

 are light and membranous, and of a texture 

 exceedingly fine and delicate. They are of great 

 extent compared with the size of the body, when 

 fully expanded : and are curiously folded when 

 not in use. For the protection of these delicate 

 organs, the parts which correspond to the upper 

 pair of wings of other insects, are here converted 

 into thick opaque, and hard plates (e), adapted 

 to cover the folded membranous wings when the 

 insect is not flying, and thus securing them from 

 injurious impressions to which they might other- 

 wise be exposed from heat, moisture, or the 

 contact of external bodies. These wing cases, 

 or elytra as they are termed, are never themselves 

 employed as wings, but remain raised and motion- 

 less during the flight of the insect. They pro- 

 bably, however, contribute to direct the course 

 of flight, by variously modifying the resistance 

 of the air.* 



In the Orthoptera, (Fig. 159), the coverings of 

 the wings, or tegmina, instead of being of a 

 liorny texture, are soft and flexible, or semi- 

 membranous. The wings themselves, being 



* The Elytra of insects have been regarded by Oken as cor- 

 responding to the bivalve shells of the Mollusca, a notion which 

 seems to be founded upon a fanciful and strained analogy. 



