SUMMARY.1.Epoicocladius flavens(Diptera: Chironomidae) has a 1‐year life cycle in the River Tud in eastern England. Pupation and adult emergence occurred from the end of May until the beginning of September. Larval growth was rapid during the summer but thereafter almost ceased until the following April.1 Unlike other chironomids, the population density ofE. flavenswas very stable and remained at around 1000m−2all through the year.2.Ephemera danica(Ephemeroptera: Ephemeridae), the host ofE. flavens, has a 2‐year life cycle. The proportion infested was about 90 for the older host year‐class and less than 30 for the younger year‐class from autumn until the following summer. A rapid increase in infestation occurred during the summer as a result of hatching of theE. flavenseggs. Of those infested, the older year‐class hosts harboured two or three larvae, on average, whereas the younger ones had only one.3. When host year‐classes were treated separately, the frequency distribution of the number ofE. flavensper host each month did not depart significantly from a Poisson distribution. Younger instars departed from a Poisson distribution more frequently than older instars.4. The majority ofE. flavenslarvae attached to the gills of their hosts. As the instars progressed, increasing proportions were found on the thorax, legs and tails. The thorax was the preferred site for fourth instar larvae.5. Enhanced chance of survival, rather than the supply of detrital food particles which collect on the host's body, is considered as an important benefit to accrue toE. flavensfrom this
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