312 THE PRACTICAL APPLICATION OF ECOLOGY 



differences are especially marked in Mropus sibiricus and 

 Chorthippus alhomarginatus, while a number of other species can 

 be arranged in descending order of their tendency to show marked 

 colour differences. The most pronounced differences are found 

 in A, sibiricus, which is an active species, tending to form dense 

 aggregations of individuals. These differences are almost equally 

 well shown in C. albomarginatus. In all cases the dark coloured 

 forms occurring collectively are interpreted as representing the 

 phase gregaria, while the pale examples found singly and sparsely 

 are regarded as representing the solitaria phase. This conclusion 

 is supported by such experimental evidence as is available. 



Thus, individuals of C. albomarginatus, reared in isolation, 

 showed a tendency to develop into the pale coloured solitaria 

 form, while those reared gregariously, in crowded conditions, 

 produced a very definite gregaria type. Apart from the colour 

 differences alluded to, the supposed phase differences are also 

 betrayed in : (1) solitaria individuals being smaller, with femur 

 and tegmen shorter ; (2) shortening of the tegmen in solitaria 

 being more pronounced than that of the femora, the ratio being 

 greater in solitaria than gregaria ; and (3) variability in solitaria 

 being always greater than in gregaria — as can be seen by comparing 

 the maximum and minimum figures. Rubtzov's data are 

 accompanied by coloured figures portraying supposed phase 

 differences, and they unmistakably suggest that we have, among 

 non-swarming grasshoppers, clear evidence of the existence of those 

 same phases that feature so markedly in the economy of locusts. 



A further phenomenon, discussed by Rubtzov, can only be 

 briefly alluded to here. He shows that a homologous series of 

 colour variations reveals themselves through many species of 

 Acrididae. These are inheritable variations and, according to 

 him, each such race possesses the potentiality, to a greater or 

 lesser degree, to exhibit such phase characteristics. The latter, 

 as it were, are super-added to the former in response to the 

 extreme conditions of individual life. In practice, it resolves 

 into the necessity, fully recognised by this author, of clearly 

 determining which race of a given species is being utilised in all 

 studies designed to explore the phase idea. 



