Behavior on approach to surface prey by larvae of Toxorhynchites amboinensis and T. brevipalpis (Diptera: Culicidae).

Behavior of Toxorhynchites amboinensis (Doleschall) and Toxorhynchites brevipalpis (Theobald) larvae (starved 48 h) as they approach and capture surface prey is described quantitatively from videotaped records. Of 106 T. amboinensis and 82 T. brevipalpis larvae observed, 84.9 and 97.6%, respectively, responded to the presence of surface prey within 2 min (most < 20 s). Once they detected prey, larvae executed initial backward swims if prey was rearwardly positioned, then approached it in a series of undulatory forward swims with progressive adjustments of body angle so that the final movements were directly toward the prey. Swims consisted of an active phase, during which propulsive bodily flexions were made and a passive one of continued inertial drift after active motion had ceased. Quantitative changes in the nature of the swims in terms of interval separating active phases and also distances, times, and speeds throughout the approach sequence were analyzed from the video tapes. T. brevipalpis responded to the presence of prey more rapidly, captured prey in less time, and approached prey more rapidly, with shorter rest periods between active swim phases than T. amboinensis. The behavior indicated that Toxorhynchites larvae are able to assess both the angle to surface prey and its distance and that they interpolate this information to optimize the approach path. The degree of refinement in this behavior indicates that it is well adapted to take advantage of the important surface food source in nature. In this phase of their feeding, Toxorhynchites larvae are active hunters and are not entirely the passive ambush predators they have seemed to be from many studies that have used other mosquito larvae as (subsurface) prey.