300 THE PRACTICAL APPLICATION OF ECOLOGY 



environmental influences acting either directly or indirectly upon 

 the insect. If this be correct it follows that a thorough ecological 

 study of each locust species in relation to fluctuations in external 

 conditions, over a period of years, would be the most profitable 

 way of arriving at an understanding of the factors regulating out- 

 breaks of these insects. 



While the menace of locust swarms can be better counteracted 

 by the prompt application of artificial control measures than has 

 happened in the past, these measures still remain hopelessly 

 inadequate when combating great invasions. Being remedial in 

 effect their application may alleviate pressing needs, but the 

 menace of subsequent swarms is left unaffected. In other words, 

 they leave the main problem unsolved, i.e., the causes of locust 

 outbreaks and the areas of their origin. 



In 1921 Uvarov advanced what has subsequently become 

 known as the phase theory of locusts. His views were first put 

 forward as a working hypothesis, but later observations conducted 

 both in the laboratory and in the field by Faure (1932) and other 

 workers have so fully corroborated its main contentions that the 

 phase theory of locusts is regarded to-day as an established 

 biological phenomenon. 



The Phase Theory. The starting point of the theory is that 

 the gregarious species of Acrididae or true locusts are polymorphic. 

 Such species are not constant in all their characters, but are 

 capable of producing a series of forms differing from one another, 

 both morphologically and biologically. In their notable discussion 

 on polymorphism, Uvarov and Zolotarevsky (1929) propose a 

 standard phase nomenclature which, although referring more 

 especially to the well-known species, Locusta migratoria, will 

 probably prove applicable to the other species also. According 

 to their interpretation a locust can exist in three unstable biological 

 phases, namely, a solitary one, phasis solitaria ; a gregarious one, 

 phasis gregaria ; and a transitional phase between these two, 

 which they term phasis transiens. These phases differ from each 

 other in morphological and colour characteristics, on the one 

 hand, and in biological features (mainly behaviour) on the other. 

 The phases gregaria and solitaria are often so strikingly distinct 



