214 LECTURE XVII. 



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. The Nepa, or the 

 Dytiscus, at the same time, may possess its organs of reptation, of 

 burrowing, and of flight ; thus, like Milton's fiend, it 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, there are, in fact, 

 associated, in Insects, equally, if not more varied, structures for 

 imbibing, seizing, masticating, and digesting nutritious substances. 

 The patience of the anatomist is taxed to the utmost to unfold these 

 delicate structures ; but his admiration is chiefly excited by the 

 discovery that they are so 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 upo%: 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 

 which operate upon the solids, and have made the Haustellata and 

 Mandihulata the primary divisions of the class. 



The composite parts of the proboscis or siphon are however fun- 

 damentally the same as those that form the strongest or most 

 formidable apparatus for mastication ; but as they are most con- 

 spicuous and most uniformly developed for the latter office, I shall 

 commence the demonstration of those complex parts of the mouth, — 

 the trophi or cibarial instruments, — as they exist in a Mandibulate 

 insect, {fig. 100.) 



Man has two jaws only, and no Vertebrate animal has more ; they 

 work up and down, or in the direction of the axis of the body. 

 Insects have also their upper and lower jaws, horny edentulous plates, 

 serving in many for little else than to close the mouth, and hence 



