30 October 2008

Why do butterflies have four wings?

ResearchBlogging.orgIn this week's PNAS is a tidy result that demonstrates what you ca get away with when you study invertebrates: butterflies and moths can still fly if their hindwings are amputated, but they can't take evasive action [$-a]. That summary tells you just about all you need to about the reported experimental result; but the rest of the article has some interesting speculation.


Photo by me.
The authors, Jantzen and Eisner, start from the premise that the large, showy wings of butterflies should (very generally) make them major targets of predation. They note, however, that butterflies are also marked by highly erratic flight - an extreme maneuverability that makes it difficult for a predator to guess where a butterfly is going to be in the future based on its current trajectory. Maybe showy wings actually act as a kind of inverse protective coloration:
A bird, we suggest, could learn or inherently know that brightly colored airborne prey, discernible from afar, is not worth the chase. Too elusive to catch and, because of their [wing] scales, too slippery to hold ... Birds might simply write butterflies off, and ... relegate them all to the category of the undesirable, treating them as they treat noxious insects that they disregard.
Jantzen and Eisner's experiment, in which they amputated butterflies' hindwings, confirms that butterflies can still fly without the second pair of wings, but fly less erratically. Does the result confirm the authors' major hypothesis, though? I'm not so sure. There are plenty of other reasons to have big, showy wings - mate attraction, or as placement for eye spots to fool predators - and plenty of butterflies and moths are comparatively small and non-showy. Jantzen and Eisner's hypothesis smells more than a bit like adaptive story-telling, though it provides some food for thought.

Reference

B. Jantzen, T. Eisner (2008). Hindwings are unnecessary for flight but essential for execution of normal evasive flight in Lepidoptera PNAS, 105 (43), 16636-40 DOI: 10.1073/pnas.0807223105

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