The effect of air resistance on the jumping performance of insects.

A spring gun was constructed to propel objects at known velocities of between 1 and 4.5 m.s-1. This was used to project insects and various models in a vertical trajectory. By comparing the height attained in air by the insects or models with the height theoretically possible in vacuo, the energy lost against air resistance was observed. Small insects have a higher frontal area to mass ratio than larger ones so have relatively more aerodynamic drag and attain lower heights. The observed effect may be expressed in terms of the drag coefficient, CD. Fleas and locusts have CD of about 1. Winged flies have CD of about 1.5 which falls to about 1 when the wings are amputated and to about 0.8 when the legs are amputated. Aptery is advantageous in jumping insects. From experiments with models, it appears that the optimal condition for small jumping insects is that the body should be as compact as possible to reduce the frontal area to mass ratio. Thus dense spherical bodies are favoured. Some species of jumping insect have densities of about 1 mg.mm-3 while some flying beetles and flies have densities between 0.3 and 0.8 mg.mm-3. The Reynolds number at which the experiments were performed was from 65-205 for fleas up to 740-2340 for locusts. The models operated in similar ranges. At a velocity which would propel a larger animal to a height of 1 m, fleas weighing 0.4 mg only reach about 0.4 m. At lower initial velocities, proportionately less energy is wasted against air resistance so the jump efficiency is higher. Most fleas jump to a height of about 0.1 m with an efficiency of 0.8 while locusts jump to a height of 0.35 m with an efficiency of over 0.9. Air resistance is thus an important scale effect in jumping insects and provides its own design constraints.