Butterflies Flutter, But Not If They Are Poisonous

Blog Author: Professor Adrian Thomas, CSO, Animal Dynamics

As the temperatures warm up the butterflies are out and about – fluttering around erratically. That fluttering is weird – an erratic wandering flight path is hardly the most efficient way to get from A to B, but it does have the advantage of unpredictability, and if you are a tasty butterfly having an unpredictable flight path may be a lifesaver. Big obvious slow-flying butterflies are an obvious target for predators, and butterflies (and moths) have evolved elaborate strategies to avoid being eaten. Unpredictable fluttering flight is one, being poisonous is another.

Lots of butterflies are poisonous – usually because their caterpillars eat poisonous plants (and they store the poison). Of course being poisonous doesn’t offer protection if you have already been eaten when the poison takes effect. The protection comes from the combination of being poisonous and having a warning colour that advertises that you are poisonous. Yellow and black stripes are perhaps the most obvious colouration and we all know we should leave wasps alone. Red and black works aswell – pick up a ladybird and you will get squirted with noxious bitter alkaloids (don’t ask me how I know they are bitter). Warning colouration is all marketing – bright coloured eye-catching high-contrast patterns, designed to catch the predators eye and stop them before they’ve done any damage.

Warning colouration seems to work, and well enough that some poisonous butterflies have given up fluttering. Instead poisonous butterflies rely on their warning colours and shift their bodyweight from thorax to abdomen – guts and gonads for growth and breeding rather than flight muscles for escape. That has the coincidental effect of shifting the poisonous butterflies centre-of-gravity back down the body relative to the centre of lift of the wings, which makes them less stable, aerodynamically. Apparently paradoxically that reduction in stability may be why they don’t flutter. When a stable flyer is hit by turbulence its stability generates restoring forces – gusts knock them off balance and their stability recovers them. The result is a wandering – fluttering – flight path. Unstable flyers don’t generate restoring forces when hit by gusts, so they follow a smooth direct flight path, but they require active control to stay the right way up in still air. Stability has aerodynamic costs – generating restoring forces causes drag – and glider pilots will carefully trim the centre of gravity of their aircraft shifting it backwards until the stability is only just acceptable. In hang gliding competition, pilots used to reduce pitch-stability so much that they suffered turbulence-induced glider-breaking nose-down tumbles, and rules have had to be enforced to maintain safe levels of pitch-stability.

In birds and pterosaurs, the efficiency gains from reduced stability (and a greater reliance on active control) have driven the evolution of flight-morphology. The first known birds (like archeopteryx) had a long, broad tail, so did the earliest pterosaurs (like ramphorynchus). Over evolutionary time birds tails got shorter, and pterosaurs tails virtually disappeared. Both groups evolved towards reduced aerodynamic stability and greater reliance on active neuromuscular control of flight – with greater manouverability and greater efficiency. You can see it in effect today – watch the swifts (unstable) and red kites (stable) soaring in thermals (turbulent). Red kites get knocked around by the turbulence, and make slow gentle manouvers, swifts are apparently oblivious to turbulence – unperturbed – carrying on making sharp agile turns to catch the insects swept up and concentrated in the thermal updrafts.

Poisonous butterflies have taken the same route, adopting warning colours, shifting the centre of gravity backwards, reducing the mass of their flight muscles, reducing their aerodynamic stability and therefore cruising straight and level through turbulence. If an oblivious predator ignores the warning colouration and catches them it will pay the cost later, and learn not to attack similar coloured butterflies in future. The poor victim pays the price, but its similarly toxic and similarly coloured relatives will be protected, and that selective advantage has led to the evolution of the most striking colours we see in nature.

If you are lucky enough to see a swallowtail butterfly admire its strong and direct flight, but don’t eat it. The larvae eat plants rich in aristolochic acid and the adults can fly confidently knowing any predator that eats them is likely to suffer kidney failure not long after. Unlikely to be a problem in the UK, since Swallowtails only breed in Norfolk. However, the other day I found a six-spot burnet moth on the way in to work. The six spots are bright red on a black background, striking in a day-flying moth, and a necessary warning: burnet moths and cinnabar moths give off are able to give off hydrogen cyanide as a chemical defence. They don’t need to flutter.

Swallowtail Butterfly
Swallowtail Butterfly

Cinnabar Moth
Cinnabar Moth


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MAY 28, 2021