Any 'reason' for a caterpillar changing into a butterfly?

A quick question, what evolutionary purpose is served by having an organism with two grossly differrent morphologies? I can understand parasites that bounce around different hosts, but why is it an advantage for insects? Why don’t we have baby flies? I know that the high school texts list it as a seperate ‘feeding’ and ‘migration’ phase, but can’t that be served by a single body type?

Also a bit of a side question, how can such a mechanism evolve? Is there a recessive genetic trait that turns insects into a soupy mess?

Quick note: No, I’m not a creationist troll; and I know that there isn’t necessarily a narrative reason for evolution (hence the inverted commas).

There is now “why” in evolution. The fact that they do, and that doing so didn’t drive them to extinction, is enough.

The joke goes that “a chicken is an egg’s way of making another egg”. In creatures that have a pronounced larval stage in their life cycle, this is closer to the truth.

Many insects do start out as “babies” similar to their adult form. Many don’t. As to “why” in the sense of “what selective pressure favored this mode”, the answer is probably something like this:
Insects as a group are highly successful because almost all insects can fly in their breeding phase, making it much easier for them to propagate. So retaining that adult morphology is highly useful. But having a separate larval form allows them to exploit food sources and econiches they wouldn’t be able to use otherwise. Some have taken this to the extreme that that they only live long enough as adults to mate and spawn; then they die.

Adult mayflies don’t even have mouth parts for eating. They do all their eating in the larval stage; the adult stage is only for sex and egg-laying, and lasts only one day, then they die.

We do have baby flies. They’re called maggots.

Not really, no. The point of the separate feeding phase is not that it is the only phase that feeds, it is that it is the only phase that feeds on that foodstuff. That increases the total amount of resources available to the species greatly.

Caterpillars for example have chewing mouthparts adapted to break up tough solid matter. Moths OTOH have a drinking tube for a mouthpart. They are quite useless at exploiting anything other than solid food. If the organisms decided to utilise a single body type they would need to make a choice between living exclusively on leaves or exclusively on nectar, they wouldn’t be able to utilise both.

That is a very undesirable state of affairs when you realise that most insects have huge juvenile mortality. There are effectively 100 or more juveniles for every adult. If all the juveniles and adults ate the same food there simply wouldn’t be the resources to support the adults any more, and reproduction would suffer.

Therefore those organisms that evolved in such a way that their offspring didn’t compete with other members of their own species until they were ready to reproduce had a massive advantage.

That’s one of the tough questions, and the answer is a long way form certain. What appears to have happened is that the ancestral insects originally had a form on hatching that was similar to but not identical to the adult. This is still seen in a lot of insects today and the juveniles are referred to as nymphs. In these first insects there was a more or less constant change between the nymph through to the adult form, with each moult producing a few more changes.

Later insect types became decreasingly divergent form this pattern, and all living insects that don’t outright metamorphose go through at least 3 fairly distinct changes. The fist is the pronymph, which is basically designed as a specialised crawling insect. That seems like a sensible enough mechanism evolutionarily, since only the final two moults can produce wings in insects there is little point maintaining a flying form when there is no chance of flight. Far more sensible to have a form that is well adapted to crawling. Following the pronymph modern insects produce a nymph. The nymph is the essentially a juvenile form of the adult, complete with wing buds etc. It is essentially a form that allows a transition from crawling and hiding to flying moving. Then of course finally there is the winged adult form.

The transition from pronymph to nymph to adult is under the control of a complex series of hormones. What appears to have happened is that within some lines of insects the hormonal changes that produced the transition between the three ancestral body types became gradually more dramatic. Instead of the pronymph being replaced over a series of several moults by the nymph which in turn was replaced by the adult the pronymph only moulted into larger pronymphs, and then in one very dramatic moult the pronymph produced a nymph form which then existed largely unchanged and then in trun produced the adult in one spectacular moulting event.Today that has been taken to the extreme where the pronymph is the caterpillar form, and the nymph form has been entirely reduced to a pupa.

It has been proposed that the original impetus for that transition was to free up m ore food for the pronymph forms. By allowing them to exists in, say, a flattened form or a burrowing form they could exploit food in crevasses or underground that would be lost far more rapidly if they began an immediate transition towards the adult form. The ideal tactic therefore was to extend the pronymph form for as long as possible during the spring and summer, and then only enter the pronymph forms during a period of aestivation during the winter, fuelled entirely by internal food reserves. Then when the warm weather returned the pronynmph emerged and went through one or two final moults to produce the adult. In that manner there was limited competition between adults and juveniles even without complete metamorphosis.

By a series of fairly gradual changes, each improving on the efficiency of each stage the current system of total metamorphosis was produced.

It might be just some cosmic accident, but it does seem that one is a supreme eating machine and the other is a supreme mating machine. One might as well ask: Why have the **same ** body type for the various stages of life.

That’s only really true of a tiny fraction of the holometabolous insects though. Most adult moths, flies, beetles, fleas etc are just as well designed for eating as adults as they are as larvae. The only difference is the food sources they exploit

Thanks for the great post Blake, I completely forgot about competition within a species.

And John Mace, I understand that sometimes evolution can be summed up as “sh!t happens” (ie, random chance that doesn’t significantly affect the chances of an organism reaching breeding age) but I was wondering what offset the energy requirements for the metamorphosis.

Your logic would only apply if they could feed off both types of food at the same time. If neither is available, then they die…

I don’t see any reason to have a “reason” for morphology… we never “needed” to evolve past bacterical stage.

That is, if EITHER is UNAVAILABLE… oops!

The way it is, it’s just increasing dependence because a greater variety of food sources is absolutely required.

That’s not the case dre2xl. Imagine how many more members of the human species a modern city could support if all children under 12 could eat lawn clippings for example. That shows why the logic applies perfectly well even if each lifeform can only feed off one food item.

I think you are getting a little confused here dre2xl between dependence on resources and availability of resources. All organisms are dependent on resources, and if those resources vanish the organism dies. However what we are dealing with here is not resource dependence but resource limitation. It is not simply sufficient in the evolutionary game to have the ability to utilise a resource, it is necessary to be able to utilise the resource with such efficiency that it doesn’t limit reproductive success in any way. That is the ultimate ‘goal’.

It doesn’t matter how many foodstuffs are available to a hemimetabolous insect like a cockroach because once the population reaches a specific level there will be insufficient food available to permit the organisms to grow and/or reproduce. The name of the game is to be able to reduce the limitations imposed by resources, not to diversify foodsources per se.

By having juveniles that feed off completely different foodstuffs holometabolous insects have effectively halved the population, or else doubled the level at which food becomes limiting however you care to look at it. It doesn’t matter that no one individual can exploit both food sources at the same time. What matters is that at any population can exploit both food sources at the same time. AT any given point in time large portions of the population are not competing with the rest of the population. The frees up large amounts of resources and thus extends the limits of the population.

You make a good point and explanation, blake–thank you very much.

Waiter! There’s soup in my fly!