An amazing grass moth mimicking… a spider?

October 7, 2013 • 7:20 am

by Matthew Cobb

Moar amazing things from Tw*tter. Spotted by @CMBuddle, retweeted by reader Dom (@HStiles1), this superb grass moth from the East Khasi hills in Eastern India, just to the north of Bangladesh.

Spinner

The photographer, rohtmig, identifies it as a member of the genus Siamusotima. It appears to be Siamusotima aranea, so called because when its wings are stretched out it looks like a spider, although rohtmig appears to disagree with this on his/her webpage. It could be another Siamusotimid.

Here’s another lovely example that was originally on Flickr here. This moth was photographed by John Horstman in Pu’er, Yunnan, China:

Horstman writes:

This moth is very similar to a recently described moth (originally from Thailand in 2005) called the Lygodium Spider Moth because it feeds on Lygodium species, an invasive Old World climbing fern, and has markings that look like a spider (possibly mimicry to protect it from predators).

Here’s another picture taken from here (no credit)

Siamusotima aranea was originally described in 2005 in the Annals of the Entomological Society of America, and it turns out the larva is also pretty cool:

The adult and larva exhibit extraordinary morphological modifications whose functions are unknown but may presumably be for defense against predators and/or parasites. The wing pattern of the adult (Figs. 1A and B) resembles legs held in a laterigrade fashion resembling those of the Thomisidae, the crab spider family (J. Miller, personal communication). There have been a few behavioral observations on adult moth species mimicking spider behavior in defense against spider predation (Manu 2003, Rota 2003, Aiello and Becker 2004), but it has not been observed in S. aranea. The larvae S. aranea have a morphological adaptation of the anal shield that does not occur in any other known stem-boring pyraloid that may provide protection from predation and/or parasitism. The anal shield is modified as a cup-like structure with a margin surrounded by setae (Fig. 4A–C), very similar to that of some beetle larvae (Fig. 4D–F), specifically Amarygmini tenebrionid larvae (Spilman 1969).

 Here’s the picture of that bizarre cup-like structure at the rear end:

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PS Yes, I was wrong about the Nobel for Phyiosology or Medicine 2013. It did not go to my three nominees, but instead to James E Rothman, Randy W Schekman and Thomas C Südhof for their work on vesicles in cell trafficking. Congratulations to those three, and here’s to 2014 for Hall, Rosbash and Young!
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14 thoughts on “An amazing grass moth mimicking… a spider?”

  1. Nice moth. It kind of looks like an assassin bug or an ambush bug (Hemiptera). Hemipterans, besides having nasty bites, often contain noxius chemicals (stink bugs) that repel predators. Spiders are much scarier to people than to birds. Of course, an optimal moth mimetic pattern may be intermediate and pool the ‘nightmares’ of all several enemies.

    2. I was surprised that it would be a better idea for a moth to look like a spider if it could, birds are perfectly happy to eat spiders. Maybe there is a very fat-arsed local bug that it’s trying to mimic.

  4. The only grass moths I know are really tiny – micromoths – that look just like a grass seed. Just wonder what animals will avoid this moth based on it looking spider-like?!

    Isn’t nature wonderful?!

  5. I am sure I had seen photos of the moth somewhere before. Was it in WEIT before?
    In any case, my first impression of the photograph of the larva is that the anal shield is a false head. The drawings do not help to persuade me of that, however.

  6. How unusual is the way the antennae are held pointing backwards and parallel? – that reduces the resemblance to a typical moth, at least.

    The sclerotised anal shield reminds me of a Shieldback Trapdoor Spider, where it’s a defense against parasitic wasps (and of course there are sneaky wasps with slim-jim ovipositors driving the adaptation). I guess the moth larva similarly inhabits a burrow or tube, or the tail-cap would be no defense at all.

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