Eggs of the mosquito Culex pipiens pipiens L. float vertically because a flexible corolla at the anterior (lower) pole has a hydrophilic lower surface and a hydrofuge upper surface. The corolla assumes different configurations in the egg raft; the surface forces operating on eggs are briefly analysed. The outer surface of the chorion is hydrofuge, but in eggs floating on water there is jacket of water surrounding the egg in the spaces between the tubercles of the exochorion, below the meshwork connecting them. The humidity over natural water can be low enough to cause continuous evaporation from the jacket; water is replenished through openings within the lower surface of the corolla. The disposition of the corolla prevents the uptake of materials confined to the water surface outside, but the mechanism could concentrate solutes in the jacket, with possible deleterious consequences. Eggs are joined together by inter-digitation of the regularly spaced tubercles. The behaviour by which the female assembles the egg raft is described in detail, explaining the form of the raft and the patterns in which eggs are packed.
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