Moths are messing with your mind

Buff Arches - it's moth masterpiece, it also exhibits a very convincing optical illusion.

I think the Buff Arches is my favourite moth - it's a moth masterpiece! Even though you know you're looking at a flat wing, the optical illusion is so convincing that you can't help but see it as two surfaces.

I like the way that evolution has provided it with a drop shadow, to give the illusion of depth, of the kind you'd produce if you were using photoshop. To see how closely I could mimic the effect I produced the image on the right.

Why would coloration like this evolve? It seems clear that breaking up the outline is the 'objective' here. But what about the other markings? Why is it that colour? Why does it have the looped markings, why does it have the line along the wings' trailing edge? Is it possible to ever know?

It's sometimes contended that every single marking on a moth or butterfly's wing, every tiny line and dot, must have an adaptive benefit. This may seem hard to believe - surely some of it is purely random?..isn't it?

To attempt to mimic the illusion I added an  area of uniform colour to the wings then added a drop shadow. The illusion is enhanced by the contrasting white line around the shadow.

The illusion is even stronger from the side.

The Buff-tip a very striking example of mimicry in moths - not only does it closely resemble a twig, but a broken twig and a silver Birch, broken twig at that

Moths are messing with your mind...well actually birds' minds.

They've got a problem.  In order to avoid daytime predators most are active at night. They are less conspicuous during the day as they are inactive, however they still have to avoid detection – that’s the problem.

Birds - and it’s mostly birds – eat moths (by day).  Birds will find it more difficult to detect well concealed moths - these will have a better chance of producing offspring. Birds are, therefore, exerting a continual selection pressure on moths.

So predators and their prey are in a contstant ‘arms race’ with each other – as the moths improve ways of avoiding being detected so birds improve their moth detection skills.

Evolution has come up with a bewildering array of different solutions - a host of designs – some subtle in their sombre beauty, others mesmerisingly showy. These designs are aimed directly at influencing the brains of birds – or more often avoid influencing them.

Burnished Brass - one of several species I catch, which have patches of metallic colouration. Having contrasting light  absorbing areas and the light reflecting areas very effectively break up the moth's outline - disruptive colouration.

Predators have a ‘search image’ – a mental template of a prey item's form. When an object in the field of view matches this search image it triggers a series of ‘prey’ responses  - identification, capture, handling, ingestion.

So adaptions which allow the moth to avoid conforming to the search image will be selected for. By far the commonest means of achieving this is camouflage.

When we look at camouflaged animals it’s easy to forget that the patterns aren’t usually ‘for us’. The pattern on a zebra for example seems conspicuous – but we have colour vision, lions only see in black and white – hence the zebra’s monochrome stripes.

While I would have thought it impossible to find an adaptive reason for each and every one of these lines and dots, even if one exists, it is maybe possible to, at least, list some of the factors that may be at play. So, here goes...

The moth itself
Phylogeny - features 'handed down' by the species' evolution
Behaviour - 'lifestyle'
Morphology - size and shape
Physiology
Cost - metabolic cost of producing a given feature
Background specificity - is camouflage targetting a specific or a range of backgrounds
Method of concealment - pigment, reflection, hairs
Day-time versus night-time factors
Seasonality - what part of the year is the adult's flight period - how long
Other strategies - fall back strategies such as startling patterns
Functions other than predator avoidance, e.g sexual selection, heat retention - there  may be trade-offs between these and the dictates of predator avoidance (and visa-versa)

Other moths
Frequency - if other moths of the same species are plentiful the predator's search image will be reinforced
Appearance - if there is little variation the search image will be reinforced
Competition - for 'camouflage space', spatially and ecologically
Other species - which similar ones are present - where moths (and other groups) are plentiful moth predators will be plentiful

Environment
Backgrounds - what backgrounds are available
Mimicable objects - which are available
Availability - of raw materials, pigments for example
Lighting conditions 
Seasonality - how does it change through the year
Homogeneity of background - may influence background matching versus disruption

The Predator
Behaviour - compare the foraging behaviour of the Treecreeper and Robin for example
Morphology - size, shape
'Startleability'
'Gullibility' - how easily is it fooled
Cognition  for example how good is it at spotting a moth
Vision
Search image - how specific, how accurate

Background Matching The moth's pattern resembles a random sample of the background it rests on. The 'goal' is to reduce the contrast between the moth and the background. This kind of camouflage is background specific.  Marbled Beauty

Disruptive Colouration The moth's colouration obscures its outline. This kind of camouflage is effective against a variety of backgrounds (even when the pattern contains colours that aren't present in the background¹). Clouded border

The Clouded border's pattern allows it to diverge from a typical moth shape and therefore helps to avoid conforming to the predator's search image. Edge detection plays a big part in the visual processing responsible for shape recognition - hence the role of camouflage in disrupting it.

