Apart from the classical work of SEIFERT (1930, 1932) (quoted by FRAENKEL & GUNN (1961) and others) in which it was shown that Triops granarius LUCAS, Chirocephalus and Artemia spp . always turn their dorsal or ventral sides respectively towards the light when swimming freely, little is known of the orientation responses of the Notostraca and Anostraca . RzbSKA (1961) mentions that, despite a high rate of evaporation resulting in considerable cooling, the temperature of the tropical rainpools in which these animals live may become as high as 41°C (106°F) . Yet it has been found experimentally that T. granarius, a dominant member of such pools in the Sudan, cannot survive for long in water at temperatures above a maximum of 40°C (104°F) whilst the lethal temperature over a period of 24-hours is as low as 34°C (93 °F) (CLOUDSLEY-THOMPSON, 1965). Survival must, therefore, depend upon orientation responses by which the animals find a cooler microclimate in deeper water, or in the mud beneath .
It was not possible to confirm this by field observations, because the desert pools containing Triops and other Branchiopoda are invariably extremely muddy and the activity of the animals themselves keeps a quantity of silt in suspension, even when the wind drops sufficiently to allow uninhabited pools to clear . As the animals can normally be seen, therefore, only when they come right up to the surface of the water, I decided to carry out some simple laboratory