The influence of substrate organic content on the growth of a stream chironomid

Stream detritus and Tipula (Diptera) feces were wet-sieved to a 63–250 µm particle size range and mixed in three proportions with ashed sand of the same size to yield food substrates of varying quality. Growth of fourth-instar Stictochironomus annulicrus (Townes) (Diptera: Chironomidae) larvae was used to indicate whether the more nutritious particles were selected from these mixtures. Respective mean relative growth rates on the substrates, in order of decreasing percentage organic matter (i.e. food quality), were: 0.0229, 0.0104 and 0.0004 mg·mg^–1d^–1. Replicates of the relatively high quality substrate consistently elicited the highest growth rates; animals grew at intermediate rates on medium quality, and slowest on low quality substrates. Thus, this collector does not appear to have selected for food quality under the described experimental conditions.