Group Effects and Development

Group effects (discussed in Chapter 8) can have a profound effect on the developmental trajectory of juvenile cockroaches and are known from at least three families of Blattaria (Table 8.3). Nymphs deprived of social contact typically have longer developmental periods, resulting from both decreased weight gain per stadium and increased stadium length (Griffiths and Tauber, 1942b; Willis et al., 1958; Wharton et al., 1968; Izutsu et al., 1970; Woodhead and Paulson, 1983). In P. americana, nymphs isolated at day 0 are one-half to one-third the size of grouped nymphs after 40 days (Wharton et al., 1968).The effect is cumulative, with no critical period. It occurs at any stage of development and is reversible at any stage (Wharton et al., 1967; Izutsu et al., 1970). Respiration of isolates may increase, and new proteins, expressed as electrophoretic bands, may appear in the hemolymph (Brossut, 1975; pers. comm. to CAN). The physiological consequences seem to be caused by a lack of physical contact (Pettit, 1940; Izutsu et al., 1970) and the presence of even one other individual can ameliorate the effects (Izutsu et al., 1970; Woodhead and Paulson, 1983). The means by which tactile stimuli orchestrate the physiological changes characteristic of the group effect in cockroaches is unknown. In termites, as in cockroaches, the physical proximity of conspecifics significantly increases the longevity and vigor of individuals, with just one nestmate as sufficient stimulus. This "reciprocal sensory intimacy" is thought to play a key, if unspecified, role in caste determination (Grassé, 1946; Grassé and Noirot, 1960).

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