312 THE MECHANICAL FUNCTIONS. 



insects have furnished entomologists with very con- 

 venient characters for their classification : on these 

 are founded the orders of Coleoptera, Ori/ioptera, 

 Rkipiptera, Hemiptera, Nemoptera, Hymenoptera, 

 Diplera, and Lepidoptera. To enter into any 

 detail in a field of such vast extent as is presented 

 by the infinitely diversified mechanism of the insect 

 creation, would, it is obvious, far exceed the proper 

 limits of this treatise. I must therefore confine 

 myself to a iew leading points in their structure 

 and modes of progression. 



In the Coleoptera, an order which probably com- 

 prehends the largest number of genera of insects, 

 the lower pair of wings (w. Fig. 150, p. 287) are 

 light and membranous, and of a texture exceedingly 

 fine and delicate. They are of great extent com- 

 pared with the size of the body, when fully ex- 

 panded ; 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 

 in 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 im- 

 pressions to which they might otherwise 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 motionless during the flight 

 of the insect. They probably, however, contribute 

 to direct the course of flight, by variously modifying 

 the resistance of the air.* 



* The Elytra of insects h;ive been regarded by Oken as corre- 

 sponding to the bivalve shells of the Mollusca; a notion founded 

 on a very fanciful and strained analogy. 



