The article on insects morphing from one environment to another does not seem to state explicitly why two environments. It appears to me that one is favorable for the growth stage and the second for reproduction and distributing the species.
Any comment on the complexity of how the DNA in a single cell can contain all the information not only to form the first form of the insect, but also the deferred second form and execute the transformation at exactly the right time?
cmoose
Link to Staff Report (which will appear officially on Tuesday, Dec 9): OK — WHY exactly does a caterpillar morph into a butterfly? - The Straight Dope
Good point. Doug does not really address why the adult would shift to a different environment or why the feeding strategy should be different.
I imagine it’s the same as with any developmental process. What triggers, for example, sexual differentiation and the development of different sex organs? What triggers the development of sexual maturity in humans so much later than the embryonic and fetal development processes?
The cell might not be all that pressed for space. A lot of a cell’s DNA doesn’t seem to do anything at all.
In fact, I don’t think erthe even has to be an advantage attaching specifically to this trait. (I mean a trait like the switching of feeding strategies.) For it could just be that the younger form, simply by being smaller, thereby occupied a different niche than the older form, and so evolutionary pressures worked on the younger form differently than the way they worked on the older form. There was nothing particularly advantageous about switching strategies per se, on this scenario. The switching of strategies, and subsequent differentiation of young and adult forms, are not selected for, but rather are accidental byproducts of other things being selected for.
-FrL-
First, I think this was an excellent answer to a difficult question. Whenever you are dealing with “why” questions in biology you are on much shakier grounds than “how” questions. Also, this column must be read as a companion piece to Cecil’s answer referenced in the question. That has much of the information about how the metamorphosis occurs, including the “packing” questions discussed in this thread. I think the evolutionary advantage of multiple ecosystems should be somewhat apparent: one can always vanish. There is some credence to the idea that the adult is better suited for distribution of the eggs/larvae to hedge against disappearance of the first environment.
I’m not sure what you’re saying, here. If you’re saying that having a more mobile adult phase hedges against the loss of any one pocket of the larvae ecosystem (while other pockets remain), that’s true enough (on an individual level, of course).
It’s pretty clear that for some species, distributing offspring is now a key selective pressure towards keeping their two-phase system. In many species the flying adults don’t feed at all, and merely mate, find a good spot for eggs and lay them (females only) and die.
But there doesn’t have to be one answer: different species can have different selection pressures. It’s certainly plausible than in some cases, merely changing size creates a different enough interaction with the environment that drastically changing shape is worth the effort. After all in many other species, the flying adults do feed extensively and live as long or longer than the larvae.
The explanation given in my invertebrate zoology class was that this is a way to keep the adults and larvae out of direct resource competition with one another. In the case of the reviled Japanese beetle, the larvae feed on roots, while adults feed on leaves and flowers.
As a current biology student, I have to agree with corigirl.
The simplest answer to why a complete metamorphosis is evolutionarily advantageous is intraspecific competition. Specifically in regards to butterflies, transforming into an adult stage with the ability to fly is also extremely advantageous when it comes to finding food, shelter, etc and avoiding predators. If the possibility to develop flight is there, then it will most assuredly be selected for evolutionarily. Think about it, who’s got a better chance of escaping a predator, a butterfly or a caterpillar? Or better yet, don’t just think about it, go out in the backyard and see which one is easier to get your hands on. I venture to say that the flying version will be much more elusive.
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Slight correction to jwalk1979’s comment on flying advantages - some species do not eat! and live only few days, just long enough to mate and lay eggs (the Monarch is a huge exception). So reproduction seems the primary advantage of the second phase. cmoose
Agreed, it’s definitely a big factor. I was confused at first when I first started biology, what with some species having a predatory larval stage. I believe that part of the definition of an “adult” organism is the stage which has the ability to reproduce.
it makes complete sense that a given insect might change from a larval stage to an adult stage to inhabit different ecosystems and to avoid competition. the answer is still incomplete, though, i think. the question i want an answer to is: why have a larval stage at all? the adult phase seems to address all the concerns raised here.
