There isn’t much insect activity in the garden at this (dry, cool) time of year but when I was ambling around it last week I saw a Green Ant which was too small – only about two thirds of the size of a normal Green Ant.
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.
Nearly a year ago I wrote about a bug which was doing a great job of pretending to be a black ant. This time I have one which similarly pretends to be one of our ubiquitous Green-ants. First, the real thing:
Now here’s the mimic …
I noticed it mostly because the antennae looked vaguely wrong for an ant. Looking more and more closely, I could see (1) that the antennae curve smoothly backwards instead of having obvious elbows and pointing forwards, (2) the body is broader and (3) it has no jaws. In fact, it is the juvenile form (nymph) of a sap-sucking bug (Hemiptera), and like all its relatives it has a piercing tube, seen here tucked up against its chest, instead of jaws.
I thought it was a new species to me – I still get them occasionally in my own garden – but through Steve in Airlie Beach I found that I did know the adult but had been baffled by its difference from the nymph. Between us we have a complete sequence showing how it changes as it matures. The one above is the youngest and is the best ant-mimic; as it matures we get …
The last of these four photos shows the adult, with its wings completely covering its abdomen. The middle two are courtesy of Steven Pearson, and I thank him for permission to use his photos and for identifying my ant mimic. It’s a Pod-sucking Bug, Riptortus sp., Alydidae, Hemiptera.
Riptortus serripes seems the most likely species since it’s the only one I’m sure has been identified in this region, but Brisbane Insects shows its nymphs mimicking black ants. Either we need to look out for another species or Brisbane Insects has confused R. serripes with its smaller cousin Melanacanthus scutellaris.
We took advantage of the Boxing Day holiday to drive down to the camping and picnic area at Alligator Creek. It is normally a popular spot but the long dry spell which only ended on Christmas Eve seems to have discouraged the campers and even the day-tripper numbers were down, so it was pleasantly quiet. We paddled in the shallows, swam in the deeper pools, clambered over the rocks and enjoyed a picnic lunch. All of us are enthusiastic about wildlife and all of us had cameras so my photographic haul for the day is only about a quarter of the total.
We all took photos of the scrub turkeys, Alectura lathami. There were plenty of them around and they were absolutely comfortable with human society – even to the point of shopping at Supré, apparently:
They are large and somewhat clumsy birds but my third photo here is misleading: the bird did not crash-land at all but was enjoying an energetic dust-bath. A far more formal portrait is here, on my Flickr photostream.
The scrub turkeys were not the first creatures we noticed on arrival: the cicadas were. Their screaming drone is characteristic of the Australian bush in summer and dominated the picnic area. After a while we saw some of their cast-off shells (here and adjacent) clinging to tree-trunks and saplings but we never did see any of the adult insects; they must have been high in the trees.
I also brought home pictures of spiders – another ant-mimicking jumping spider, a tiny yellow spider which had somehow defeated a green-ant plus a couple of others – flies (1, 2), dragonflies, damselflies and a marvellously camouflaged mantis:
A post about an earlier visit to the same park shows the scenery and some more of the fauna, while this link will take you to a composite collection of my Flickr photos of the wildlife.