Bird-dropping (Celaenia excavata)

Physical description and identification

The bird-dropping spider is an Australian member of the orb-weaver family that no longer weaves an orb as an adult. It belongs to the genus Celaenia and was described by Ludwig Koch in 1867. Along with the bolas spiders it belongs to the araneid lineage of moth specialists, and like them it replaced the web with chemistry — but it uses no bolas at all, a distinction worth keeping straight.

Adults

• Size: adult females are about 12 mm in body length, a solid but not large spider. Males are dwarfs of around 2.5 mm and are hardly ever noticed.
• Colour: mottled chalk white, black and grey, glossy in places and matt in others, exactly the texture of a fresh dropping on a leaf. The pattern varies between individuals, and the dark marks on the back are sometimes read as a tiny skull — the origin of the name death's head spider.
• Eyes: eight small eyes in two rows, none enlarged, set on a low carapace that is largely hidden beneath the overhanging abdomen.
• Distinctive features: the abdomen is broad, hunched and roughly triangular, with a shallow depression running along the midline — the excavation the species name refers to. The forelegs are long, thick and spiny; at rest they are folded tight against the body so the spider has no spider-shaped outline at all.

Eggs and egg sacs

The egg sacs are unmistakable: hard, brown, marble-shaped spheres about 12 mm across, hung together on stout silk lines so a cluster looks like a bunch of small dried fruit. A single female may produce up to about thirteen of them, each holding roughly 200 eggs, so one spider can leave well over two thousand young on a single bush. The sacs are tough enough to weather months outdoors and are often found long after the female has gone.

Spiderlings

Spiderlings emerge pale and darken as they mature. They disperse by ballooning on silk and, unlike their mother, they do build a web: a small, neat, circular orb of the ordinary araneid type, which they use for a season before abandoning web-building on reaching maturity. Growth proceeds by repeated moults.

How she hunts

The adult female builds no capture web. Shortly after dark she moves to the edge of a leaf or twig, hangs beneath it by her two back legs with the long spiny forelegs held ready, and releases volatile compounds that imitate the sex pheromone of certain female moths. Male moths track the scent upwind. When one comes within reach she seizes it in mid-air with her forelegs, bites, and settles down to feed. There is no bolas, no sticky droplet and no net; the whole apparatus is a scent and a pair of arms. The orb web that defines her family survives only in the juveniles.

Where the bird-dropping spider lives

Celaenia excavata is native to Australia and is recorded across the eastern, southern and central parts of the continent, including Queensland, New South Wales, Victoria, Tasmania and South Australia. Other members of the genus reach New Zealand.

It occupies foliage rather than the ground: open forest and woodland, heath and wetland margins, regrowth scrub, and — often enough to have earned the name orchard spider in Australia — fruit trees and garden shrubs. Anywhere moths fly at night and there are leaves to hide under by day will do. Note that in North America the name orchard spider refers to a completely different animal, the slender green Leucauge, so the term is best avoided outside Australia.

Are bird-dropping spiders venomous?

Yes, in the ordinary sense that nearly all spiders are venomous: Celaenia excavata uses venom to subdue the moths it catches. It is not medically significant to people or pets. The venom is tuned to insects, no antivenom exists or is needed, and no serious envenomation has been recorded from this species. It is not related to the Australian funnel-webs or the redback and carries nothing comparable to those venoms.

Can bird-dropping spiders bite?

She can, but almost never does. The spider is slow, reclusive and committed to her disguise: her defence against being noticed is to stay perfectly still, and if that fails she drops on a dragline. Verified bites are essentially unrecorded, and the few anecdotal accounts describe a brief pinch with slight local redness that fades within hours, with no lasting effects. Wash the site with soap and water, apply a cold pack if it is sore, and seek medical advice for any bite that swells dramatically or spreads, which would point to an allergic reaction rather than to the venom.

Ecological importance and behavior of the bird-dropping spider

This is a specialist predator of night-flying moths. Because the lure copies particular pheromone blends, a single spider draws in a narrow set of species, most of them male — a very precise form of pest control where the moths concerned are orchard or garden pests. The disguise does double duty, hiding her from daytime predators while costing her nothing at night, since moths navigating a scent plume are not looking at her.

Natural predators: birds are the principal threat, and the mimicry is the answer to them. Wasps take a share — both spider-hunting wasps that paralyse adults and parasitic wasps that attack egg sacs — along with lizards, ants and larger spiders.

Prey–predator dynamics: one point in older accounts needs correcting. Adult females build no capture web at all, so the claim that birds find them easily because their webs sit in bird-rich forest has it backwards: the whole point of the bird-lime disguise is that a visually hunting bird passes them by. It is the juveniles, sitting in small conspicuous orbs, that are most exposed.

Seasonal behaviour: adults are found mainly through the warmer months. Egg sacs appear from late summer into autumn and often persist on a bush through winter, with spiderlings emerging and ballooning in spring. Most individuals complete a life cycle in about a year.

Relationship with humans: entirely benign and quietly useful. She does not come indoors, strings no web across a path, and cannot realistically be bitten by accident. In an orchard she is worth having.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The species name excavata refers to the hollow running along the midline of the abdomen, not to any digging — this spider never touches the soil.
• The name death's head spider comes from dark markings on the back that in some individuals resemble a small skull.
• Bolas spiders of the American genus Mastophora evolved a near-identical bird-dropping disguise on the other side of the Pacific — the same problem solved the same way twice.
• The egg sacs outlast the spider. A cluster of brown marbles on a twig in winter is often the only trace that a female spent the previous summer on that bush.
• Because the lure is chemical, she catches the same moth species a pheromone trap in the same orchard would catch, which is how her hunting method was first worked out.

In summary, the bird-dropping spider is an orb weaver that traded the web for a disguise and a scent. She spends the day looking like something no bird wants to eat, and the night calling in male moths with a counterfeit of their own mating signal — an elegant piece of natural engineering that is completely harmless to people.