PHASE TRANSFORMATION IN LOCUST BIOLOGY
Authors: KENNEDY J. S. 
Published: January 2008

Uvarov's phase theory of locust plagues stated, first, that these insects were continuously variant physiologically and morphologically, in both sexes and at all stages, between two extreme types orphases: solitariawhich led a solitary sedentary life and was produced when the individuals were dispersed, andgregariawhich was highly gregarious and active and was produced when the population was dense; and, secondly, that this reversible process of phase transformation provided a sufficient explanation of the intermittency of plagues.

The first point is generally accepted with some qualification. The second point has been the object of mounting criticism and is now generally rejected; this appears to be justified in so far as the previous emphasis on phase change meant neglect of the more fundamental question of changes in numbers. The connected claims that phase change is but a secondary concomitant of locust outbreaks, that the only relevant aspect of it is the behavioural change, and that even that change is merely quantitative, do not appear to be justified.

The phase differences in post‐embryonic development, and in adult morphology and metabolism, suggest a greater influence of the corpus allatum (juvenile) hormone insolitariathan ingregariathroughout life, for which there is some direct evidence from implantation experiments. This provides the starting point for an hypothesis of the nature of phase variation: that phasesolitariais a more juvenile andgregariaa more adult type, in terms of morphology, physiology (including behaviour) and ecology.

It is further suggested that the main difference between adults and juveniles, among terrestrial animals in general, is that adults find growth resources, while juveniles exploit them. The adult has more elaborate sensor i‐motor equipment and spends accumulated resources in more activity, while the juvenile is more vegetative. On this view, finding the appropriate mate and oviposition site are strictly adult functions, but the development of the sexual products is a functional reversion toward juvenility. Thus there is evidence that phasegregarialocusts are more adult and less vegetative thansolitaria, not only behaviourally and morphologically but also in their lesser fecundity and longevity.

Change of phase towardgregariais brought about by mutual nervous stimulation. Visual, mechanical and chemical stimuli all contribute among fully gregarized individuals, but their relative importance during the process of gre‐garization is in doubt. The nature of the central nervous process in gregarization is also in doubt; conditioned reflex formation has been suggested. The result of it is to accentuate adult characters as defined, including locomotor activity, and strikingly to concert not only locomotor responses but also other processes such as pigmentation, sexual maturation, mating and oviposition, among the individual insects.

Inheritance of the effects of mutual stimulation has been demonstrated in both physiological and morphological characters; the egg stage being no barrier but rather a key point of divergence. Within one generation the effects are considerable but limited, and more than one generation is needed to complete gregarization.

The two phase extremes differ only quantitatively in respect of the isolated components of their behaviour such as the tendency of hoppers to march or of adults to embark on long migratory flights. But these responses are weak insolitariaand usually overridden by opposing ones to extraneous stimuli, while they dominate the behaviour ofgregaria.Thus the resultant behaviour patterns of the two differ qualitatively.

Water is a limiting and variable factor in the ecology of locusts, and their habitats are patchy and ephemeral. The two phases of one species may exhibit similar seasonal cycles of geographical redistribution, travelling on the wind between adjacent regions offering complementary wet and dry seasons. Within one such region thesolitariahabitat offers the most favourable conditions for multiplication within a small compass. Thesolitariamode of life is well adapted to exploit this, as thegregariais not.

Given sufficient multiplication within thesolitariahabitat, the return of unfavourable conditions can entrain mutual stimulation and gregarization to the point of swarming out into the largergregariahabitat. There multiplication cannot proceed at the high rates possible inside thesolitariahabitat, but the active, opportunistic mode of life of thegregarialocusts in swarms appears to be well adapted to their habitat, since they can maintain themselves even in regions where thesolitariahabitat does not occur at all. Eventually the swarms dwindle and disperse, but individuals from them revert to phasesolitariaand re‐colonize its habitat.

The outstanding peculiarity of locusts, it is concluded, is that they veer between two forms, two modes of life and two habitats, in response to the vagaries of their environment. The intermittent plagues cannot be explained by that fact alone but neither can they be explained without it.