Abstract: During their life cycle, most organisms undergo ontological shifts which include changes in size and possibly also in habitat. In general, as an organism grows in size, its place within the community structure changes. These changes in size also allow for cannibalism which is a common factor in many species and ubiquitous in aquatic and most terrestrial food webs. However, no previous study has examined the consequences of the predatory interactions which occur between and within cannibalistic species. Here, we use an experimental approach using the two cannibalistic dragonfly species, Anax junius and Aeshna umbrosa to test the relative strength of intra- and inter-specific predation and their interaction effects, as well as their effect on community dynamics and structure. Since these organisms have a wide size distribution the interaction of large A. junius and A. umbrosa can be examined as well as its effect on the prey community (including smalle r A.umbrosa ). We found that the intra- and inter-specific interactions between the predators significantly affected the prey community. These two dragonfly larva are not substitutable but together reduce the risk of predation upon the prey community. These results are vital to understanding proper food web structures. A community is not dependant upon species alone but also upon the size distribution within a community.