bunkering down

Some insects and spiders have got ‘home delivery’ of their meals down to a fine art, says MARTYN ROBINSON

Recently, while socially isolating in remote western New South Wales, I had the chance to observe some invertebrates you’d be familiar with, or with their relatives at least.

The first are pitfall ants, or funnel ants, in the genus Aphaenogaster. They are named for the entrance to their underground nest, which you find in arid areas but sometimes also suburban lawns. The entrance hole can be the diameter of a 20-cent coin or larger, and the rim is bordered by a wall of loose sand. When unwary crawling insects step over the rim, the loose sand denies them a grip and they tumble into the pit, whereupon they are overpowered by the ants and fed to their larvae. The ants, on the other hand, seem to come and go easily, perhaps because they step more carefully.

An insect that builds a similar trap – ironically for catching mainly ants –

ABOVE, LEFT TO RIGHT

Camouflaged burrow of trapdoor spider (Aganippe sp.); bathplug-like door of that burrow held open; entrance to burrow of brown trapdoor spider

(Arbanitis rapax); entrances to pitfall ant nests, and antlion pit (lower right). is the antlion. It’s the larva of a grey lacewing in the Myrmeleontidae family. You’ve probably seen these traps around, even if you haven’t recognised them for what they are. Almost any patch of dry, dusty sand or soil protected from rain will have them. As with the pitfall ant nest, a small invertebrate starts to tumble into the antlion larva’s pit, but in this case the larva hurls jawfuls of sand at it to hasten the process, until it drops to the bottom and the larva grabs it.

On Australia’s east coast, most people with lawns or garden beds would be familiar with the brown trapdoor spider’s burrow, although they might not know what made it. These burrows are often mistaken for those of funnel-web spiders, even by some exterminators. The difference is the Sydney funnel-web spider almost always builds its burrow under some sort of cover, and the entrance is marked by a lot of obvious white silk, while the harmless brown trapdoor spider in the Arbanitis genus makes its burrow out in the open. Contrary to the common name, brown trapdoor spiders never have a door at the entrance to the burrow. However, they have relatives in the Arbanitis and Aganippe genera and others that build a close-fitting lid to the burrow, giving the entire group, whether their burrows have doors or not, this name.

In both cases, the spider waits at the burrow’s entrance at night with its feet at the rim. Where the burrow has a lid, it is slightly raised, still concealing the spider. When prey comes close to the entrance, the spider pounces out, grabs it, and drops back down into the burrow.

The female brown trapdoor spider spends her entire life – sometimes longer than a decade – in the same burrow, but the male, once mature, must wander in search of a mate, which is how he spends the last days or months of his life. So if you find a number of large brown spiders drowned in your swimming pool in the warmer months, you may have brown trapdoors living nearby.

Martyn gardens mainly on Sydney’s Northern Beaches