The emergence of midges (Diptera : Chironomidae) from a wet gravel-pit.
Emerging adult chironomids were captured from a wet gravel-pit in the Midlands in 1975 and 1976. The emergence period in 1976 was earlier than in 1975 and this change was attributed to the higher temperatures during the early part of 1976. Forty-seven species were identified of which twenty-one were found in both years. However, only eighteen species represented more than 5% of the monthly emergence from any one trap. The timing and location of emergence of the common species is discussed in relation to the physical parameters of substratum, depth of water and vegetation at each trap site. Similarity indices were used to compare samples from each trap site and revealed a progressive change in the chironomid assemblage with depth, caused by a succession of species and a change in the structure of the assemblage with ‘equitability’ decreasing with increasing depth. The predominant species in 1975 were Cricotopus sylvestris, C. intersectus and Procladius choreus with a number of Chironominae present in both May and June. A trap in the littoral zone caught maximum numbers earlier than traps in open water. In 1976 the above species were still predominant and Tanypus punctipennis and T. vilipennis were also numerous. The Chironominae became proportionally more abundant as the season progressed. The abundance of emerging midges was low compared with other surveys due to the restricted emergence period, imagines being abundant only in June, and also due to the low numbers emerging.