3603 Many animals undergo profound changes in behaviour that adapt them to changing needs and conditions at different stages in their life histories. Neuromodulation of neuronal networks or intrinsic changes in the amounts of neurotransmitters within these networks are two means of effecting such behavioural change, as has been reported to accompany the onset of sexual maturity or reproductive status (Fabre-Nys et al., 1997; Broad et al., 2002), the development and metamorphosis of insects and amphibians (Homberg and Hildebrand, 1994; Kloas et al., 1997; Takeda, 1997; Sillar et al., 1998; Lehman et al., 2000a,b; Consoulas et al., 2000; Mercer and Hildebrand, 2002), and during caste differentiation (Sasaki and Nagao, 2001, 2002) and the division of labour between workers in social insects (Taylor et al., 1992; Wagener-Hulme et al., 1999; Schulz and Robinson, 1999). In the shorter term, neuromodulation by octopamine and serotonin in insects and Crustacea is associated in complex ways with social status arising from agonistic encounters (Kravitz, 2000; Sneddon et al., 2000; Stevenson et al., 2000), and serotonin has an important role in regulating the sensitivity of photoreceptors between night and day (Cuttle et al., 1995; Hevers and Hardie, 1995). Locusts undergo an extreme form of phenotypic plasticity that is driven by population density, which results in extensive but reversible changes in many aspects of morphology, physiology and behaviour (Uvarov, 1966; Simpson et al., The Journal of Experimental Biology 207, 3603-3617 Published by The Company of Biologists 2004 doi:10.1242/jeb.01183 Desert locusts (Schistocerca gregaria) can undergo a profound transformation between solitarious and gregarious forms, which involves widespread changes in behaviour, physiology and morphology. This phase change is triggered by the presence or absence of other locusts and occurs over a timescale ranging from hours, for some behaviours to change, to generations, for full morphological transformation. The neuro-hormonal mechanisms that drive and accompany phase change in either direction remain unknown. We have used high- performance liquid chromatography (HPLC) to compare amounts of 13 different potential neurotransmitters and/or neuromodulators in the central nervous systems of final instar locust nymphs undergoing phase transition and between long-term solitarious and gregarious adults. Long-term gregarious and solitarious locust nymphs differed in 11 of the 13 substances analysed: eight increased in both the brain and thoracic nerve cord (including glutamate, GABA, dopamine and serotonin), whereas three decreased (acetylcholine, tyramine and citrulline). Adult locusts of both extreme phases were similarly different. Isolating larval gregarious locusts led to rapid changes in seven chemicals equal to or even exceeding the differences seen between long-term solitarious and gregarious animals. Crowding larval solitarious locusts led to rapid changes in six chemicals towards gregarious values within the first 4·h (by which time gregarious behaviours are already being expressed), before returning to nearer long-term solitarious values 24·h later. Serotonin in the thoracic ganglia, however, did not follow this trend, but showed a ninefold increase after a 4·h period of crowding. After crowding solitarious nymphs for a whole larval stadium, the amounts of all chemicals, except octopamine, were similar to those of long-term gregarious locusts. Our data show that changes in levels of neuroactive substances are widespread in the central nervous system and reflect the time course of behavioural and physiological phase change. Key words: desert locust, Schistocerca gregaria, phase transition, HPLC, solitarious, gregarious, polymorphism. Summary Introduction Substantial changes in central nervous system neurotransmitters and neuromodulators accompany phase change in the locust Stephen M. Rogers 1,2, *, Thomas Matheson 1,† , Ken Sasaki 1,‡ , Keith Kendrick 3 , Stephen J. Simpson 2 and Malcolm Burrows 1 1 Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK, 2 Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK and 3 Laboratory of Cognitive and Developmental Neuroscience, Babraham Institute, Babraham, Cambridge CB2 4AT, UK *Author for correspondence (e-mail: Smr34@cam.ac.uk) † Present address: Department of Biology, University of Leicester, University Road, Leicester LE1 7RH, UK Present address: Human Information Systems, Kanazawa Institute of Technology, 3-1 Yakkaho, Matto, Ishikawa 924-0838, Japan Accepted 12 July 2004