''Phrynarachne decipiens'', the bird-dropping spider, is a species of tropical

crab spider The Thomisidae are a family of spiders, including about 170 genera and over 2,100 species. The common name crab spider is often linked to species in this family, but is also applied loosely to many other families of spiders. Many members of th ...

from Malaysia and Indonesia (

Sumatra Sumatra is one of the Sunda Islands of western Indonesia. It is the largest island that is fully within Indonesian territory, as well as the sixth-largest island in the world at 473,481 km2 (182,812 mi.2), not including adjacent i ...

and Java). It mimics a bird dropping in its appearance and the way it behaves.

Description

The bird-dropping spider is a master of deception. It crouches stationary on a leaf or other level surface and exhibits an elaborate combination of form and colour, the posture it adopts and the character of its web so as to simulate accurately a patch of bird's excreta. The underside of its abdomen is chalky white and its legs black. It weaves a small irregular white web on the surface of a prominently placed leaf and adopts an upside-down pose near the centre of the web with its legs folded, anchoring itself in place with some spines on its legs. The effect of this is to create the impression of a semi-solidified bird's dropping with a white raised centre with black specks, a surrounding thinner, more liquid portion and even a drip effect on the lowest margin ending with a little knob. The mimicry is enhanced by the fact that the spider emits an odour not unlike bird excreta.

Discovery

Here is how the Scottish naturalist Henry Ogg Forbes described how he first came to discover the spider: Later Forbes found another specimen on Sumatra and sent it back to Britain. He was a believer in Charles Darwin's theory of evolution through natural selection but found it difficult to understand how the

mimicry In evolutionary biology, mimicry is an evolved resemblance between an organism and another object, often an organism of another species. Mimicry may evolve between different species, or between individuals of the same species. Often, mimicry f ...

of a variable object like a bird dropping could have evolved. The arachnologist Reverend Octavius Pickard-Cambridge brushed his doubts aside. The spider was not attempting to mimic the dropping and the web spun on the surface of the leaf was solely to anchor the spider in a position where it could await the arrival of winged prey. The fact that it then in some way resembled a bird dropping was fortuitous and natural selection merely acted to enhance the similarity.

References

{{Taxonbar, from=Q2420403 Thomisidae Spiders of Asia Arthropods of Malaysia Spiders described in 1883 Taxa named by Henry Ogg Forbes