Figure 4. Capture rate of insects as a function of the Deborah number.

Flies (empty squares), ants (filled squares). The Deborah number (De) is defined as the ratio of the fluid elastic relaxation time λ (see Fig. 3b) to the typical half-period of the swimming stroke of insects in the fluid τ. For each insect category, the capture rate decreases rapidly when De<1, suggesting that trapping occurs when the elastic forces created by the insect's movements have no time to relax.