I think the last part of your question was kind of answered in the article, but to elaborate… The larval stage is basically just the developmental stage that mammals go through in the womb taking place outside of the body and outside of parental control. I would guess that due to size and life span, among other things, developing insects don’t have enough resources provided by an egg or placenta to allow for full development into adulthood. Therefore, they need a stage before all the advantages of adulthood can be reached, that allows them to seek out their own resources, since the parents are not involved in that process. Think about the difficulty that some human parents have in providing for the handful of children that we produce. Now imagine popping out a few dozen or even hundreds of offspring and you can see how these animals’ early stages must be exponentially more independent than a human’s during the required time to reach adulthood.
There is also the practical problem of growth in an exoskeleton. A distinct larval form allows the adult to exist in ecological niches (especially involving flight) that are difficult or impossible to combine with molting.
There are a lot of good points here, but some people are missing something which is, whether the Dennis A. meant it or not, implicit in the question.
Complete metamorphosis (laying aside hemimetamorphosis for a moment) is not just another trait like antennae or an exoskeleton or a tweed jacket. Holometamorphosis (to recap, this is what most people think of as “metamorphosis”, the complete breakdown and reordering of the body in a crysalis) is a huge gamble and not one which can be easily explained away. The organism is static and defenseless for a significant period of time leaving it wide open to predation and the transformation takes an enormous amount of energy. An organism needs a very good reason to do something like this.
Clearly the purpose is to occupy two distinct niches at different times because this is exactly what is achieved, but this is not an explanation in itself, it still leaves us with the original question, why do it? What makes changing niches so valuable that it is worth the risk and expenditure?
The answer is different for different species, but the common factor is specialisation. Even some ametabolous insects (don’t metamorphose) are masters of specialisation (see ants) and the greater question that comes out of this one which also bugs (pun intended although, of course, insects are not bugs) biologists is “why specialise?” Again, it is energetically expensive, so it must have a definite purpose. The best answers I have seen essentially come down to the fact that specialised organisms do what they do better.
So what I’m saying, in a very roundabout way is that a holometabolous insect wants to do two different things really really well (depends on species what those are). In the vast majority of butterfly species, for instance, caterpillars eat like crazy and butterflies eat little or nothing and mate like bunnies for their short lifespan. In this case, the caterpillar is ideally suited to be camouflaged against its energy rich food source, probably greenery, uses very little energy (crawling and eating is pretty low-energy) but doesn’t get very far. It builds up a nice little energy store and then becomes a colourful butterfly. The butterfly has bright colours so it will attract mates but also predators.
It can fly which uses a huge amount of energy but that doesn’t matter because it won’t live very long anyway and it’s much more important to get those eggs into a safe place, away from the vegetation which has most likely already been ravaged by larval forms onto new pastures (remember many insects go through several generations every summer). There is no way that some kind of caterpillar-butterfly hybrid could fulfil either of these roles nearly as effectively (just look at Hans in A Bug’s Life, thus the metamorphosis.
It’s also worth noting that this probably isn’t only a solution to more general insect issues like independence of development or intraspecific competition (strictly speaking it would have to be kin competition to have any evolutionary impact) as there are ametabolous insects which deal with these problems in vastly less energy intensive and risky ways. Natural selection is a harsh mistress, she doesn’t have time for unnecessary messing about. Many species have increased their survivability by overcoming other problems at the same time, changing food sources is a convenient thing to do since you’re already messing with your morphology, but then some organisms do that without metamorphosing.
Eek, bit of an essay there, my apologies but I hope it is helpful for stimulating further discussion.
I generally agree with Beebop’s points (should we call you **Butterflybop **for this thread?) , though I’d phrase the question not so much as “Why specialize” but as “If caterpillars are well-specialized for one niche, why do they go through all the effort of re-specializing for a different one?”
It is clear that finding mates and distributing eggs is a major advantage for metamorphose in many species, because adults in those species don’t feed at all or feed minimally. In other species, other benefits may be more important. In fact, given the number and variety of insect species, I’d guarantee there’s a species that gain some major advantage besides mating and dispersion (but I’ll echo** Beebop**: there’s no evolutionary advantage to an adult in avoiding competition with juveniles that aren’t your offspring those of very close relatives
Finally, there are also certainly some insect species that gain no real advantage to metamorphosing, but simply haven’t existed long enough for natural selection to re-order such a major part of their physiology and development.