1. The egg, vermiform larva, and the nymphal stages of ph. gregaria are described. It is shown that six stages occur, or seven instars including the vermiform larva. These stages can be distinguished by (a) the number of antennal joints, (b) the degree of backward prolongation of the pronotum, (c) the differentiation of the terminal abdominal structures. Measurements of the pronotum and femur served to distinguish the stages in material from the same swarm, but the absolute measurements cannot be used to distinguish material from different sources.

2. The colour pattern of ph. dissocians is analysed, and colour changes are indicated as divergences from the gregaria pattern. Forms of ph. dissocians observed in the field are grouped into several types the relationships of which are illustrated, and the environmental factors which produce them are discussed.

3. Hoppers believed to be of ph. congregans could not be distinguished from certain forms of ph. dissocians.

4. Cage experiments in which hoppers were reared under varying degrees of crowding are described. It is strongly indicated that activity which is induced by crowding influences the development of gregaria colouring. The green colouring of dissocians hoppers appears to be associated with high humidity conditions and the presence of green food. Other colours of dissocians hoppers are conditioned by those of the environment.

5. An aberrant pallid type of hopper is described and its origin is ascribed to the effects of parasitism by Nematodes and possibly also by Dipterous larvae.

6. Certain inferences regarding the processes in the formation of the colour pattern are inferred from a consideration of the effects of parasitism and of dissocians producing factors.

7. The colourings of young adults of ph. gregaria a few days after the ultimate moult and of types taken from large swarms in August are described. It is shown that colour changes occur gradually in the field, the conspicuous pattern of light and dark coloration of the young adult being obscured and finally hidden by the development of a bright red pigmentation.

8. The colouring of young adults of ph. dissocians is described. These show a pattern of light and dark coloration similar to that of the young adult of ph. gregaria, from which it is indistinguishable, immediately after the ultimate moult. In ph. transiens no marked colour change takes place throughout the life of the individual except the development of a purple coloration on the hind wings.

9. Factors influencing adult colouring are discussed. Adults crowded in cages and derived from hoppers reared under crowded conditions did not undergo the colour changes observed in the field. Material taken from large swarms in the field and showing some development of swarm colouring were maintained under crowded and uncrowded conditions. The red pigment of the body changed to a dark brown in both cases. Under uncrowded conditions no further change took place except a conspicuous development of purple coloration on the hind wings in all individuals. Under crowded conditions the brown pigment spread, its final distribution resembling that of the red pigment in later swarm types from the field, but the red colouring of the hind wings was developed only to a very slight extent in a few individuals. In the case of individuals isolated completely the swarm colouring was lost entirely.

10. It is inferred that swarm colouring in the adult is associated with crowding and possibly with activity and that development of the red and purple coloration on the hind wings in gregaria and transiens is affected by different factors and possibly differs in composition.

11. Biometrical data obtained from a variety of types is tabulated and discussed. Material varied from types taken from very large and dense swarms to what might be described as “solitaria beginning to congregate.”

12. It is shown that certain measurable characters, especially the relative size of the sexes, the degree of development of the femur in relation to the wing, and the relative length and degree of constriction of the pronotum differ greatly in gregaria and transiens types and may be used to distinguish them.

13. In the case of material from very large and dense swarms and from very small loose swarms no differences in the characters on which the types could readily be distinguished were observed. It may be that these small groups were derived from larger swarms.

14. Interesting aberrations occur in the case of a swarm believed to have been one of the earliest to migrate any considerable distance after the commencement of swarming. The quantity of material was insufficient to permit of very reliable comparison but it would appear that in these forms the pronotum of the female was generally not shortened or constricted to the same extent as in extreme gregaria, although the construction of the pronotum of the male approximated fairly closely to that of more extreme gregaria