Figure 3.18 Fluctuations in thoracic temperature and oxygen consumption during warm-up and flight in the carpenter bee, Xylocopa capitata (Hymenoptera, Anthophoridae).

Source: Nicolson and Louw. Journal of Experimental Zoology 222. © 1982. Reprinted by permission of Wiley-Liss, Inc., a subsidiary of John Wiley & Sons, Inc.

juveniles of holometabolus species). Many adult insects such as all worker and soldier ants, and all flightless species have little other choice. Ballooning is open to small caterpillars, and passive dispersal by water or by wind is occasionally possible for small, desiccation-resistant, or hydrophobic species (Coulson et al. 2002). For the rest, pedestrian locomotion is the only alternative to flight. It is perhaps surprising then that pedestrian locomotion and its costs have been investigated in such a small range of species and stages: worker ants (Lighton and Duncan 2002 and references therein), the caterpillars of a moth species (Casey 1991), larvae and adults of a calliphorid fly (Berrigan and Lighton 1993, 1994), the adults of a large tropical fly (Bartholomew and Lighton 1986), and a few cockroach and beetle species (Herreid et al. 1981; Bartholomew and Lighton 1985; Lighton 1985; Kram 1996; Rogowitz and Chappell 2000). Moreover, at least some of these studies have been undertaken using treadmills and 'motivational hardware' (Lighton et al. 1987) that might result in inaccurate estimates of transport costs (Section 3.1).

The costs of pedestrian locomotion are much lower than those of flight (Berrigan and Lighton 1994; Roces and Lighton 1995), but can still be very high in limbless stages and those that rely on a hydraulic skeleton (Casey 1991; Berrigan and Lighton 1993) (Fig. 3.20). Costs of transport vary both with body mass and with speed of locomotion although the form of this variation is controversial. Costs of transport can be calculated in three ways. Gross cost of transport is the metabolic rate (hereafter MR) of an insect walking (or crawling) at a given speed and is clearly dependent both on SMR and locomotion speed. Net cost of transport is equivalent to MR-SMR, and is also dependent on speed. However, Taylor et al. (1970) showed that the costs of transport decline with increasing speed, eventually reaching a constant value, which they argued is equal to the slope of the regression relating mass-specific metabolic rate (ml O2 g"1 h"1) and running speed (kmh"1). This theoretical value is the minimum cost of transport (Mmn or MCOT) and is independent of SMR and speed, at least at the highest running speeds. Whether these speeds are actually reached by insects is not clear, but seems unlikely in cockroaches (Herreid et al. 1981).

Ln mass

Figure 3.20 The allometry of minimum cost of transport in insects, showing elevated costs of transport in limbless species. Source: Berrigan and Lighton (1993).

Ln mass

Figure 3.20 The allometry of minimum cost of transport in insects, showing elevated costs of transport in limbless species. Source: Berrigan and Lighton (1993).

Nonetheless, MCOT is now widely used for examining the costs of transport in insects, as it is in vertebrates, and is commonly used when the scaling of these costs is investigated.

In endothermic vertebrates, MCOT scales negatively with body mass either as Mmn = 8.46 M"0 40 (Taylor et al. 1970), or Mmn = 3.89 M^0 28 (Fedak and Seeherman 1979). Whether the minimum costs of transport of insects conform to this relationship has not been resolved. Early studies suggested that this is indeed the case (Herreid et al. 1981; Lighton 1985; Lighton et al. 1987), while later work has indicated that, at least in ants, minimum costs of transport are much lower than the vertebrate relationship would suggest (Lighton and Feener 1989; Lighton et al. 1993d; Lighton and Duncan 2002). Indeed, Lighton and Duncan (2002) conclude that estimates of MCOT based on vertebrate scaling equations yield several-fold overestimations for insects. Thus, at present, and in the absence of a broader range of data, it seems safest to conclude that minimum costs of transport are lower in insects than in endothermic vertebrates, but that these costs vary significantly with the form of locomotion adopted.

In ants, load carriage generally results in a reduction in running speed and an elevation of the costs of transport (Lighton et al. 1993d; Fewell et al. 1996; Nielsen 2001). In some species, the costs of load carriage are equivalent to carrying an extra amount of body mass (i.e. internal and external loads carry the same energetic price) (Bartholomew et al. 1988; Nielsen 2001), whereas in others external loads are less costly (Lighton et al. 1993d). The ways in which loads are carried also have an effect on both the cost and efficiency of carriage. This is nicely illustrated in a comparison of army ants and leafcutter ants. Workers of the former species carry light, energy-rich loads slung between their legs and load costs are low relative to those incurred by leafcutter workers, suggesting that this load carriage method is mechanically more effective. However, as the load ratio increases, the costs of carriage relative to SMR increase more rapidly in the army ant workers than in the leafcutters, making heavier loads relatively less expensive for the latter, whose long legs help them to balance bulky loads (Feener et al. 1988). Thus, the way in which loads are carried is likely to have a marked influence on relative leg length. Indeed, this relationship might be much more important in determining the relationship between body size and leg length (Nielsen et al. 1982), than the costs of long leg length for movement through rugose environments as predicted by the size-grain hypothesis

(Kaspari and Wieser 1999). To date, there has been no examination of costs of movement by different sized ants through planar and rugose environments, and field tests of ant body lengths in environments differing in rugosity are equivocal (Yanoviak and Kaspari 2000; Parr et al. 2003).

The costs of foraging, in relation to the benefits, are central to foraging theory, and range from negligible in some ants such as seed harvesters to values that are comparable to the costs of high energy input foragers such as bees (Fewell et al. 1996). These benefit to cost (B / C) ratios have strong effects on foraging strategy. Ants with low B/C ratios are generally sensitive to foraging costs and vary their behaviour in response to foraging costs, whereas those with high B/C ratios choose strategies that maximize net foraging gains per unit time. These strategies can be thought of as those which maximize efficiency and those that maximize power. They are closely linked to habitat conditions and resource type, and have a considerable influence on reproductive strategy (Fewell et al. 1996).

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