Carrying unbalanced weights in hovering flight: effect of prey position on load maximization in wasps

With a simple mathematical model, it was recently shown that wasps which carry their load in flight forward their center of gravity suffer a reduction of the theoretical maximum load that can be carried. Here, we 1) implement the model considering the effect of an additional factor (the presence of induced wind), and 2) apply the model to determine the reduction of maximum load suffered by wasps of the genera Oxybelus and Sceliphron, which carry their resource load posteriorly or anteriorly to the center of gravity along the longitudinal body axis. The induced wind has a negligible effect on maximum load, within the approximation level of the analysis. Assuming that the load is located along the wasp longitudinal body axis, in the unbalanced situation (MmaxU) is strongly reduced (of about 75% in Oxybelus or about 45% in Sceliphron) when compared to balanced situation (Mmax). A way to face this severe reduction could be that to increase the angle x which the straight line connecting the wasp with the load centers of mass makes with the horizontal line; for example, at x=80º Oxybelus would suffer a reduction of maximum load of about 40% of Mmax. As a consequence of these results, weight of prey collected by Oxybelus females, on average, resulted lower than the Mmax but heavier than the MmaxU at x=45º, and did not differ from MmaxU at x=80º. This last situation seems suitable to represent the fact that prey is often maintained below the wasp body axis. As an additional result, we estimated, for Oxybelus, a particular high value of the lift coefficient is necessary. This suggests the presence in Oxybelus (but not in Sceliphron) of non-steady aerodynamical effects.