A bird! A plane! A…moth that’s a spider? Whaaaaa?????

If you check out THIS link, you’ll see a pretty interesting example of mimicry…or is it?  This moth is a member of the family Crambidae, a very large group of insects which includes a quite a lot of diversity in both form and function.  Siamusotima

Lygodium Fern Food source for caterpillar stage.

aranea has been dubbed the “Lygodium Spider Moth”.  It’s not the Lygodium that’s the interesting part of this little guys name…nope, it eats the Lygodium fern.  That makes sense.  It’s not the moth part so it must be the “Spider” part!  Yes, there are some that look at this moth and see a spider.  What do you think?

At first glance it can be quite convincing.  The patterning on the wings while in resting pose gives it that creepy spider vibe, right?  But is it really a mimic?  Mimicry has been called “…the greatest post-Darwinian application of natural selection.” (Fisher, 1930)  And it truly is!  Organism ‘A’ looks like something that you don’t want to eat…you don’t eat it…it survives and procreates, making more things that look like something you don’t want to eat!  Crazy, I know.  But that’s what natural selection is.  Organisms which have traits that lead to it and its offspring being more likely to survive to make more offspring is natural selection.  Do that enough times and you have evolution of a new species.

So we all agree, mimicry is cool, right?  But when is a mimic really a mimic?  The Viceroy Butterfly is generally agreed to be a mimic of the Monarch Butterfly.  Everyone loves the Monarch so they must love the Viceroy too…I wonder how many amateur butterfly nuts have Viceroy’s in their collections mislabeled as Monarchs…but I digress…again.  Looking at the image below, we can easily see how these two butterflies are similar.  In order for mimicry to be an actual “thing” observed in the organism there has to be evolutionary pressure that leads to its occurrence.  The Viceroy and Monarchs survive because they feed on plants that have chemicals that pool up inside the insects which makes them unpalatable to their predators.  Look alike, eat nasty food, taste nasty, predators get the message to not eat them.  Environmental events/pressure cause traits in better adapted individuals to be more likely to survive, so they do..

Monarch Butterfly | Viceroy Butterfly The Viceroy was once believed to be an example of Batesian Mimicry (non-toxic to the Monarch’s toxic) but it has recently been observed that the Viceroy is also unpalatable. This qualifies the relationship as Mullerian mimicry.

How can that sort of idea be translated into a moth coming to look like a spider?  There’s really not enough information yet to make a determination.  Some ideas I have rolling around in my head include:  Spiders are predators so perhaps it discourages other small arthropod predators from wanting to eat the moth…but that probably would do nothing to prevent larger invertebrates or vertebrates (lizards, birds, etc) from wanting to eat them.  As I am not familiar with habitat in which this insect evolved I simply cannot say what pressures there have been which could lead to this.

For the moment, I remain skeptical if this is really mimicry or not.  It could just be another instance of humans getting a little excited about cute fuzzy creatures.

Lygodium Spider Moth (Siamusotima aranea, Musotiminae, Crambidae)

For more information on mimicry and natural selection, check out these two sources:

Insect Mimicry

Mathematizing Darwin


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