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Larvae of the subfamily Tanypodinae are in general regarded as predators. Actual predation has been observed directly in only a few Tanypodinae species, but their behaviour and mouthpart morphology suggest that all Tanypodinae ingest food in the same way and thus are all predators. This view is reflected in most autecological databases. There remains uncertainty for some species, most notably for Monopelopia tenuicalcar (Kieffer 1918). The uncertainty stems from the lack of direct observations, while gut content analysis points to non-animal food sources. A laboratory experiment was carried out in which larvae of Corynoneura sp. were offered to M. tenuicalcar in a set of Petri dishes. All predator and prey larvae were collected from the same locality, where they were the most abundant members of early spring littoral community. M. tenuicalcar showed clear predatory behaviour. In most cases (84 out of 86) the predator larva pierced the larva of Corynoneura and sucked its inner body content instead of engulfing it. Only in two cases did the predator engulf the whole victim. In all cases the seizing and processing of the prey was the same, with the ingestion of the food carried out by strong sucking. Obviously, if the chitinous structures of the prey fit the mouth opening of the predator, it was engulfed completely, otherwise the prey larva’s internal contents were just sucked out. A video of the exceptional case of engulfing the whole prey is available at https://youtu.be/o6-8dA1XDy0. The strategy of sucking out prey may explain why body remnants like head capsules or claws of prey have not been found in the guts of Monopelopia larvae. Instead, the guts of Monopelopia larvae that pierced Corynoneura were green as they were full of algae “stolen” from the intestines of the grazing victims. Piercing and sucking out prey by Tanypodinae may be more common than has been expected before. Due to this, gut content analysis seems to be unreliable for the determination of feeding strategy in Tanypodinae.