Abstract
The water snipe fly, Atherix ibis, is known for its eggbearing females aggregating in clusters. They are most frequently found as flat crusts under the outer parts of bridge ceilings above the water. There is no persistent pattern in the cluster distribution either in relation to the current direction or to the sun. Females aggregate in early summer in short periods of warm and calm weather only. Females are attracted to an aggregation site, probably by pheromones. The flies fasten to the surface and to each other by minute Velcro-like structures on their feet. The eggs are shed inside the cluster. After 9–12 days the eggs hatch, and the larvae leave clusters and drop into the water. There is no increased temperature inside the cluster. There are no signs of larval feeding on either dead flies or siblings in the cluster. The majority of larvae leave the cluster during night time, also when the normal day–night rhythm is eliminated by experimental black-out.
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