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Knapp and Casey 1986). All the factors influencing the thermal relations of caterpillars—behaviour, setae, colour, aggregation, and tent making (Casey 1993)—converge in M. americanum. Caterpillar setae increase the effective diameter of the animals and reduce convective but not radiative heat transfer (Casey and Hegel 1981; Kukal et al. 1988). Dark colouration enhances heat gain during communal basking. Silk tents retard heat exchange and facilitate the clumping of caterpillars to form a composite body with a reduced surface area to volume ratio. A dramatic example of the latter benefit is seen in the anomalous emperor moth, Imbrasia belina (Saturniidae), which increases in body mass from 4 mg to 16 g during larval development (Klok and Chown 1999). Aggregations of instars II-III maintain similar Tb to solitary individuals of instars IV and V, and their Tb is much higher than operative temperatures of single models of the early instars. The caterpillars do not bask, but the increase in their effective body mass enhances heat gain (see also Stevenson 1985), as well as decreasing water loss (Section 4.4.2). Bryant et al. (2000) compared the thermal ecology of gregarious and solitary nettle-feeding nymphalid butterfly larvae: exposed gregarious larvae of two species (Aglais urticae and Inachis io) maintained high and regulated Tb, while the concealed larvae of two other species were essentially thermocon-formers. The incorporation of larval thermoregulation into models of development time allowed these authors to estimate that thermoregulation extends the range of A. urticae and I. io in Britain by 200 km northwards (Bryant et al. 2002). Whether thermoregulatory patterns, and the other physiological benefits of aggregation such as reduced water loss (see Chapter 4), are a cause or a consequence of the evolution of gregariousness is not entirely clear. Gregariousness in caterpillars probably evolved as a consequence of the advantages of egg clustering (Courtney 1984; Clark and Faeth 1998), with physiological benefits in the caterpillars perhaps maintaining this behaviour (Klok and Chown 1999). However, several other hypotheses have been proposed to explain caterpillar aggregations, including predator avoidance (Sillen-Tullberg 1988) and the advantages associated with group feeding (Fitzgerald 1993; Denno and Benrey 1997).

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