When I was wandering along the bank of Ross River near the Bowen Road bridge a few months ago, I looked down, saw a perfectly ordinary looking ants’ nest and a moment later thought, “Hey! That’s odd! That would be under water at high tide!”
Jumping spiders are undeniably cute, even to people who ‘don’t like spiders’. They display a fearless, active curiosity about the world around them out of all proportion to their diminutive stature, and they have those big eyes which somehow prompt a gush of affection even across the huge gulf between two-metre anthropod (1) and five-millimetre arthropod. They are also common enough in our gardens and houses to be observed often, are enormously varied (500+ species) and display a wide range of hunting habits. Some of them (getting to the point of this post at last) mimic ants.
This ant, one of many I saw on the strappy leaves of a clump of what may have been flax lilies in the Wallaman Falls camping ground, is very similar to the common Polyrhachis ammon but is a bit smaller at 4-5mm. I am reliably informed that it is closely related even though we can’t be sure of its exact identity; we will have to call it just Polyrhachis.
When I saw this spider I thought, as I was intended to, that it was another of the ants. It was the same size and coloration … but its movements weren’t quite right: spiders dart and pause, while ants tend to keep moving. Its head looked too big, too. When I disturbed it, it stretched out its front legs which had been tucked up beside its head and it suddenly was clearly a spider (eight legs, no antennae), not an ant (six legs plus antennae).
So why do spiders imitate ants? Fadia Sara Ceccarelli of JCU studied them and puts it this way:
The aggressive nature of ants, and their possession of noxious chemicals, stings and strong mandibles make them unfavourable prey for many animals. The resemblance of a similar-sized arthropod to an ant can therefore also protect the mimic from predation.
Myrmarachne is an ant-mimicking salticid spider genus, whose species associate closely with their model ant species. The behavioural reactions of Myrmarachne to ants were analysed, including instances when there was contact between the spider and the ant. In Townsville the salticid Cosmophasis bitaeniata and one Myrmarachne species associate with Oecophylla smaragdina workers. The Myrmarachne mimics the ant visually, and Cosmophasis bitaeniata mimics the cuticular hydrocarbons of the O. smaragdina worker ants. Cosmophasis and Myrmarachne also mimic ants through certain types of behaviour, such as the “antennal illusion” and bobbing the opisthosoma up and down.
She goes on to say that the spiders avoid contact with the ants and manage to avoid being attacked by them, so it is clear that the mimicry is defensive, helping them avoid attacks from predators. An American study of a similar ant-mimic found that it was an effective strategy:
Ant-like appearance (myrmecomorphy) has evolved >70 times in insects and spiders, accounting for >2,000 species of myrmecomorphic arthropods. Most myrmecomorphic spiders are considered to be Batesian mimics; that is, a palatable spider avoids predation through resemblance to an unpalatable ant – although this presumption has been tested in relatively few cases. Here we explicitly examined the extent to which Peckhamia picata (Salticidae), a North American ant-mimicking jumping spider, is protected from four species of jumping spider predators, relative to nonmimetic salticids and model ants. … We found that mimetic jumping spiders were consumed less than a third as often as nonmimetic jumping spiders, suggesting that Peckhamia does indeed gain protection as a result of its resemblance to ants…
Mine is not a Myrmarachne but a Ligonipes, one of the other four genera of ant-mimicking jumping spiders occurring in Australia according to Ed Nieuwenhuys (the others being Judalana, Rhombonotus and Damoetas) but the same arguments must apply.