Spiders and the ant-walk
(appeared in July 2017)

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It is not only how you look that counts, how you move is important too, says S.Ananthanarayanan.

When living things wish to be left alone, they find it useful to look like something unappetising or something that is likely to hurt. Looks, however, extend beyond colour and shape. Things are also recognised by how they get around.

There are many examples in nature where species which have no defences against predators have evolved to resemble related species that do. In most examples, however, it is through colour or skin patterns that the mimic species advertise similarity to another species. Paul S. Shamble, Ron R. Hoy, Itai Cohen and Tsevi Beatus from Cornell University, USA, describe in their paper in the Proceedings of the Royal Society, an assessment of a species of spider which makes use of features of movement, rather than colour, to pass off as a substantially dissimilar animal.

The evolution of animals to share features with other species which predators have reason to avoid was first studied by Henry Walter Bates, a British scientist who worked with Amazonian Butterflies. The Heliconid butterflies of the Amazon, also known as the Passion Flower butterflies, live in groups and shelter from the rain in shrubs of the Passion Flower. This plant has toxic leaves which keep it safe from insects. But the caterpillars of the Heliconid have developed resistance and use the toxins in the leaves to make the butterflies themselves poisonous to eat! Snacking on a Heliconid leads to such discomfort that those who have had a taste steer clear thereafter.

What Bates found more interesting is a related butterfly species which does not have this kind of protection. The related species has evolved to have wing shape and markings deceptively similar to the Heliconid butterfly. Predators that have learnt to avoid the Heliconid then also stay away from the related, but quite palatable cousins! This kind of ‘borrowed’ protection, which has been found in many more instances, is known as Batesian mimicry.

A well-known instance is the Indonesian Papillio butterfly, whose females mimic other, foul tasting species. Another instance is of the Eastern Coral snake, a relative of the cobra and the mamba and found in some states of the USA. This snake is venomous and has characteristic colouring to announce itself. But the harmless Scarlet King snake has evolved almost identical markings and stays safe in the shadow of its deadly look alike!

There has been much research into the genetic trail that leads to visual similarities and the groups of genes that control pigments in mimics and models have been identified. Environmental forces that induce genetic selection and evolution of mimicry have also been analysed. The Cornel University researchers observe that while these studies of protective mimicry have focused on static traits, like colour and patterns on the coats or wings of living things, the importance of their dynamic traits, or how the animals move, have also been recognized. Back in the nineteenth century, Henry Walter Bates had observed that butterflies which evolve to have wings like related species also flit with similar action and are indistinguishable in flight. Recent studies of how animal brains work to recognize things have also highlighted the role of movement as a visual clue, the paper says. However, it is only in recent times that the dynamic aspect of mimicry has been rigorously investigated, the paper says.

Although the benefits of being visually similar should lead to the resemblance of mimic species to the models being nearly perfect, it has been observed that the mimics are often only poor copies, a phenomenon known as ‘imperfect mimicry’. The quality of perceivers’ detection equipment has hence become important, as has the need to understand different aspects of visual appearance, including the dynamic, the paper says.

Ant mimicry

A common form of dynamic mimicry, the paper says, is the mimicry of ants. Ants have powerful defenses, like strong jaws, a poisonous sting, chemical arsenal, general aggressiveness and nest-mates to help. Ants are hence a fine model to mimic and many species of spider have done so. Spiders, the paper says, although they lack the defenses, particularly the chemical weapons, of ants, have their own tools of offence and are a feared lot too. The family, Saltacidae, of jumping spiders, the paper says, are themselves the model for mimicry by other species. Moths and some flies sport patterns on their wings to appear like the spiders’ legs and they wave their wings to give the impression of a spider raising its forelegs, to keep foes or competitors away.

What sets jumping spiders apart from others is their ability to make very quick jumps over distances many times their own length. The spiders have four pairs of eyes, for precise location of prey and they have a hydraulic system of powering their legs, for perfectly guided jumps, for hunting, and to escape attack. For all this, jumping spiders need to avoid predators too and to this end many varieties of spiders are known to mimic the movement methods of ants.

Unlike other instances of Batesian mimicry, ants are no related species of spiders, but “are separated by significant differences in morphology, behaviour and hundreds of millions of years of evolutionary history,” the paper says. The spider is a stocky arachnid with eight legs and two body segments while the ant has six legs, two antennae and three body segments separated by narrow constrictions. “Jumping spiders are solitary predators famous for visually driven behaviours. They typically stalk their prey carefully, leaping towards their targets from many body lengths away. Ants, however, are social, opportunistic foragers whose worlds are dominated by chemical cues,” the paper says.

A basic difference between the cases of the spider-ant mimicry and instances of butterflies, snakes and some others would lie in dimensions and the speed of movements of the animals. Larger and slow moving animals allow clear visualising. Close visual resemblance of a mimic to the model is hence essential. As ants are small and make swift, darting movements, however, a predator may find it difficult to form a well resolved visual image and would rely on the rhythm and trajectory of motion to identify and differentiate possible prey. This has been suggested as the reason that spiders have evolved to carry out movements that appear like those of an ant. The suggestion, however, has not been followed through with precise, high speed recording of the spiders’ movements, to show that this is truly as case of protective, locomotor mimicry.

The Cornell researchers used multiple high-speed cameras to track and compare leg movements of freely moving animals in three dimensions. The cameras took pictures at the rate of 1,000 to 4,000 frames a second and made 27 recordings of Myrmarachne formicaria, a jumping spider that is considered a one of the best examples of ant-mimicry, 15 recordings of ants and then 23 of spiders that did not mimic ants.

The results showed that “the movement of the ant-mimicking jumping spider, M. formicaria, is similar to that of ants both at short, single-step timescales and at long, full trajectory timescales,” the paper says. The mimic trajectories showed regular, curved, wavelike shapes, with a wavelength of about ten body lengths. Ants following a trail also moved in the same regular, wave shapes, ten body lengths apart. In following a pheromone trail, ants cross the trail, till it seems to fall off and then come back, again to cross till it gets weak and so on. While this is a path with a purpose for ants, the same path for spiders, which follow no trail, is clearly to ‘move like an ant’. And further, while ants continuously held their two antennae aloft in front of the body, jumping spiders moved swiftly, in bursts, but when stationary, for about 100 milliseconds, they raised their front legs, “generating antennal illusion,” the paper says.

The paper discusses how the 100 millisecond spells of antenna mimicry may be good enough to deceive many observing predators. Spiders, which are capable of walking with six legs, still use all eight legs except when stationary. This suggests that it is during these 100 stationary milliseconds, when observers can make out sufficient detail, that it would be best to mimic ant-like forelimb behaviour, the paper says.


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