Tuesday, February 18, 2014

Why look like a spider?

"Why did the fly fly? Because the spider spied'er." - Anon.

Not every resemblance is a case of mimicry. An image of the Tephritid fly Goniurellia tridens with spider-like or ant-like patterns on its wing went viral in recent times and some have pointed out that this might be a case of wishful thinking. A little more than a week ago, I was observing a tiny day-flying moth in the Western Ghats of India and its behaviours. It seemed to make quick movements after landing and before coming to rest and then it would wave its hind legs slowly. It also made slight shifts of the forewings giving it the distinct impression of a jumping spider (Salticidae). 

Here are some not so brilliantly captured videos:

I cannot find a matching Salticid model but the posture and appearance make the mimicry seem perfectly plausible. Unlike the ants, which are dreaded by most of the smaller majority it is harder to think of a jumping spider as being recognized and avoided.

Here is a Salticid as seen from the side

Salticus scenicus from California by Kaldari (Wikimedia Commons)

Roger Kendrick has kindly identified this as belonging to the family Glyphipterigidae
Now there are ways to test if the resemblance is just accidental or if it really enhances the survival of the mimic. When I posted this on Facebook, I was pleasantly surprised to know that the Tephritid pattern which is quite widespread has actually been put to some test by an old friend, Dinesh Rao and his colleagues. Some of the earlier studies on the Tephritids with experiments involving wing transplantation suggests that the behavioural part is vital and that this seems to be specifically to avoid predation by Salticids. It seems like much more work is needed and we may well find many more cases of Salticid mimicry. But why only Salticids, is it because they are the most active on vegetation? And then we have some Salticids themselves being mimics of ants.
Brenthia sp. (from Rota & Wagner, 2006)

Here are some more suspects -
http://bugguide.net/node/view/433093 (another moth)
http://www.flickr.com/photos/orionmystery/8266380277/ (Amphientomidae, Psocoptera)

See also the excellent and interesting videos on http://naturedocumentaries.org/4450/predator-mimic-fly/
Rear end of Amycle sp. (from Zolnerowich 1992)

PS:  Have just examined the history and it seems that Tom Eisner was the first to suggest spider mimicry in Zonosemata.

Many other "microlepidoptera" have the habit of raising or waving their hindlegs. Given that salticids are considered masters of taking indirect routes to their prey, it seems like waving legs from the blind hind end might have a benefit not unlike that of the false heads of lycaenids. Perhaps salticids choose to circumvent this false head and come into the visible zone of the potential prey's zone of vision, thereby making them less liable to fall prey.

PS: In July 2015, during the monsoons, I noticed this common tetrigid with prominent white (maxillary?) palps making these jumping-spider-like motions. The grasshopper itself moves in jerks and it seems that the resemblance might have a value in escaping predators.


Dinesh Rao for pointing out the work he is doing and allowing me to post without fear of sounding like a complete crank.

A salticid peers from the cover of a fern (Wynaad)