112 TRUE TALES OF THE INSECTS. 



would have to migrate regularly, in order to procure food. 

 The irregularity seems to depend on three things : that 

 the multiplication of locusts is kept in check by parasitic 

 insects ; that their eggs — which were supposed to be 

 comparatively easily affected by climatic influences — may 

 lie hidden in the soil for years and yet hatch out in a 

 favourable season ; and that the disposition to migrate 

 — though some locusts appear essentially migratory — is 

 only effective when immense numbers of individuals 

 are produced. 



Every animal necessarily meets with checks of one 

 kind or another to its undue multiplication, and the 

 balance of power does not always lean to the side of 

 the enemies of the locusts. During a year when the 

 locusts are not numerous the abundance of the parasites 

 may decline, and the bird destroyers of the locusts may 

 greatly fall off in numbers ; so that the locusts get on 

 the rising side of the scale, as it were, and for a time 

 may increase rapidly, while the enemies are much 

 inferior to them in numbers. If there should come a 

 year when very few of the locusts hatch, then the next 

 year the parasites will be greatly reduced, and if then 

 large numbers of locusts from eggs that have been in 

 abeyance should hatch out, the parasites will not be 

 present in sufficient numbers to keep them in check ; 

 and by the following year the increase in number of 

 the locusts may be such as to give rise to a swarm. 



