''Phrynarachne decipiens'', the bird-dropping spider, is a

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

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 Malaysia ( ; ) is a country in Southeast Asia. The federation, federal constitutional monarchy consists of States and federal territories of Malaysia, thirteen states and three federal territories, separated by the South China Sea into two r ...

and

Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania between the Indian and Pacific oceans. It consists of over 17,000 islands, including Sumatra, Java, Sulawesi, and parts of Borneo and New Guine ...

(

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 Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...

). 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 Henry Ogg Forbes LLD (30 January 1851 – 27 October 1932) was a Scottish explorer, ornithologist, and botanist. He also described a new species of spider, '' Thomisus decipiens''. Biography Forbes was the son of Rev Alexander Forbes M.A. (182 ...

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 Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended fr ...

's theory of

evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

through

natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charle ...

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 Arachnology is the scientific study of arachnids, which comprise spiders and related invertebrates such as scorpions, pseudoscorpions, and harvestmen. Those who study spiders and other arachnids are arachnologists. More narrowly, the study of sp ...

Reverend

Octavius Pickard-Cambridge Octavius Pickard-Cambridge Fellow of the Royal Society, FRS (3 November 1828 – 9 March 1917) was an England, English clergyman and zoologist. He was a keen arachnologist who described and named more than 900 species of spider. Life and wor ...

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 Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill the ...

. 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