372 LECTURE XVIT. 



LECTURE XVII. 



INSECTA. 



The extraordinary powers of locomotion possessed by insects, the 

 variety of elements which they can traverse, their aptitude to gain 

 access to every situation where organised matter may be obtained, 

 prepare us to expect that they should manifest all the modifications 

 of the digestive system which may be required for the assimilation 

 of the different kinds and conditions of the solids and fluids of plants 

 and animals. 



One insect preys upon another ; pursues and attacks, like the 

 falcon, on the wing ; but, with better mastery over the air-element, 

 it can tear to pieces and devour its prey without alighting : another 

 insect, sedentary and inactive, imbibes the juices of a plant : a third 

 eats its way into the hard wood : a fourth burrows in the earth for 

 roots or worms. 



Some traverse the surface of the earth with a succession of steps 

 too swift for definition ; some by leaps so extraordinary, as to have 

 excited the powers of the dynamical calculator from the earliest 

 periods. The waters, also, have their insect population ; some 

 swiftly cleaving the clear element, some gyrating on the surface, 

 whilst others creep along the bottom. Nor are the activities of the 

 aquatic insect confined to that lower sphere. Nepa and Dytiscus 

 possess organs of natation, of reptation, of burrowing, and of flight. 

 Like Milton's fiend, each is qualified for different elements, and 



" Through strait, rough, dense, or rare, 

 With head, hands, wings, or feet, pursues its way, 

 And swims, or sinks, or wades, or creeps, or flies." 



With such diversified powers of attaining food are associated, in 

 Insects, as varied structures for imbibing, seizings masticating, and 

 digesting nutritious substances. The patience of the anatomist is 

 taxed to the utmost to unfold these delicate complexities ; but his 

 admiration is chiefly excited by the discovery that they are clearly 

 referable to a common type. 



The most marked modifications of the digestive organs relate 

 rather to the physical condition than the chemical constitution of the 

 food ; depend more upon its being solid or fluid, than upon its being 

 of a vegetable or animal nature. Some entomologists have separated 

 all the insects which suck the juices of plants and animals from those 