Most moths that have background matching patterns, also exhibit some disruptive markings, especially near the wing edges.¹   Green Arches

Mimicry Here the object is not to avoid detection, but following detection, to be perceived as an irrelevant object - often a twig or leaf or distasteful object such as, in this case, a bird dropping. Bird droppings make a good 'target object' for micro moths to evolve a likeness to. They occur commonly, they are on a similar scale and they are distasteful. Plum tortrix

The Angle shades folds its wings in a way that enhances the similarity to a dead leaf, as well as having disruptive markings.

This list of factors is possibly just scratching the surface, but I think it's clear that there will be a whole web of interconnecting factors determining the moth's appearance. These factors will be in flux, some will be traded off against others.

Take one example of a factor -  the moth's 'lifestyle' - how it feeds, how it moves about, its larval foodplants - these will directly influence the moth's morphology - its size and shape. This will in turn influence which backgrounds it's possible to match, which objects it's possible to mimic (there won't be an unlimited range of backgrounds and objects available) this will, further, be determined by the the moth's habitat.

It's fascinating to speculate how there could be effects in the opposite direction - that's to say the kind of camouflage strategy the moth adopts could influence the moth's behaviour - a feedback loop.

Buff-tip

The eye is often brought up as an example of something that seems too difficult for evolution to arrive at 'by chance'.  Or in the case of the Buff-tip, how do you get from a Buff-tip ancestor to something that not only mimics a broken twig but a silver birch broken twig?

The answer is - you don't get from one to the other in a 'great leap forward' but incrementally, bit by bit.  So you come to look more and more like the thing you're imitating.

"That wouldn't work as you'd have to have intermediates that only look a bit like a stick", is a possible objection.

In the same way that having some vision, with an intermediate form of eye, is better than having no vision at all, looking a bit like a stick is better than not looking at all like a stick. The benefits might only be marginal, but they're benefits all the same. When your competitors have no vision at all, or no form of camouflage you're in evolution's pole position.

In the land of the blind the one eyed man is king, and in the land of conspicuous moths, one that's a bit 'sticky' is also king.

You might start off by fooling predators only in poor light or a long way off. But, by small steps, you'd graduate from this to full blown, accurate mimicry.

The illustrations in my moth book are so lifelike they almost seem to jump off the page!
The Blood Vein, another camouflage solution that natural selection has come up with

Peppered moth

I'm very lucky to regularly catch Peppered moths in the trap, it's almost a 'poster child' (or should that be 'poster moth') for evolution.

It is the first recorded and experimented case of  natural selection and remains "one of the clearest and most easily understood examples of Darwinian evolution in action".

Moths often to show considerable variation within the same species, this variation means that natural selection has 'something to work with' Small pheonix

During the Victorian era trees were blackened due to pollution. The light coloured form of the moth became conspicuous, while the dark form became better camouflaged and so the dark form came to predominate in industrial areas.

The light form didn't turn into the dark form - the dark form, which was there already - gained a selective advantage and so produced more offspring.

Yet another camouflage solution to the predator avoiding problem - the Scorched wing - a moth I'd only seen in photographs and illustrations. In my job as a web designer I use graphics programmes a lot and the pattern on the wings immediately suggested a photoshop-style 'radial blur'. Well I saw my first Sorched wing last month and it didn't disappoint - a fabulous looking moth.

Other strategies -  the Poplar hawk moth looks a lot like a dead leaf when resting. If it fails to escape detection it has a fall back strategy - it attempts to startle a predator, giving an opportunity to escape. The moth in the short clip below certainly startled me! The red under wing markings (probably) serve no camouflaging purpose - they are a kind of 'break glass in case of emergency'.

Other moths make no attempt at camouflage, especially day flying ones, instead they adopt warning colours - advertising their distastefulness or imitating other dangerous insects.

The function of these markings is presumably to immitate eyes and startle a would-be predator. The Spectacle.

Not all moth markings serve a predator avoiding function. Some moths are sexually dimorphic so sexual selection may play a role Bee moth.

So is it possible to ascribe a function for every minute feature of a moth's wing? Probably not, that's certainly not to say that no such reason exists - it possibly does. However the web of factors, influences, effects, is probably much too complicated to be unravelled.

Still, I'm looking at moths with fresh eyes now. Things of great beauty - certainly - but also trying to get just a glimpse of a glimpse of the reason that things are the way they are.

¹ H. Martin Schaefer, Nina Stobbe  2006, Disruptive coloration provides camouflage independent of background matching

NB I've only discussed adult moths in this post, most of this applies to all stages of a moth's life cycle.

Thanks to Andrew Caddy for this thoughts on the Burnished Brass

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