The desert locusts exhibit extreme density-dependent phase polyphenism (Uvarov 1966, Pener 1991). They usually exist as scattered individuals within the recession area and such individuals do not cause much damage. Solitarious nymphs are green and actively avoid each other and adults are pale gray or beige when immature, with males becoming pale yellow upon sexual maturation (Steedman 1990). When ecological conditions favor an increase in population density, locusts change their behavior to be gregarious and form a cohesive group. Concentration of populations is shown to be affected by local vegetation structures (Collet et al. 1998, Despland and Simpson 2000) . Nymphs develop conspicuous black and yellow coloration. Gregarious adults are bright pink and become bright yellow upon sexual maturation. Morphometric ratios change as a result of crowding (Uvarov 1966). Nymphs form large bands that march, roost, and feed cohesively. Adults form large swarms that can travel a long distance. Swarms move in a downwind direction. Swarms fly by day and settle on vegetation at night. Sexually mature adults exhibit group mating and group oviposition, that result in a large egg-bed.
Click 'play' button on the right to see how gregarization occurs.

Schistocerca gregaria is one of the most well-studied organisms in terms of the effect of rearing density in laboratory settings. When solitarious nymphs are crowded, they begin to exhibit gregarious behavior within four hours (Roessingh and Simpson 1994) . Among all possible sensory cues, tactile stimuli have the strongest effect in shifting phase (Roessingh et al. 1998) . Gregarious behavior can be induced by stimulating mechanosensory receptors located on the outer surface of hind femur (Simpson et al. 2001) . When crowing persists, the chemical composition of nymphal pheromone changes, which keeps the group cohesive (Obeng-Ofori et al. 1993, Obeng-Ofori et al. 1994) . Upon crowing, black pigmentation develops, which may a result of [His7]-corazonin expression (Tawfik et al. 1999) . Background color also changes from green to yellow within an instar and a full color change is visible after molting. Morphometric ratios may take several generations to change from solitarious to gregarious ratios, but it is shown to be affected by [His7]-corazonin (Hoste et al. 2002, Maeno et al. 2004) . The rate of sexual maturation increases upon crowding (Norris 1952) . The phase status is transmitted across generations. Ovipositing females can determine the phase status of offspring based on the density at mating and oviposition by producing a gregarizing factor probably from accessory glands via egg foam (McCaffery et al. 1998) . Phase expressions are reversible and not always coupled. Thus, it is possible to observe solitarious-looking locusts behaving gregariously and vice versa.


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Tawfik, A. I., S. Tanaka, A. De Loof, L. Schoops, G. Baggerman, E. Waelkens, R. Derua, Y. Milner, Y. Yerushalmi, and M. P. Pener. 1999. Identification of the gregarization-associated dark-pigmentotropin in locusts through an albino mutant. Proceedings of the National Academy of Science of the United States of America 96: 7083-7087.
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