Development times of Diamesa incallida and its numbers of generations per vear were studied using in situ reanngs in conjunction with continuous collection of egg masses from the field The evidence suggested that the crenophilous D incallida laid eggs throughout the year and produced eight to ten generations per year, depending on water temperature (76°C to 80°C) This is the first proof for a muitivoltine Diamesinae as well as for continuous egg laying behaviour by a non–troprcal chironomid The comparatively large mature larvae (96 mm, 710 μ.g a f d weight) and the numerous generations per year indicate that D incallida is a highly productive component of crenal communities The importance of rearing chironomids over a sufficient number of generations is emphasized as a basis for the reliable interpretation of field data
I. Ash-free dry weights and dry weights of each larval instar were determined for nine chironomid taxa using unpreserved material. Larval weight (w ^g) was related to body length measurements (/ mm) by the equation \n w = \n a + b \n I.2. For a given taxon, the constants In a and b were dependent on larval size and shape, respectively: b was found to increase with larval length, while taxa with characteristically stout larvae yielded higher values for In a.3. Comparison of measurements of freshly-killed chironomid larvae with measurements taken after preservation in 70% ethanol showed no ehange in body shape due to preservation. Length measurements from larvae preserved in this way are therefore suitable for use in the calculation of larval biomass when the relationship to the unpreserved weight is known.4. Maximum larval lengths and mean body proportions (length:width ratios) are reported for thirty-two chironomid taxa.
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1. Streams in the seasonal wet and dry tropics have highly dynamic discharge regimes. Our study stream, located in mid‐western Brazil, drains into the Pantanal, South America’s largest wetlands, and is characterized by frequent spates in the rainy season and several weeks of interrupted flow in the dry season.
2. In order to understand how these seasonal flow patterns influence the aquatic fauna, floating litter was studied because: (i) this habitat is exposed to the current, and thus is likely to be most affected abiotically and biotically by changes in flow; and (ii) this habitat is abundant in unaltered tropical streams. Studies were conducted in a third‐ and a fourth‐order reach. Mayflies were chosen as study organisms because they were frequent and species‐rich on floating litter, and because they should to be responsive to changes in current velocity.
3. In the course of 15 months, covering one rainy and two dry seasons, mayflies showed pronounced seasonal patterns at family and genus level, which were evidently driven by discharge. Two periods of high mayfly densities were observed in the course of one year. One maximum coincided with the peak of the rainy season, the other with the peak of the dry season, and both were distinct in faunistic composition. At high current velocities the leptophlebiid Farrodes sp. was dominant and Leptohyphes sp., Acerpenna sp. and Paracloeodes sp. were frequent. In the dry season, when the river was reduced to isolated standing waters, 86–93% of all mayflies were Caenis sp.
4. Altogether sixteen mayfly genera from the families Baetidae, Caenidae, Leptohyphidae, Leptophlebiidae and Oligoneuriidae were recorded. Two genera are new to science, the baetid Aturbina was recently described (among others, from material from our study river), Acerpenna and Paracloeodes are new records for South America, and Miroculis and Terpides are recorded for the first time south of the Amazon.
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