One of the trickiest things to wrap one's head around about insects and other arthropods* is also one of the most basic - how they grow. We tend to forget just how different arthropod growth is from our own - I've even known people who work with arthropods regularly to have it slip their mind.
*Other than that a scorpion's anus is at the very end of its tail next to the sting, not under the base of the tail as we chordates might tend to imagine.
For us as vertebrates, growth to maturity is fairly continuous. We start out small, we get steadily bigger. Take a balloon, blow it up, and you've got a fairly good representation of how we grow (yes, I'm massively simplifying things, but bear with me for a moment). Arthropod growth, on the other hand, is more like a series of balloons of different sizes all one inside the other, with the smallest balloon on the outside and the largest balloon at the centre. Start blowing up the balloons, and you'll only be able to blow it up to the size of the smallest balloon. If that smallest balloon breaks open (like an insect moulting its skin), then the balloons can inflate to the size of the second-smallest balloon. And so on and so forth, until you reach the largest size. Instead of growing in size continuously like we do, arthropods grow in steps - an extended period of no obvious increase in size, then a moult followed by a near-instantaneous increase as the animal swells up to fill its new skin, then another period without obvious growth. The change between moults can be drastic, as most obviously shown by the holometabolous insects with their radically different larval and adult stages. Even if the differences in morphology are not so drastic, separate instars (life cycle stages) may occupy distinctly different size ranges, with little or no overlap, and may have very distinct ecologies.
It's not that the arthropod is not growing at all between moults. A new layer of cuticle is being grown inside the old layer, albeit sort of crinkled up so that it can fit. Once the new cuticle has finished growing, the animal enters the pharate ("cloaked") state until the old cuticle is shed to reveal the new. Sometimes, the pharate period will be minimal, and the old cuticle will be shed pretty much as soon as the new one is ready. At other times, though, the pharate period will last for a considerable time. If conditions aren't right for the arthropod to move on to the next stage in its life, its growth may be effectively put on hold. Desert spiders may remain as subadults almost indefinitely, waiting for the rains to come before they moult into mature adults (and if the rains don't come one year, they can wait as subadults until the next). Caterpillars may moult into pupae at the beginning of autumn, but not emerge as butterflies until some time in the next spring when the flowers they feed on are beginning to bloom. If environmental conditions suddenly deteriorate, vertebrates are forced into the awkward position of having to maintain growth despite their reduced food supply. Arthropods, on the other hand, can afford to wait things out.
On the other hand, vertebrates have some liberties that arthropods do not. Most arthropods have a set number of moults in their life cycle, and as a rule they do not reach maturity until the very last moult. The flipside, of course, is that once an arthropod does reach maturity, that's it. They are unable to resume growth should the occasion arise (this is not necessarily a problem because many, if not most, arthropods do not live long as mature adults). [Update: A couple of readers have pointed out that some arthropods do continue to grow and moult after maturity, but this is not the norm. Arthropods being such a mind-bogglingly enormous group, any attempt to make generalisations leaves one bound to make an idiot of oneself.] Contrary to what the cartoons may suggest, little ants do not grow into big ants. Both are fully adult, both are as big as they're going to get. In those ant species that have different sized castes, it's easy to imagine otherwise, but that's simply not the case. Big ants hatched out from their pupa as big ants, little ants hatched out as little ants. Similarly, if the queen of an ant colony were to die, it would not be possible for one of the workers to develop a functional reproductive system and take her place - sterility is a one-way trip.
Christopher- super-helpful post. Thanks!
ReplyDeleteYour balloon inside a balloon analogy is the best I've heard. I'm going to be stealing and using it liberally. Thanks.
ReplyDeleteThe illustration shows five nymphal stages. I assume that the number of nymphal stages is constant in any one species: does it vary from species to species (in which case it would be a "life history trait" relevant to phylogenetic analysis), or is it pretty much constant across (at least hemimetabolous!) insects?
ReplyDeleteThe number of instars is often constant within an arthropod species, but different taxa may have different numbers of instars. At least some arthropods do have varying numbers of instars within a species, however. For instance, different environmental stimuli (such as warmer temperatures) may cause a nymph or larva to moult through to maturity earlier than normal. In at least some Opiliones with polymorphic males, different male morphs result from individuals reaching maturity at different ages. Also, while I did say that most arthropods only have a single sexually mature instar, there are some exceptions to that too (barnacles, for instance, lose and regrow their genitalia between breeding seasons).
ReplyDeleteYou can also get apparent variations in life cycle that are not the result of actual changes in the number of instars. Arthropods go through a number of embryonic instars before hatching out their eggs. In some taxa hatching may be delayed so that some of the larval instars also happen in the egg, so the larva is more advanced when it emerges (and hence will go through fewer visible instars before reaching maturity). One of the most extreme examples of this is found in some flies where the larva is retained within its mother and not "laid" until it has already reached the pupal stage.
Great summary. You could add this caveat though: in a group as large and diverse as the arthropods there are exceptions to pretty much every generalization.
ReplyDeleteFor example, female tarantulas continue to molt and live long after sexual maturity.
Great post! It's amazing how sometimes we forget or don't know the most basic life cycle facts about organisms (something botanists are highly guilty of, too!).
ReplyDeleteSo, in the realm of my ignorance about arthropods, there is a question I've been confused about for years: what is meant by 'growth' in arthropods? Is it measured by an increase in cell number, or by an increase in volume? This has distinct implications for when 'growth' actually occurs.
One version I've heard is that these two definitions of growth in arthropods are distinct - that cellular growth is constant, but volumeterically occurs in discrete stages.
Is this the case? Hopefully, you can clear up this conundrum for me!
Mossy Rock: You're right, growth as in cell division is a distinct process in arthropods from growth as in increase in volume. New cells and structures are grown inside the outer cuticle between moults, but volumetric increase only occurs during the moulting process.
ReplyDeleteIf you want a really extreme version of this, look at mushrooms. Mushroom-producing fungi grow their fruiting bodies in miniature within a 'bud' ('bud' is not the correct term, but I can't remember the correct term and 'bud' is close enough for now), and then the cells within the mushroom are pumped full of water, swelling them to many times their original size. This is how mushrooms are able to become visible overnight (and, of course, why "to mushroom" means "to grow quickly").
Mushrooms have 'buttons'. :)
ReplyDeleteBut, buds in most overwintering woody flowering plants also follow this pattern. I didn't even think of the parallel before - good call! The amazing, sudden, flush of leaves during spring, happening right now in my area, is due to the influx of water pumped into extremely small cells of the leaves that already developed over the winter. Watching buds fatten in spring after a long winter is surefire harbinger of green things to come.
Oh, how I love that such phylogenetically diverse groups use the same mechanism! Any others out there in the diverse world of multicellularity? I assume ferns must also do this, their croziers appear a little too rapidly in spring to be just cell division. Brown algae, red algae - they have structures *termed* buds, but do they have multicellular buds that function this way?
I don't think that red and/or brown algae have buds in that sense, but I could very easily be wrong.
ReplyDelete