202 



THE HARMONIES OF NATURE. 



weight of the insect, that the pressure of the air alone 

 is more than sufficient to sustain it without exertion, and, 

 as it were, to set the force of gravity at defiance. In the 

 Bibio febrilis the foot is furnished with three suckers, and 

 in the Cymbex lutea with five. Many other species, amongst 

 which is the common wasp, are similarly furnished with cushions 

 and analogous suckers, which enable them to ascend vertically 

 on glass. 



Most of the perfect in- 

 sects have their body most 

 admirably organised for 

 flight. In all its parts it 

 is traversed by numerous 

 air-tubes, communicating 

 with several external open- 

 ings or spiracles, and not 

 only branching out into 

 numberless ramifications, 

 so as to penetrate even 

 the smallest and most 

 delicate organs, but fre- 

 quently also dilating into 

 vesicles or sacklike expan- 

 sions, the size and number 

 of which is always in exact 

 proportion to the powers 

 of flight ; their development 

 being most considerable in 

 the bees, flies, and butter- 

 flies, while there is not 

 the slightest trace of them 

 in the wingless larvae, or 

 in insects that constantly 

 reside on the ground. 

 Thus by a most beautiful mechanism the tracheae or air- 

 vessels not only abundantly supply the insect with all the 

 oxygen needed for its active habits of life they not only act as 

 lungs or respiratory organs, but diminish at the same time the 

 specific gravity of its body, and enable it to support itself on 

 the wing with less muscular effort. The spiracles, stigmata, or 



Tracheal System of Water Scorpion. 

 a head, 6 first pair of legs, c first segment of the 

 thorax, d second pair of wings, e second pair 

 of legs, / tracheal trunk, g one of the stigmata, 

 h air-sac. 



