Factors Influencing Fluid Handling

The rate and efficiency with which an insect can transport nectar from the floral nectar reservoir and through its proboscis depends on the physical properties of the nectar solution, the modality of fluid feeding, the geometry of the feeding apparatus, and the dynamics of muscle contraction [25]. Betts [88] was the first to recognize the importance of viscosity in limiting nectar ingestion rates in honeybees, and Baker [89] hypothesized that similar biophysical constraints may have influenced the evolution of the dilute nectars found in hummingbird flowers. Early biomechanical analyses [26,90] employed the Hagen-Poiseiulle relation to describe how the rate of nectar intake, Q, varies with viscosity, proboscis length, L, food canal radius, R, and the driving pressure gradient, P:

One prediction derived from Equation 9.1 is that the nectar intake rate declines linearly as proboscis length increases. Thus, based on this simple analysis, an obvious disadvantage to a long proboscis may be a slower nectar intake rate. Alternatively, long-proboscid insects may compensate for this handicap by developing proportionally larger pump muscles and/or increasing the radius of their food canal. Presently, no published studies have addressed these possibilities, but preliminary data from 33 species of euglossine bees suggest that nectar intake rates decline with tongue length after the confounding effects of body size have been removed [91].

In seeking to maximize their rate of energy intake, insect nectarivores must select from a variety of floral resources. One constraint faced by these foragers is that nectar viscosity increases exponentially with sucrose concentration, and Equation 9.1 tells us that nectar intake rate declines with viscosity. Thus, the rate of energy intake will be maximized at some intermediate concentration (Figure 9.8). Because the pressure drop P varies with fluid properties [92], the position of this optimal nectar concentration will depend on the precise mechanism of force production.

Researchers have identified two primary mechanisms of fluid transport during nectar loading: capillary-based lapping and suction feeding (see Section 9.2). Lapping insects such as ants (on extrafloral nectars [48,93]), bees [42,48-50,93], hummingbirds (Trochilidae), and nectar-feeding bats (Phyllostomidae: Glossophaginae) dip their hairy tongues (or glossae in insects) into the nectar solution whereupon liquid is drawn up via capillary forces and subsequently unloaded internally via "squeezing" or suction from the cibarial pump [25,26,49,94]. Suction feeding, which depends solely on a pressure gradient generated by fluid pumps in the head and along the intestinal tract, occurs primarily in the Lepidoptera, Diptera, and some Hymenoptera (Table 9.2). Many flies use a primitive sponging mode of nectar feeding where nectar is first taken up by the spread labella and later sucked into the food canal. The loading phase of sponging likely depends on both capillary forces and suction pressure generated by the spreading labella.

These two mechanisms of feeding lead to different predictions regarding the value of the optimal nectar sugar concentration [25]. Daniel et al. [24] used A.V.

Sucrose (%) Viscosity (mPas) Sucrose (%)

FIGURE 9.8 Relationships between energy intake rate, nectar intake rate, viscosity, and sucrose concentration. Because viscosity increases exponentially with sucrose concentration (A) and volumetric nectar intake rate declines with viscosity (B), energy intake rates will be maximized at intermediate sugar concentrations (C). Graphs are calculated for a 150-mg insect using the suction feeding model of Daniel et al., Oecologia, 79, 66, 1989.

Hill's classic model of muscle contraction dynamics to describe the behavior of the cibarial pump musculature in butterflies. This model predicted an optimal range of sucrose concentrations between 31 and 39% (sugar weight to total weight) depending on parameter estimates. Empirical studies with eight lepidopteran species and numerous other insects have largely confirmed these predictions (Table 9.4). Remarkably, although proboscis length influences the absolute rate of energy intake for suction feeders (see above), the sugar concentration that maximizes energy flux is predicted to be independent of proboscis length [24].

Using a capillary pressure term to examine the mechanics of lapping by bees, Kingsolver and Daniel [25] predicted that optimal nectar sugar concentrations for lappers should be greater than those for suction feeders. Indeed, maximal energy intake rates for lapping bees and ants are at sugar concentrations nearly 15% (w/w) higher than those for suction-feeding insects (Table 9.4). Because the frequency and amplitude of glossal extension in hymenopterans relies on passive mechanical properties [48], Borrell [31] suggested that as tongue length increases, lapping ceases to be an effective mechanism of fluid transport. One consequence of the evolution of greatly elongated proboscides in the Diptera and Hymenoptera may have been a downward shift in the sugar concentration that maximizes the rate of energy intake.

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