Commensalism is widespread among the Chironomidae. Animal hosts include the Ephemeroptera, Plecoptera, Trichoptera, Megaloptera, Hemiptera, Odonata, Diptera, Mollusca and fishes. In general, benthic organisms with relatively large body size and intermediate mobility appear to be chosen most frequently as hosts. The evolutionary advantage of a commensal mode of life in the Chironomidae is discussed. Better feeding opportunity, increased mobility, protection from disturbance and reduced predation risk are likely to constitute the four aspects of major importance in establishing commensal relations. It is proposed that the Chironomidae have always been under the pressure of both limited mobility and high predation due mainly to their morphological characteristics. Under these circumstances commensalism is considered as a convenient strategy to solve both the mobility and predation problems at the same time. In particular, association with large‐sized hosts is thought to make the chironomid commensal less vulnerable to a variety of freshwater predators which are in general gape limited. The evolutionary path to parasitism seems to have been more difficult in the Chironomidae, with fewer taxa being recorded as parasites than commensals. The parasitic mode of life requires that the benefit of feeding directly on the host's bodily resource exceeds the cost incurred in the form of reduced fitness of the host and hence a reduced host population, the situation that a limited number of ectoparasitic chironomid species appear to tolerate.
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