99 

 INSECT ASSOCIATIONS, 



H. H. WALLIS, M.A. 



Within the last few decades, considerable advance has been 

 made in the study of ecology. The relations of structure and 

 distribution of plants to the physical factors of their environ- 

 ment have been studied, and we are well acquainted with the 

 term ' plant association.' This seems but the starting point 

 of a line of investigation which, though involving detailed 

 research and the compilation of masses of statistics, will 

 ultimately lead to a much better understanding of the habits, 

 distribution, structural metamorphosis and interdependence 

 of animals. 



The factors which determine the structure and local 

 distribution of plants are soil, temperature, altitude, water 

 supply, prevailing winds, etc. Upon these depend the 

 association of plants. Regional distribution and geographical 

 distribution seem to be similarly dependent, with the intro- 

 duction of further factors, such as barriers and agencies of 

 distribution. 



While much work has been done on the regional distribution 

 of insects in the way of county lists and the records of in- 

 dividual collectors, more remains to be done in the, study of 

 the insect from the ecological standpoint. Why is Papilio 

 machaon restricted to the fen district in England ? On the 

 continent it occurs commonly in woods, on mountain slopes 

 up to an elevation of 5000 feet, and less abundantly at even 

 higher altitudes. In the past it was of much wider distribu- 

 tion in England : Stephens gives localities near London in 

 1827. The food plant it most affects, Peucedanum palustre, 

 is certainly uncommon, except in the marshy districts of 

 Eastern England, from Suffolk to Yorkshire ; but it will also 

 eat Angelica sylvestris, Foeniciilum vulgar e and Daucus carota, 

 which are abundant in many localities. The Milk Parsley 

 and Angelica are marsh-loving plants ; the Swallow Tail 

 butterfly is, in England at all events, a marsh-loving insect, 

 and there our knowledge ends. 



No doubt there are many cases in which the non-appearance 

 of the insect is determined by the absence of the food plant 

 of the larva, or, in the case of parasites, by the absence of 

 the host, but in many instances other factors will determine 

 the distribution of the insect, especially if of catholic tastes 

 in the matter of food. Carabus glabratus seldom occurs below 

 the 500 feet level. Is this insect a survival of a glacial 

 period? As the ice retreated from the valleys, did this 

 beetle forsake the warm lower levels for the cooler hills ? 

 What factor determines the relegation of Dytisctis punchilatus 

 to the mountain tarns of the Western Highlands ? Why does 



1921 Mar. 1 



