A Staff Report from the Straight Dope Science Advisory Board

OK — WHY exactly does a caterpillar morph into a butterfly?

December 9, 2008

In reference to your excellent response to today's question, "How exactly does a caterpillar morph into a butterfly?" [staff report, 9/16/2008]: I have long wondered about the next logical question, namely why does a caterpillar morph into a butterfly? What is the evolutionary advantage? I have searched far and wide for the answer, getting a "Who knows?" from one entomologist and tepid theories from the Internet. What say you?

SDSTAFF Doug replies:

If a biological trait is sufficiently novel and dramatic, then sometimes it can be a problem understanding how it might have come about through evolution. But while the complete metamorphosis undergone by most types of insect – egg to larva to pupa to adult – is certainly dramatic, it's not exactly novel: there's a whole range of differing metamorphic processes seen in various insect species, so it's possible to look for patterns that might explain how the trait came into being.

For example:

(1) Among the hemimetabolous insects – those that undergo incomplete metamorphosis, from egg to nymph to adult – a few pass through a "pseudopupal" stage, a kind of rest stop between the last active nymph stage and adulthood.

(2) In the really archaic aquatic insects, such as mayflies and dragonflies, the nymphs and adults are fairly different in appearance compared to other hemimetabolous groups. Mayflies are the only insects that have two adult stages, both with wings (the second adult stage is much better at flying than the first).

(3) In the more primitive holometabolous insects (holometabolous = undergoing complete metamorphosis) such as dobsonflies (which are aquatic), the immature forms and adult forms are not nearly as different from one another than is the case in the more advanced holometabolous insects.

On putting a number of such observations together, a fairly clear and logical pattern emerges: when the immature and adult forms of a single insect species live in different habitats and/or have different feeding strategies, there is an evident utility to the two forms' being different in appearance. So the problem that complete metamorphosis solves is this one: If you spend your entire life in one basic form, then that one form cannot possibly be adaptively optimized for two different sorts of environments. That is, if you have a nymph that's perfectly designed to live underwater, an adult that's essentially similar in form to the nymph isn't going to be very well designed for living out of the water. A dramatic metamorphosis between larva and adult means that natural selection can act independently to optimize both the immature and adult stages for their particular environments without requiring any messy compromises. If a maggot doesn't need legs, eyes, or a recognizable head, then the maggot is free to evolve in that direction along its own pathway (toward whatever is best for the maggot), while the evolution of the body of the adult fly is subject only to the pressures acting on that specific life stage. You can have related species with effectively identical larvae but very different adults, or nearly identical adults but very different larvae. The uncoupling of the immature and adult evolutionary pathways makes a lot more possible for those species that can pull it off.

Of course, this sort of benefit applies to any physical transformation as a creature grows from the earliest stages of life to adulthood. A lot of organisms transform during their development, sometimes fairly dramatically overall, but it's usually a gradual process. Look at amphibians, mollusks, fish, or echinoderms. In a lot of such species, if all you were shown were the newly hatched forms and the full-grown adults, you'd never be able to match them up. Heck, even in mammals, the only real reason we don't get to appreciate the radical transformation is that most of it happens while the animal is still inside the mother's body. But in all these cases, there are no sudden jumps from one form to another, but rather a smooth curve connecting the various developmental phases. Complete metamorphosis in insects is a developmental process that permits the near-elimination of those intermediate stages in the transformation. That's a lot more efficient, and permits a remarkable degree of fine-tuning and specialization.

I hope that's not too tepid for you.

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