Wednesday, 10 December 2008

#57 A Bird-dropping spider breeds and dies

Spiders appear in numbers overnight, almost magically, as summer approaches. I am always keen to discover new spiders in my backyard, and to further observe habits of the regulars.

The Bird Dropping Spider is not exactly a regular in my garden; I have only seen two specimens. Last summer I was lucky enough to be able to keep tabs on a mature breeding female.

A Bird Dropping Spider in my garden

I believe the species of Bird Dropping Spider I have found in my Hunter Valley rural garden is probably Calaenia excavata (formerly C. kinbergi) from family Araneidae.

It is no puzzle as to how this odd little spider got its common name; it is a master of camouflage as it rests with legs folded against its body throughout daylight hours looking remarkably like a blob of bird excrement, therefore escaping the attention of predators like wasps and birds.

"The Bird-dropping Spider also uses mimicry of a quite different sort to capture its prey, which consist almost exclusively of male moths. At night the Bird-dropping Spider hangs from the edge of a leaf or twig on a short silk thread, its forelegs outstretched. While doing this it releases a chemical scent (pheromone) that mimics the airborne sex pheromone released by female moths to attract their mates. The unfortunate male moths that are attracted by the spider's deceiving pheromone eventually flutter close enough to the spider to be grabbed by its strong front legs." Ref: Australian Museum Online.

White and brown colouring mimics a bird's dropping

Underside of Calaenia excavata (Bird Dropping Spider)

The female Calaenia excavata's body grows up to 12mm, while the tiny male is only 3mm. Unfortunately, I did not find the male (it is possible he became a meal after mating), but of course it could be easily overlooked.

Egg sacs that I observed were from 8 to 10mm, spherical in shape, brown with black markings, and paper-like in texture. Six egg sacs were constructed over a period of about 4 weeks, and were suspended in a Leptospermum shrub, loosely connected by silk. The female remained with her unhatched egg sacs until she died.

I did not take the effort to observe the nocturnal hunting techniques of this clever spider, so the only time I saw her outstretched legs was when she was at the end of her life. Shortly after I took the following image, the spider dropped from her position to the ground, and was lifeless.

Female Bird Dropping Spider close to the end of her life

Despite checking the egg sacs most days, I did not see the spiderlings emerge and disperse. I did, however, break open an empty egg sac to inspect the interior. A tiny escape hole was evident.

Bird Dropping Spider Calaenia excavata empty egg sacs

These odd, yet remarkable little spiders are not considered dangerous. I look forward to finding more living and breeding in my garden. They are apparently commonly found in citrus orchards, so if you have a couple of citrus trees in your backyard, you have more chance of finding the Bird Dropping Spider than I have.

Esperance Blog has an excellent entry on the Bird Dropping Spider, Calaenia excavata, over the other side of the country.

Spider identification links


Duncan said...

Nice pictures Gaye. As I noted on Snail's blog I never see them nowadays.

Denis Wilson said...

Hi Gaye
Nice post.
I have never photographed a Bird Dropping Spider.
I wonder if the neat round hole was in fact a predator's entry hole?

Mosura said...

Great photos! I remember the spiders from N.S.W. but have never seen them here. Not sure if we even get them - I'll have to check that out.

Gouldiae said...

G'day Gaye,
A beautifully detailed entry on a delightful species. Thanks.

Gaye from the Hunter said...

hi Duncan, Denis, Mosura and Gouldiae,

once again, I have been away from my computer. My apologies for my delay in responding.

Denis, I don't believe the hole is a predator's hole, as the same hole is present in the egg sac shown in Esperance Blog. I will take more notice if I am lucky enough to spot a nesting this season.

Thank you all for dropping by, and for leaving a comment. It is nice to know who is reading about my nature observations.


Anonymous said...

Being new to Australia, we live near Perth my other half has just found one of these spiders in our garden complete with eggs which the other half collected and put in a jar. He was worried stiff it was poisonous and you have alleyed our fears. Thanks for the info. We have left put it back safe and sound x

Vivienne said...

We have found two of these spiders and eggs in our backyard in the last fortnight. One, my husband took over the road out of the way with the eggs and the other is living above on garage door frame. We know it moves around at night because it is often in a different position in the morning, albeit in the same place on the garage door frame. We live in Perth.