Spiders that look like ants

A team of scientists led by AM's Dr Helen Smith has described two new species of ant-mimic spiders from the previously monotypic genus Anatea. 

Animals that mimic ants are called myrmecomorphs, and there are many examples among spiders. In general, it is thought that these ant mimics primarily gain protection from predators, many of which find ants distasteful. Among spiders, a few myrmecomorphs are also known to prey upon their models.

We don't know much about the habits of spiders in the genus Anatea, but many spiders in this group of Redback relatives - called hadrotarsines - do eat ants so it is quite likely Anatea species do too. Although ant mimicry in Anatea seems highly likely, it is not yet proven. At least to humans, Anatea do rather resemble ants and this was noted when the genus was described from type specimens found among a collection of ants back in 1927.

The unusual thing is the form of the body modification that gives an ant-like appearance. One problem for a spider trying to look ant-like is that ants have three body sections, while spiders are normally thought of as having only two, joined by a little 'stalk' called a pedicel. Most ant-mimicking spiders gain ‘extra’ sections by having the front and/or rear sections of the body elongated and apparently subdivided, either by a physical constriction of with patches of colour. Many Anatea spp. however, have an elongated pedicel, a character not seen to such a degree in any other known species of spiders.

Most ant mimic spiders also use behavioural ruses, such as ‘losing' the extra pair of legs by waving them like antennae; we might expect that Anatea do this as well, but there are no reports of the behaviour of the previously described New Caledonian species in the literature. The new Australian species are so difficult to find in the field that we also lack observations, in fact one species is only represented by a single specimen. Observing these animals is made more difficult by their size – at 2 to 3 mm in length, recognising a spider from an ant when they are running in leaf litter is very difficult – and once a spider is scooped up into a collecting net it will often drop any behavioural disguise.

We have placed the two new Australian species in the existing genus Anatea because that seems the best match until we know more about any of these animals. Until this paper only one -  the New Caledonian species, A. formicaria - was officially described, and we add several more records for that species. But we also record several undescribed species from the island, all of which seem to mimic different species of ants. The type species, Anatea formicaria, has a moderately elongated pedicel with a bump which may mimic the ‘nodes’ seen on the petiole of many ants. Some undescribed New Caledonian Anatea species have a short but swollen pedicel, one has a long pedicel with spines; our new Australian species have long pedicels without ‘nodes’ or spines. The third pair of legs is also unusually long.

One of the new Australian species is named in honour of Queensland Museum entomologist Dr Geoff Monteith, who has contributed enormously to our knowledge of tropical spiders through his collecting activities. The other is named for its amazingly elongated pedicel.

The New Caledonian species seem to be far easier to collect that the Australian ones, despite that there has been far more collecting effort in some of the areas of north Queensland that our new species come from. This indicates that there may be behavioural differences between Anatea from the two countries. This, together with some morphological differences such as the long third legs, means that we are not totally confident that our new Australian Anatea really belong in this genus. As with so many other small species, we just need to do far more work.

Publication citation: Smith, HM, M.S. Harvey, I Agnarsson and GJ Anderson, 2017. Notes on the ant-mimic genus Anatea Berland (Araneae: Theridiidae) and two new species from tropical Australia. Records of the Australian Museum 69(1): 1–13.