1. The results of experiments in a temperature gradient showed a definite temperature ‘preference’ on the part of hoppers (nymphs) of all stages. This ‘preference’ was constant from instar to instar but varied with the preconditioning temperature.
2. The rate of movement of first-instar hoppers was shown to increase in a linear manner with temperature up to 25° C., above which the rate fell off. It is suggested that these are quantitative data supporting Kennedy's (1939) remarks on negative thermokinesis.
3. Experiments in 12 l. cages showed that group formation depends on a patchy temperature field rather than on any particular temperature, and that environmental conditions are more important than mutual responses of the hoppers. Hoppers less than 3 days old, as well as older ones, formed groups under the conditions of patchy temperature.
4. The experiments suggested that surface temperatures are more important than air or body temperatures in the initial formation of groups.
5. Basking groups induced by local radiant heat in a large cage did not differ in form from the groups in the 12 l. cages formed in the absence of radiant heat.
6. Surface texture was shown to be unimportant in group formation, hoppers always collecting on the hotter surface even when temperature differences were of the order of only 1° C.
7. The groups were shown to be in a very dynamic state, with hoppers continually coming and going. The average time spent in a group by any one hopper was 6 min. 46 sec.
8. Formation of basking groups in the field depends on the physiological state of the hoppers, rather than on any definite temperature.