I METAMORPHOSES OF INSECTS 29 



gravity by enlarging its bulk, rendering it better able 

 to support itself on the wing with little muscular 

 effort. That this is the use of the sacs may be 

 inferred from their entire absence in larvae, and in 

 insects that are not addicted to forsake the ground ; 

 while they are largest and most numerous in volants 

 having the longest and most powerful flight. It 

 would be impossible to exaggerate the advantage of 

 this beautiful provision to the humble-bee, which is 

 compelled to be constantly on the wing. Its large 

 and cumbrous body is fitted for all purposes of 

 strength, but would undoubtedly occasion it inconve- 

 nience and fatigue, were the apparently unwieldy 

 structures not lightened in this remarkable manner. 

 The development of the vesicles begins at about 

 the period when the larva ceases to feed, preparatory 

 to change to pupa. When an insect is quiet, or 

 is walking, it breathes slowly, and there is little 

 air in the tracheae. The body is heavy, and must 

 contain more air before the creature can fly. Its 

 act of respiration at this moment resembles that of 

 birds under the same circumstances. It elevates 

 its elytra, the anterior* pairs of spiracles are opened, 

 and the air rushing into them, is extended over 

 all the vesicles. The air-bags expand, which in- 

 creases the bulk of the body, rendering it light in 

 relation to the bulk of air it replaces, so that when 

 the spiracles are closed at the instant the insect 

 makes the first stroke with its wings, it is able to 

 rise, and endure sustained and forcible flight with- 

 out excess of exertion. Explanation is here obtained 

 of the long and elevated voyages undertaken by 

 hordes of locusts, which are not conspicuous for 



