n6 TRUE TALES OF THE INSECTS. 



wing probably with little muscular effort. Their body, 

 we have seen, contains elastic air-sacs in connection 

 with the tracheae, which, constantly filled and refilled, 

 float them up in the air, and at the time of flight, it may 

 be presumed, the internal balloons have room for perfect 

 expansion, as previously explained. Thus the insects 

 spend but little muscular force in their aerial movements, 

 and, instead of really flying as does a bird, are borne 

 along chiefly by the wind. Suppose a swarm has risen 

 from the ground on a clear sunny morning, and is being 

 wafted straight on their course by favouring winds. 

 Opposing winds suddenly prevent them from moving 

 on in the direction they desire to go. Instantly they 

 drop, and wait for a change. With a return of the 

 wind to its original quarter, if during the warm part 

 of the day, often, with wonderful unanimity, all 

 again take wing, and fly off towards their intended 

 destination. 



An apparently inexplicable point in the natural history 

 of the migratory locusts is their disappearance from a 

 spot they have invaded. A swarm will arrive in a 

 locality, deposit there a number of eggs, and then pass 

 on. But after a season or two there will be few, or 

 none of the species in the spot invaded. This appears 

 to be partly owing to the want of food, and consequent 

 mortality among the young after hatching ; but in other 

 cases they in turn migrate after growth they disappear 



