Ants are fierce predators. They move in their hundreds, ruthlessly searching for prey and devouring anything in their path. When attacked, they can swarm the offending predator, often overpowering it with their strength and numbers. Ants are therefore risky prey – but is this something that other invertebrates can exploit to their own advantage?
Spiders have their own suite of adaptations to avoid predators. They can run fast, jump high and even self-amputate their own legs to escape, and some species do something even cleverer: they disguise themselves as ants. They wave their forelegs around like antennae and even their thin, tapered bodies are shaped like those of ants. It has the desired effect; most predators mistake them for ants and leave them alone. Some of these myrmecomorphs – ant-mimics – even move like ants. Spiders run in a stop–start fashion, turning on the spot, whereas ants typically move continuously, changing direction while walking. Yet the degree of similarity between the movements of these spiders and the ants that they mimic remained untested until Ximena Nelson and Ashley Card, from the University of Canterbury in Christchurch, New Zealand, decided to investigate.
The team compared the locomotion of four species of Myrmarachne ‘ant spiders’ with that of ants and some non-ant-like spiders from the same local habitat. By filming them walking and analysing the movements, Nelson and Card confirmed that the movements of the ant-mimic spiders were remarkably similar to those of ants, with longer bouts of continuous movement in contrast to the stop–start motion of their non-ant-mimic cousins.
As this ant-like movement appeared to be so accurate, the authors surmised that locomotion must be an important part of the mimicry. To test this, they created two animations of the ant-mimic spiders – one moving like a spider and the other like an ant – and showed them to some predatory spiders to measure their responses. Ideally, the team would have liked to investigate the benefits of the movement mimicry on a predator that dislikes ants, such as Portia fimbriata, a predatory jumping spider, but they are so wary of anything that even resembles an ant that they won't even approach an ant-like animation, making it impossible to test their reactions. Instead, the team presented the animations to a local predator that specialises in eating ants – another jumping spider, Sandalodes bipenicillatus – to see whether it would be fooled. The reactions of these spiders were pretty conclusive; they spent more time looking at the animation with ant-like movement, and were more likely to visually track and attempt stalking than when shown the animation with spider-like movement. So, it seems as though the mimicked motion of the Myrmarachne is enough to fool the specialist ant predator, suggesting that it would also be a convincing enough disguise to protect them from attack by species such as P. fimbriata.
In some ways, these results raise more questions than they answer. Previous work has shown that Myrmarachne’s physical resemblance to ants is enough to deter ant-hating predators. So if these spiders mimic ants so accurately that they risk being eaten by specialist ant predators, why do it? Nelson and Card suggest that it could be due to the way predators size up their prey, simultaneously processing multiple characteristics such as number of legs, colouration and movement. Depending on experience, the predator may require more of these characteristics to tick the ‘ant-like’ box for them to be convinced by the disguise, possibly leaving these ant-mimics in the clear. This work also emphasises the need to understand more about how invertebrate predators identify their prey, which in turn would help researchers learn how these adaptive traits came to be. However, in the meantime, we must doff our caps to these spidery masters of disguise.