SWARMING OF LOCUSTS 305 



Applied Entomology, Series A, for further information and 

 references to the hterature concerned. 



Factors Governing Swarming. The first impulse for a locust 

 to increase in numbers appears to be provided by favourable 

 meteorological conditions. This is automatically followed by 

 evolution towards the swarming phases. Once swarming has 

 taken place a crisis is reached, a decline supervenes, and this 

 usually leads to the solitary phase. 



In the case of Locusta migratoria, sub-sp. migratorioides, the 

 solitary phase has an enormous area of distribution, whereas the 

 gregarious phase only occurs in certain regions. The areas where 

 the latter phase can be produced are characterised by the extensive 

 development of reeds, bamboos and similar tall plants. It would 

 appear that such vegetation provides the most suitable food, but it 

 is also possible that the connection between the gregarious phase 

 of this insect and reed-beds may be due to the climatic conditions 

 associated with the latter. Such a uniform environment, providing 

 an abundance of food and shelter, is very suitable for mass 

 development of the locust, and the appearance of large numbers 

 of hoppers in a favourable year leads to transformation into the 

 swarming phase. The stimulus to increase in numbers appears to 

 be provided by dryness in tropical countries, while in temperate 

 lands a high temperature is of great importance in this respect. 

 Once the dense migrating swarms have left their breeding grounds, 

 the latter remain almost free until a fresh increase in numbers 

 results. Migrating swarms usually settle on lands very different 

 from whence they came. Difference of conditions appears to 

 reduce the numbers of the generation which results from their 

 egg-laying, and transformation into the solitary phase supervenes. 

 The activities of parasites, birds, and other natural enemies 

 increase at this period and further accelerate the transformation. 

 The hoppers of the solitary phase do not differ in their general 

 behaviour from those of other solitary grasshoppers, and do not 

 exhibit any tendency towards gregariousness. 



The actual stimuli which promote the migratory flight are, 

 according to Uvarov, physiological. Lack of food can be dis- 

 missed, since the vast stretches of reed-beds in their breeding 



