Selection for evasive mimicry imposed by an arthropod predator

It has long been hypothesized that a species that is relatively easy to catch by predators may face selection to resemble a species that is harder to catch.  Several experiments using avian predators have since supported this “evasive mimicry” hypothesis.  However, the sudden movement of artificial evasive prey in each of the above experiments may have startled the predators, generating an avoidance response unrelated to difficulty of capture. Additionally, in the above experiments, the catchability of prey was all or nothing, while in nature predators may occasionally catch evasive prey or fail to catch slower species, which might inhibit learning. Here, using mantids as predators, we conducted an experimental test of the evasive mimicry hypothesis that circumvents these limitations, using live painted calyptrate flies with modified evasive capabilities as prey. We found that mantids readily learned to avoid pursuing the more evasive prey types. Warning signals based on evasiveness and their associated mimicry may be widespread phenomena in nature.  These findings not only further support its plausibility but demonstrate that even arthropod predators can select for it.

Methods
Training phase
The training phase of our experiment involved repeatedly presenting two similar-sized flies, one with thorax painted black and one with thorax painted white, to individual mantids (Tenodera sinensis). The presentations (seven in total) were made every second day over a period of two weeks. The two presented flies were clipped and painted as part of a 2 (Black B or White W) x 2 (Clipped C or Not Clipped N) treatment design i.e. only the white fly was clipped (BN/WC; treatment 1), only the black fly was clipped (BC/WN; treatment 2), both flies were clipped (BC/WC; treatment 3), neither fly was clipped (BN/WN; treatment 4). Each mantid only experienced a single training treatment and there were 20 mantids per treatment. 

Presentations of flies to mantids took place in sequential order based on treatment, so that flies were first given to a mantid in treatment 1, then a mantid in treatment 2, then 3 and 4.  After setup, the flies and mantids were checked every 20 minutes over a three-hour duration. Once a mantid had caught one fly, the remaining fly was removed. If no flies were caught after three hours both were removed. Since it takes mantids of the size (L3-L4) used in this experiment more than 20 minutes to consume a fly, the mantids were never able to consume both flies between consecutive checks. Note that failed capture attempts (or the precise timing of successful captures) were not recorded in the training phase, only the fly type that was successfully caught. 

Test phase
The test phase was conducted once for each mantid and involved presenting unclipped (BN/WN)  yet tethered flies and observing which colour of fly, if any, was first attacked. The mantids were tested following the same sequential order of treatment as above and monitored in the same manner as in the training phase.