Stupid Question ™
May 17, 2004
By John Ruch
Q: What is the evolutionary advantage of insect metamorphosis, and how did it come to be?
—Dave Williams, Columbus, Ohio
A: While not every aspect of biology is the result of selective advantage, there can be no doubt that something gave metamorphosis an evolutionary thumbs-up.
More than 80 percent of known insect species undergo complete metamorphosis, suggesting it gives a strong selective advantage across a wide range of habitats. Even more persuasively, the types of insect that undergo complete metamorphosis have many more species than those that don’t, or that only undergo partial metamorphosis. That suggests a meaningful adaptation.
And, of course, the metamorphosis tactic is used by many other types of animals, including many shellfish and amphibians. Something about it is working.
Metamorphosis is a development process in which the juvenile and adult forms of an animal are distinctly, and sometimes wildly, different. The physical difference between an adult human and an infant human is minor; the difference between a caterpillar and a butterfly is so astonishing they could well be conceived of as entirely separate creatures.
In “complete” (or “holometabolous”) metamorphosis, the insect has four life stages: egg, larva, pupa and adult. The larva is essentially a worm with very simple body parts, no wings and no sex organs. As the larva molts, or sheds its skin to keep growing, it may change its form several times—say, with legs, without, and back again.
Finally, the larva will become a pupa. This is (almost always) a semi-hibernating stage in which the insect holes up and undergoes major transformation. When it emerges from this state, it is in adult form, complete with complex eyes, wings and all the other options.
The classic example of complete metamorphosis is the butterfly larva hatching as a caterpillar, entering a pupal cocoon, then emerging as an adult butterfly.
In “incomplete” (or “hemimetabolous”) metamorphosis, the larva often just looks like a wingless, baby adult—much more analogous to the human situation. But other hemimetabolous insects, such as dragonflies, have larvae (known as nymphs or naiads) that don’t look much like the adult and live in the water.
Some simple insects—especially the very few wingless varieties—have no metamorphosis at all, or with such slight changes they aren’t counted.
The stages of metamorphosis are always tied to molting, the shedding of skin insects undergo as they grow. Insects have hard exoskeletons that can only stretch so much. If the insect is to grow, the exoskeleton has to shed a layer and grow a new, larger layer underneath that works it way out (it starts out soft, then expands and hardens).
In and of itself, this molting is extremely sophisticated, involving the absorption of excess dead tissue and other complex biological processes. The stages between molts are known as instars.
In the few insects that don’t undergo metamorphosis, the insect simply gets bigger with each instar. But in the other insects, an instar almost always involves a significant biological change in addition to growth.
Molts are caused by the release of a hormone. (What triggers the hormone release is unknown.) Another hormone regulates whether the insect continues on as a (perhaps slightly changed) larva, or becomes a pupa/adult.
The specific evolutionary history of insects is largely unknown. But we can guess at the general evolutionary advantage metamorphosis had over plain old molting.
The best guess: It allows the larval form and the adult form to both evolve almost separately, adapt on their own tracks, and even live in entirely different places in which the two forms don’t compete with each other for food and other resources.
In this light, metamorphosis can be seen as a division of the labor of perpetuating the species, since the larval form basically does nothing but eat, while the adult form has sex and reproduces.
It is conjectured that complete metamorphosis evolved from incomplete metamorphosis, as a specialization and extension of the embryonic phase of development to produce a larva focused on long-term feeding.
An example of this remarkably extreme division of labor and resources is the mosquito, in which the larvae live in water and eat plankton, while the adults fly around sucking blood. It’s the kind of specialization we would expect from two entirely different types of animal—but here it’s all in one.
One specific theory for the way metamorphosis could have developed is buried insect eggs hatching larvae that found underground foods the adult form wouldn’t eat. Selective pressures then favored such a division of resources.
And a specific theory for what the selective pressure could have been is climate.
The earliest undebated insect fossils are about 300 million years old, but insects are surely much older. The first fossil evidence for the types of insects that today undergo complete metamorphosis dates to about 290 million years ago.
That’s when a significant climate shift, including winter conditions, prevailed over former swamplands of millions of years before. A pupal stage could be a way to survive the cold; separate adaptation between the larval and adult stages could be a way to survive food scarcities.
Indeed, many insects today survive bad conditions in pupal form, or have complex larval cycles that seem reactive to the climate. (However, it must be noted there are a few examples of insects in which the larvae and adults share food sources.)
I mentioned the idea that complete metamorphosis may have come from incomplete metamorphosis. So why did incomplete metamorphosis evolve?
One speculation is that it was driven by the pressure to grow wings, which require at least one major body change to appear (and typically several in the incomplete metamorphosizers today). This strikes me as a weak and even backward argument; metamorphosis is just as likely (if not more likely) to have enabled the formation of wings.
It is probable that incomplete metamorphosis came about under the same division-of-labor/resources pressure, and that complete metamorphosis is just a more specialized (and apparently more effective) version of it.