104 INSECTS. 



thoroughly acquainted, as we shall again and again 

 have to refer to them hereaftei'. 



The wings of insects, when present, are invariably 

 attached to the two posterior segments of the thorax, 

 which are strengthened in every possible manner, so 

 as to afford a support of sufficient density and iirm- 

 ness to sustain the violent exertions of the muscles 

 employed in flight. Few things are met with in 

 Nature more admirable than these structures. They 

 present, indeed, a combination of strength and light- 

 ness absolutely beyond anything of human invention, 

 and as instruments of flight they much surpass the 

 wings of birds, both in the power and precision of 

 their movements. 



In the dragon-flies, by far the most powerful fliers 

 in the insect Avorld, all four wings are of equal size, 

 and consist of a thin membranous expansion of great 

 delicacy and of glassy appearance, supported at all 

 points by a horny net-work (Fig. 69) ; these insects 



~^o 



Fig. 69.— "WING OF dragox-flt. 



can fly in all directions, backwards, and to the right 

 or left, as well as forwards, with equal facility. 



The substances employed as food by insects are 

 various in proportion to the extensive distribution of 

 the class. Some devour the leaves of vegetables, or 

 feed upon grasses or succulent plants ; others destroy 

 timber, and the bark or roots of trees ; while some, 

 more delicately organized, are content to extract the 

 juices of the expanding buds, or si]3 up the honied 

 fluids from the flowers. ]\rany tribes are carnivorous 

 in their habits, armed with various weapons of de- 

 struction, and carry on a perpetual warfare witli 

 their own or other species; and, again, there are 

 countless swarms appointed in their various sjDheres 



