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IntroductionThe forms of the head and front legs of British corixids have been used by Butler (1923), Southwood and Leston (1959) and by Maean (1965) for species and sex determination, but the functional significance of these differences is largely unknown. Reynolds (1975) has reviewed previous literature on the feeding habits of various Palaearctic species but the morphological causes of these observed differences are largely unknown for most species. A study of the functional morphology of the front legs and of the head in relation to feeding would help to fill these gaps in existing knowledge.This paper falls naturally into three sections. The first reviews the work of previous authors on the feeding habits of corixids and describes the range of feeding habits amongst the commoner British species. The second section describes the adaptations of the front legs of these species to different, types of feeding habit. The third section surveys the modifications of the front tarsus for grasping the female as necessitated by the adaptations of the front legs to different feeding habits.

When observations began the population of Watch Lane Flash consisted of Gammarus duebeni and Sigara lateralis. Gammarus tigrinus was then introduced; probably during a transfer of water from a nearby canal. G. tigrinus rapidly increased in numbers and at the same time G. duebeni and S. lateralis decreased to zero. There followed a partial recolonisation by corixids but the species present were Sigara concinna and Sigara dorsalis. Throughout the study period corixid populations were stable in two immediately adjacent water bodies, one of which also contained G. duebeni. Thus the changes described were associated with the presence of G. tigrinus. The reasons for the success of G. tigrinus and its effects on other gammarids and corixids are discussed.

Key words: d e n s i t y d e p e n d e n t , d e n s i t y i n d e p e n d e n t , m a c r o i n v e r t e b r a t e s , c o m m u n i t i e s , p o p u l a t i o n d y n a m i c s , a b i o t i c f a c t o r s A b s t r a c t