I am not actually an ant at all, but looking like one may save me from critters willing to eat harmless vegetarian bugs like me but unwilling to attack nasty-tasting, big-jawed ants.
Starting again, a bit more seriously: these photos show a couple of insects I found together in my garden recently. The first is about 6mm long, the second about 8mm. My first and only thought was that they were ants, although the transparent rim around their bodies looked strange for an ant (and just as strange for anything else) and so did the bracket-shaped shoulder-piece of the larger one.
I uploaded both photos to my Flickr photostream and the good people from the Field Guide to Australian Insects and Encyclopedia of Life Images soon put me on the right track: “I’m guessing that these are close to the Broad-headed Bugs (Alydidae). Both seem to be immature stages and the wings have yet to develop completely. The adult would have wings covering the abdomen,” and, “not an ant but an Hemipteran nymph … it vaguely resembles other ant-mimic bugs in the family Nabidae I know from Europe,” and, “Definitely an ant-mimicing heteropteran nymph. We (Aussies) have numerous species spread among several families that have ant mimics as nymphs,” and, “Semi-transparent seams give the impression of several constrictions, where there are none!” (Clicking on the images on this page with take you to Flickr, where you can see the whole discussion as well as full-size photos.)
In this light, the puzzling features weren’t so odd: I know a native cockroach and a beetle which have transparent edges to their coloured carapaces, and the ‘shoulder-piece’ is simply a pair of wing-buds.
Once the right track, I succeeded with an image search. The best match I found was on Brisbane Insects’ invaluable site, Rhyparochromidae – Seed Bugs, which told me that Rhyparochromidae is a family closely related to Lygaeidae (Seed Bugs, Milkweed Bugs and Chinch Bugs). They are small dull brown or blackish bugs which eat seeds; many of them are flightless. When (if) my bugs become adults, they should look something like this or this (not my photos), with wing patterns mimicking the ant-like body segmentation.