CLASS V. INSECTS. 299 



cornea is all over divided into lenticular facets, which, 

 through the microscope, appear like a beautiful piece of 

 lattice-work, each section presenting objects inverted. 

 This mechanism sufficiently supplies the place of the 

 crystalline humour, which is never found in insects. 

 Spiders have generally eight eyes ; and flies may be said 

 to have as many organs of sight as there are perforations 

 in the cornea, or external covering of the eye. To con- 

 template objects, animals in general are obliged to turn 

 their eyes different ways ; but those of flies are so con- 

 trived as to embrace every neighbouring object at one 

 view. 



The mechanism of the feet of insects is no less worthy 

 of attention. The hind legs of amphibious insects, such 

 as water-beetles, which are sometimes obliged to live on 

 land as well as water, are formed with commodious flat 

 joints ; while gristles, which are placed at the extremity 

 of the limb on each side, supply the place of oars. In 

 those insects which are formed for leaping, such as the 

 grasshopper and the cricket, the legs are strong and 

 brawny ; those, on the contrary, which use their claws 

 for perforating the earth, have these members peculiarly 

 fitted for that purpose. There are even some tribes of 

 insects which transport themselves from one place to 

 another, in a manner not yet understood ; for those gene- 

 rated in stagnant waters are often found in new pits or 

 ponds ; and spiders frequently soar with their webs to the 

 summits of the most lofty towers. 



Winged insects are provided with tendons to expand and 

 strengthen the instruments of flight. Those that are fur- 

 nished with four wings, as the ephemeron, or day-fly, use 

 the outermost pair rather as cases to defend the interior, 

 than as auxiliaries in flight. When the insect is at rest, 

 the inner wings are generally closed up ; nor is it without 

 some effort that the little animal is capable of unfolding 

 them. Such insects, on the contrary, as have only two 

 wings, are supplied with little balls, or poisers, united to 

 the body under the hinder part of each wing, which serve 

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