160 .London Insects. 



obstacles which they meet in their comings and 

 goings on the dull earth, and so every available 

 corner of the trunk is packed as tightly as possible. 



But with creatures which are to have the power of 

 lifting themselves from the ground the case is dif- 

 ferent. What they most require is lightness, and so 

 the hollows of the bones, which in men and quad- 

 rupeds are used as bottles and casks for holding 

 marrow and other liquids which are wanted to oil the 

 joints and keep the machinery in working order, are 

 in birds turned into dry chambers filled with hot air, 

 lighter, of course, than the cooler air outside, into 

 which the bird has to rise. 



But, even with this beautiful contrivance for 

 lightening it, the solid inside frame of bones on 

 which beasts, and birds, and fishes are built is a dead 

 weight to lift ; and so in insects, in which the power 

 of flight is carried to a far higher perfection than 

 even in the Swallow or Frigate Bird, different 

 arrangements are made. The heavy skeleton is dis- 

 pensed with altogether, and instead of it the sup- 

 ports for the body are given by a light, stiff outside 

 skin only ; and, instead of having their fluid parts 

 aired as in beasts by little lungs in the middle of the 

 body, they suck in the air through openings, not 

 confined as in men and beasts to mouth and nose, 

 but dotted about the body in many parts, varying in 

 different insects back, sides, head, and tail into 

 tubes (" trachiae," they are called), which run back- 

 wards and forwards in every direction through trunk 

 and limbs. 



The four powerful, wide-spreading wings of the 

 Dragon Fly, " the Eagle among insects," look as if 

 made of the thinnest goldbeater's skin stretched on 

 wire, stiff at the base and front edges of the wing, 



