Sitting in the back garden yesterday, I glanced down to see an ant wandering along the edge of my table – or so I thought. But it wasn’t moving like an ant: they are purposeful, even if we may not divine their purposes, and this maybe-not-an-ant was wandering rather slowly and aimlessly. At a closer look, its antennae weren’t very convincing, either Continue reading “The ant that wasn’t”
Castle Hill dominates the central Townsville skyline but Mount Stuart takes over that role from anywhere further up Ross River. From Mundingburra all the way up to Kelso and across the river to the university and the hospital, Mount Stuart looms large.
That doesn’t mean people visit it very often, of course, but a road leading off the Charters Towers road just beyond the city winds up to a lookout beneath the radio masts. When I drove up a couple of days ago I was welcomed by a resident peacock (perhaps the same one who met me five years ago) and, after taking in the magnificent views over Magnetic Island, the Palm group and coastline all the way to Hinchinbrook, I wandered around the loop track.
The summit is a difficult environment for plants and animals alike: very exposed, very dry, and with only a thin covering of soil where there is any soil at all. Vegetation is ‘open woodland’ with a decent covering of tussocky grass, but most of the trees are tiny. Ants seem to be the most abundant invertebrates but there were other insects to be found as well as the spiders which prey on them.
* P.S. The ant has been identified by my friendly local expert as Meranoplus sp.
Lower down the mountain
Conditions half-way down the mountain are not quite so arid and I found more creatures per square metre than on the summit.
- The two kinds of paper wasps are the two commonest around Townsville. More about them here and here.
- The Spiny Orb-weaver pictured, Gasteracantha fornicata, is usually seen less often than its black cousins, Gasteracantha sacerdotalis, but outnumbered them on this trip.
- “St Andrew’s Cross spider” is a common name which is applied loosely to several similar species. Argiope keyserlingi is the best known, A. picta is encountered from time to time, and this one may be unknown to science – which is a little bit exciting.
- Golden Orb-weavers (Nephila sp.) are usually very big and not so brightly coloured but I have seen others around 12-15mm, like this one, in Western Queensland – at Aramac and Porcupine Gorge, for instance.
Spiders are both predators and prey and sometimes we see one in each role in the same, fatal, encounter.
The Daddy-longlegs (Pholcus sp.) in these photos lives quietly between my computer and the wall, waiting for anything edible to come by. (It is probably a direct descendant of this family, since there are always a few in the area.) Jumping spiders, on the other hand, are roving hunters and this little brown one (I thought it might be Servaea or Simaetha sp. but have been reliably informed it is a juvenile Hypoblemum) was on the prowl when it blundered into a strand or two of web.
That had just happened when, by pure chance, I reached around to plug in a camera lead and saw what was going on. The daddy-longlegs, outweighed two to one, took care to stay at a safe distance as it used those long legs to further entrap its prey in silk. The process took a couple of minutes and the jumping spider never even looked like getting away. When it was secure, the daddy-longlegs finally came down to sink its fangs into its victim.
Most of us, I think, automatically (and not always logically) choose sides in a conflict like this. If it’s between a lion and a deer, we tend to sympathise with the deer; if a kookaburra and a snake, we’re on the bird’s side. Cat and gecko? Bird and butterfly? Bird and spider? We don’t always side with the hunter, or with the hunted, or with the vertebrate against the invertebrate, or the mammal against the reptile. What do we think about spider vs spider? Or spider vs fly? More interestingly, perhaps, why do we think whatever we think?
The other ‘small death’ I saw yesterday was, in fact, spider vs fly: one of our larger jumping spiders, Mopsus mormon, had captured a soldier fly:
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.