Metamorphosis!

Before I get into details about more individual insects and groups, I thought it might be a good idea to go over the different types of metamorphosis, or how insects develop.  As you probably know, insects go through several different stages of development and can look very different in each stage.  Let’s look a little deeper into how it works!

Because insects have their skeleton on the outside of their bodies (they have an exoskeleton), they can’t grow the way humans do.  Their skeleton only stretches so far, so if they need to get any bigger, they need a bigger skeleton.  So, they grow a new, bigger skeleton under the smaller one.  This skeleton is soft and flexible so that the whole thing can fit under the hard, outer skeleton.  Then they molt.  When an insect molts, it breaks out of the smaller skeleton and stretches its soft, bigger skeleton out by inhaling a lot of air and pushing fluids around inside it’s body.  When it’s fully expanded, the skeleton hardens (or sclerotizes).  At some point, an insect undergoes a final molt and become an adult.  Once an insect is an adult, it no longer molts.  This means that insects only molt when they are pre-adults, or immatures.

Insects molt several times during development, but the number of molts depends on the insect.  Dragonflies molt many times as they develop, some molting 20 times or more.  Other insects molt only a few times, such as the giant water bugs that undergo 5 molts.

Let’s get back to metamorphosis.  We’ll start with the really simple insects, the ones that don’t have wings.  These include the silverfish and firebrats, though the springtails and some other technically non-insects are sometimes included in this group.  These insects have a type of metamorphosis called ametabolous.  The word ametabolous is Greek and basically means “doesn’t change.”  In most insects,  the immatures look different from the adults to some degree.  In aematbolous insects, all non-egg immature stages look very similar to the adults except that each stage is just a little bigger than the previous one.  This is illustrated in a simple diagram of the life cycle of ametabolous insects:

Ametabolous metamorphosis

Ametabolous metamorphosis

Notice how these insects look alike, just different sizes.  Because the ametabolous insects do not have wings, it can be a little difficult to tell when a silverfish is an adult or an immature.  Only the adults can reproduce though, so if they’re capable of producing offspring, they’re definitely adults.

Other insects undergo incomplete metamorphosis, including the true bugs, grasshoppers and crickets, and dragonflies.  Insects that have this sort of metamorphosis are called hemimetabolous, which is Greek for “partial change.”  In this type of metamorphosis, the immatures look a lot like the adults, except they do not have wings.  Here’s a diagram illustrating this type of metamorphosis:

Incomplete metamorphosis

Hemimetabolous metamorphosis

Can you see the difference between this type of metamorphosis and ametabolous metamorphosis above?  Here, the immatures look a lot like the adult, but there are obvious differences between them.  If you don’t see the difference between the stages in this diagram, focus on the image at the top and the image at the center-right.  Notice how the wings are stretching out past the body in the center-right image and not in the image at the top?  The insect with the full wings is the adult.  All non-egg stages without complete wings are immatures, though many hemimetabolous insects do have wing buds visible.  The immature, non-egg stages of hemimetabolous insects are usually called nymphs.

The last type of insect metamorphosis is the one most people already know, the kind butterflies, flies, beetles, and many other insects use: complete metamorphosis.  These insects are called holometabolous insects, which means “whole change” in Greek.  The immatures of holometabolous insects look very different from the adult stage and are called larvae.  Most larvae are worm-like in shape, hence the “worm” portion of the common names of many insect larvae (e.g. mealworms, glowworms, bloodworms, etc).  Holometabolous insects also have an extra stage in development, the pupa, also commonly known as the cocoon or chrysalis.  When an insect pupates, or turns into a pupa from a larva, it undergoes a complete rearrangement of its tissues so that what emerges looks completely different from what went in.  Here is an illustration of the life cycle of a butterfly, probably the best known of all insect life cycles:

Holometabolous metamorphosis

Holometabolous metamorphosis

The caterpillar on the left lives on and eats plants while the adult butterfly on the right flies around a lot and eats flower nectar.  The pupa at the top is where the transformation from the larva on the left to the butterfly on the right occurs.  Each stage looks and often acts very different from the others.

So there you have it!  All insects undergo some sort of metamorphosis, but which type they use depends on what type of insect they are.  In general, the more primative the insect (or the further back in time it evolved), the less complex its metamorphosis.  The really primative insects, the ones that never even developed wings, use ametabolous metamorphosis and don’t change shape much between stages.  The most advanced insects use holometabolous metamorphosis and drastically change shape between stages.  The rest of the insects fall somewhere in between and use hemimetabolous metamorphosis, a style of metamorphosis intermediate between ametabolous metamorphosis and holometabolous metamorphosis.

I will finish this post with one final note about the words larva and nymph.  There is some debate among entomologists about whether we should call all immature stages larvae to make things more simple (they do, after all, both refer to immature stages in insects) or whether we should continue to call the immatures of hemimetabolous insects nymphs and those of holometabolous insects larvae.  There are many good arguments for and against both sides.  I personally think that we should continue to use both words, nymph and larva.  There is a strong scientific evidence that suggests the equivalent of the larval stage of holometabolous insects occurs within the egg in hemimetabolous insects.  Hemimetabolous insects have nymphs and not larvae because their “larval stage” is complete before they hatch.  There is also evidence that suggests that the pupa of holometabolous insects is equivalent to all of the nymphal stages of hemimetabolous insects combined.  The holometabolous insects  don’t have nymphs because their “nymphal stage” occurs within the pupa.  (For any scientists out there who might be interested in learning more about this, please see the excellent 1999 Truman and Riddiford article “The Origins of Metamorphosis” in Nature Vol 401, pp 447-452.)  There is thus a valid scientific reason for continuing to use both words.  I also like to use both because whoever I’m talking to about an insect immediately knows what type of metamorphosis it uses (and whether it’s primative, intermediate, or advanced) based simply on the word nymph or larva.  Although it’s slightly more difficult to learn two words originally, they are easier to use in the long run because they convey so much information in two short, simple words.

All this to present background information for my next few posts!  Next time, I’ll discuss a group of hemimetabolous insects that have many nymphal instars (or stages between molts in the immature stage) and some of my favorite insects: the dragonflies and damselflies.

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Text and images copyright © 2009 DragonflyWoman.wordpress.com

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