Butterflies wing it

Benjamin Jantzen and Thomas Eisner have asked what at first sight seems a very odd question – does cutting off the wings of butterflies and moths affect their ability to fly? Now two things that we all notice about butterflies are that they are difficult to catch when flying and their wings are very large by comparison with their bodies. They are obviously highly manoeuvrable and this makes them a challenge for predators.  Copyrighted Image Jupiterimages A butterfly

Butterflies have two pairs of wings and they are not always of equal size. In some species the hind wings are much smaller in area than the front pair. Jantzen and Eisner have shown that the pairs of wings do not have the same role in flight. The front wings primarily drive flight – remove them while leaving the hind wings intact and the butterfly can’t fly. However, removing the hind wings while leaving the forewings intact does not prevent the insect flying nor does it seem to reduce endurance or altitude. The insects still manage the same flight trajectories as they did before, but the speed of flight is reduced and, crucially, acceleration. In short, their ability to manoeuvre is compromised.

So what is the explanation for this observation? Clearly there has to be an evolutionary advantage to the possession of two pairs of large wings, if flight is possible with one pair. The selection pressure that favoured the large hind wings is almost certainly predation. Speed may help if you can outfly a predator, but if you turn faster and zigzag more often than the predator, escape is possible.

Butterflies and moths have forewings for flight, hind wings for manoeuvrability and speed. You can sometimes see this for yourself on a summer night when bats are hunting moths around street lamps and the moths zigzag down towards the ground so that their echo is lost amongst the echoes from the ground, before the bat can catch up with them.

Oh, and please don’t go out and pull wings of butterflies yourself!

Find out more about butterflies in episode 6 of Breaking Science.