LOCUSTS 293 



ascertained that in both Algeria and Xorth America large swarms 

 occur usually only at considerable intervals. In North America 

 Eiley thought ^ the average period was about eleven years. In 

 Algeria the first invasion that occurred after the occupation of 

 the country. by the French was in 1845, the second in 1864, 

 the third in 1866, since which 1874 and 1891 have been 

 years of invasion. These breaks seem at first strange, for it 

 would be supposed that as locusts have great powers of increase, 

 when once they were established in any spot in large numbers, 

 there would be a constant production of superfluous individuals 

 which would have to migrate as regularly as is the case with 

 swarms of bees. The irregularity seems to depend on three facts : 

 viz. that the increase of locusts is kept in check by parasitic 

 Insects ; that the eggs may remain more than one year in the 

 ground and yet hatch out when a favourable season occurs ; and 

 that the migratory instinct is only effective when great numbers 

 of superfluous individuals are produced. 



It is not known that the parasites have any power of re- 

 maining in abeyance as the locust eggs may do ; and the bird 

 destroyers of the locusts may greatly diminish in numbers during 

 a year when the Insects are not numerous ; so that a dispro- 

 portion of numbers between the locusts and their destroyers may 

 arise, and for a time the locusts may increase rapidly, while the 

 parasites are much inferior to them in numbers. If there should 

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

 year there will be very few parasites, and if there should then be 

 a large hatching of locusts from eggs that have remained in 

 abeyance, the parasites will not be present in sufficient quantity 

 to keep the destructive Insects in check ; consequently the next 

 year the increase in nimiber of the locusts may be so great as to 

 give rise to a swarm. 



It is well established that locusts of the migratory species 

 exist in countries without giving rise to swarms, or causing any 

 serious injuries ; thus Pachytylus cinerascens — perhaps the most 

 important of the migratory locusts — is always present in various 

 localities in Belgium, and does not give rise to swarms. When 

 migration of locusts does occur it is attended by remarkable 

 manifestations of instinct. Although several generations may 

 elapse without a migration, it is believed that the locusts when 



1 Sep. Entomologist, 1885, p. 229. 



