The Adaptive Significance of Territoriality in Filter-Feeding Larval Blackflies (Diptera: Simuliidae)

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Abstract
I report the results of descriptive and experimental studies of territorial behavior in filter-feeding stream insects. Blackfly larvae are sometimes evenly spaced on a substrate, suggesting that individuals may react negatively to the presence of other larvae. Direct observations indicate that larvae often behave aggressively towards neighbors. Three kinds of evidence support the hypothesis that this aggresive behavior is associated with the defense of filterable food resources. First, aggressive behavior is initiated almost exclusively towards upstream neighbors, which are the only larvae capable of altering the availability of filterable particles being delivered to an individual by the current. If an individual succeeds in aggressively displacing its upstream neighbor(s), it ceases to behave aggressively, and returns to its normal filter-feeding behavior. Second, following the successful displacement of such neighbors, the individual''s short-term ingestion rate (measured indirectly) increases significantly. Third, the frequency of territorial behavior declines when the availability of filterable particles is experimentally increased. Taken together, these results indicate that larvae defend a local feeding site in order to maintain higher ingestion rates than would otherwise be possible, and that the net benefits of such aggressive behavior decline when food availability is high.