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wings also increase their visibility to aerial predators. Tropical butterflies are broadly divided into two groups where predator avoidance is concerned. The first group comprises unpalatable species. Defensive chemicals may be sequestered to make them unpalatable, and these species show warning colouration and fly slowly and regularly. Palatable species, on the other hand, fly fast and erratically, which can be energetically expensive (Dudley 2000). The impact of insectivorous birds and bats has thus selected for enhanced flight performance. This has been the subject of some elegant research involving neotropical butterflies, in which predation shapes morphology, flight patterns and thermal physiology (Chai and Srygley 1990; Srygley and Chai 1990). Palatability is assayed by means of the feeding response of an insectivorous bird, the rufous-tailed jacamar (illustrated in Dudley 2000): few butterfly species are intermediate in acceptability to this bird (Srygley and Chai 1990). A phylogeny of butterflies showing the distribution of palatability status is presented by Marden and Chai (1991).

Palatable butterfly species tend to be restricted to times and habitats with high solar radiation, while the reduced demand for rapid flight in unpalatable species favours cooler microhabitats and lower Tth (Srygley and Chai 1990). The proportion of palatable species increases in more open microhabitats, and they show higher activity during the middle of the day, resulting in significantly higher Tth (by 4°C). The lower Tth of unpalatable species is associated with broader spatial and temporal niches. Body mass itself is not correlated with palatability, but fast-flying palatable species have a greater thoracic mass, shorter bodies and shorter wings, and the centre of mass is more anterior, leading to better maneuverability in flight (Chai and Srygley 1990). This increase in flight muscle performance is associated with higher body temperatures. For 53 species of butterflies in the families Papilionidae, Pieridae, and Nymphalidae, Fig. 6.12 shows that palatable species have higher temperature excesses (Tth — Ta) and lower indices of body shape (= ratio of body length to thoracic width) (Chai and Srygley 1990). Body shape was a good predictor of Tth, flight speed, and the response of captive jacamars. In these neotropical butterfly assemblages, Srygley and Chai (1990) also

Body shape

Figure 6.12 Relationship between temperature excess (Tx)

and body shape (ratio of body length to thoracic width)

for 53 species of butterflies of varying palatability.

Source: American Naturalist, Chai and Srygley, 135, 748-765.

© 1990 by The University of Chicago. All rights reserved.

003-0147/90/3506-0001S02.00

Body shape

Figure 6.12 Relationship between temperature excess (Tx)

and body shape (ratio of body length to thoracic width)

for 53 species of butterflies of varying palatability.

Source: American Naturalist, Chai and Srygley, 135, 748-765.

© 1990 by The University of Chicago. All rights reserved.

003-0147/90/3506-0001S02.00

found that thermal characteristics of some mimics resembled those of their unpalatable models. Marden and Chai (1991) measured flight muscle ratios (the ratio of flight muscle to total mass) of 124 species of butterflies and diurnal moths in Costa Rica, and found significant effects of palatability, mimicry, and sex. Some unpalatable Mullerian mimics 'had so little flight muscle that they could just barely counteract gravity'. However, decreasing the flight muscle ratio in two pierid butterflies by adding tiny weights resulted in minor effects on survival, even for the more palatable species (King-solver and Srygley 2000).

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