Functional types of diving beetle (Coleoptera: Hygrobiidae and Dytiscidae), as identified by comparative swimming behaviour

The swimming behaviour of ten species of diving beetles was studied with a video image analysing system, with the aim of testing the interpretation of their size and shape as functional characters reflecting adaptations to different swimming strategies. Velocity, sinuosity of the trajectory, and the relation between the two were studied in an unobstructed aquarium and, for the four largest species, in an aquarium with vertical sticks. Species predicted to be poor swimmers had the lowest average and maximum velocities. A globular species considered to have high manoeuvrability swam in highly sinuous trajectories, and could maintain this sinuosity at a wide range of velocities. One of the larger species, considered to be adapted to high speed swimming, also swam in highly sinuous trajectories, but only at slow velocities; its swimming pattern was considered to be the product of behavioural rather than morphological constraints. For two of the largest species, there was a significant decrease in sinuosity at higher velocities, whilst another was found to have a good compromise between velocity and manoeuvrability. In the aquarium with obstacles all the species reduced their maximum velocity, while the effect on sinuosity varied between species. Although the space limitation of the aquarium and the lack of motivation could have prevented some species from reaching their maximal velocities, a good agreement was found between the predicted and the actual swimming characteristics.