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Why do insects metamorphose?

Most of insects undergo some kind of transformation process during their life cycle in order to reach adulthood -also known as imago phase- (e.g. butterflies). This process is named metamorphosis, although its essence is far from that of metamorphosis performed by amphibians. But, have you not ever wondered why they do this transformation? Which are the sense and the origin of the metamorphosis of insects?

Learn more about the different types of metamorphosis, the origin and sense of these transformations through this article.

Metamorphosis: what is that?

Metamorphosis of the Old World swallowtail (Papilio machaon) (Picture by Jens Stolt).

Metamorphosis is a biological process by which animals develop after birth involving huge transformations and/or anatomical restructurations (both physiological and anatomical) until reaching adulthood.

There are different groups of animals that develop by this process, however most of them don’t share the origin nor the nature of these transformations. Thus, while amphibian metamorphosis takes place by reorganization of youth preexistent organs, in insects it takes place a breaking of tissues and also the appearance of totally new cell clusters.

Ecdysis or molting

First of all, we must talk about molt in order to comprehend the metamorphosis of insects. What means molting? And why is it an essential process for insects and arthropods as a whole?

Every single animal regenerates its external tissues in some way, i.e., those tissues that are in contact with the environment and that protect the organism from external pressures. E.g. mammals regenerate their epidermal tissues periodically; a lot of reptiles shed off their skin frequently; but, what’s about arthropods?

Arthropods, which include the hexapods (group in which we can find all insects), are externally covered by a more or less hard exoskeleton. In contrast with other external animal tissues, the exoskeleton doesn’t detach progressively, and its lack of elasticity restricts the organism growth. So, this element becomes a barrier that limits their size while growing, and is for this that they have to break it and leave it away in order to keep on growing. This kind of molting is known as ecdysis, which is typical of ecdysozoa (arthropods and nematoda).

Take a look at this video of a cicada molting!:

Do all hexapods metamorphose?

The answer is NO. However, it’s necessary to go deeper into the explanation.

All hexapods molt in order to grow, but not all them undergo radical changes to reach adulthood (when they become able to breed). Thus, we can split hexapods into two main groups:

AMETABOLOUS HEXAPODS (No metamorphosis)

This group includes those hexapods traditionally known as Apterygota or wingless hexapods (Non insect hexapods –proturans, diplurans and colembolas- and wingless insects as Zygentoma or also known as Thysanura –e.g. silverfishes or Lepisma-) and Pterygota or winged insects that have suffered a secondary loss of their wings.

Specimen of Ctenolepisma lineata (Zygentoma) (Wikimedia Commons).

Since they have no wings at any moment of their life cycle, the youth phases of this kind of hexapods almost have no differences from the adult ones. Thus, the youth development is simple and they don’t undergo huge changes to acquire the adult physique; that is, there is no metamorphosis at any point of their life cycle. This kind of development is also known as direct development.

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Direct development or ametabolous development (Picture from asturnatura.com).

Ametabolous hexapods can molt tens of times throughout their development (e.g. 50 times in silverfishes, more or less), even when they become sexually mature.

INSECTS THAT METAMORPHOSE

This group includes Pterygota insects or winged insects (except for the ones that have secondarily lost their wings).

Specimen of Sympetrum flaveolum (Picture by André Karwath)

In contrast of the ones which have been explained above, the youth phases of metamorphic insects are very different from the adult ones; so, after several successive molts they undergo their last change, through which it emerges a winged adult able to breed. After reaching this phase, these insects become unable to molt again.

Types of metamorphosis in insects

So, only Pterygota insects undergo a truly metamorphosis, thanks to which they become winged insects and also reach sexual maturity. But not all these insects perform the same kind of change.

There exist two main types of metamorphosis: the hemimetabolous one (simple or incomplete) and the holometabolous one (complex or complete). Which are their differences?

Hemimetabolous metamorphosis

In the simple, incomplete or hemimetabolous metamorphosis, young insects go through several successive molts until reaching adulthood (or imaginal) stage without going through a stage of inactivity (pupa) and/or stop feeding.

Just after hatching, we referred the newborn as a nymph, which resembles a little to the adult ones (but still not having wings nor sexual organs). Usually, nymphal phases and the adult ones don’t share feed sources nor habitat, so they occupy different ecological niches; in fact, most nymphs have aquatic habits and they go to live on land after reaching maturity (e.g. mayflies).

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Adult specimen of the species of mayfly Ephemera danica (Imagen de Marcel Karssies).

In this kind of metamorphosis, nymphs go through some successive molts thanks to which wings are gradually formed and their organism becomes bigger. Finally, nymphs perform their last molt, after which the adult emerges: a winged organism that is able to breed.

Take a look to this scheme that sums up this process:

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______Hemimetabolous development of a _______grasshopper (imagen extraída de ________________asturnatura.com)

These insects are also called Exopterygota (from Latin exo- = “outside” + pteron = “wings”), because in these organisms the wings are progressively and visibly formed at the outside part of their body.

Holometabolous metamorphosis

In general terms, it’s considered the most radical metamorphosis in insects and also probably the most well known transformation by all of us. The most famous example is the one performed by lepidopterans (butterflies and moths); but there are also more insects that are holometabolous, such as coleopterans (beetles), hymenopterans (bees, wasps and ants) and dipterans (flies and mosquitoes).

In the complex, complete or holometabolous metamorphosis, insects are born as larvae, that is, a premature stage that doesn’t resemble anatomically nor physiologically to the adult. In addition, they don’t share feed sources nor habitat, as it is the case of hemimetabolous organisms. As in hemimetabolous insects, these larvae go through successive molts until reaching the size enough to undergo the metamorphosis, when they perform their last molt.

Beetle larva (“Curl grub” by Toby Hudson – Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons).

After their last larval stage, larvae enter in a stage of inactivity, moment they stop feeding and remain motionless. This stage is known as pupal stage (when they become a pupa or a chrysalis in butterflies). Usually, larvae begin to resemble to the adults at the end of this stage due to the anatomical modifications that take place and also to the appearance of new organs and tissues.

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Pupal stage of Cetonia aurata (Coleoptera) (“Cetoine global” by Didier Descouens – Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons)

Once the transformation process ends, the organisms leave that motionless state and acquire their adult form that has wings and is totally mature.

In summary, the scheme of this process could be:

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Holometabolous development of a lepidopteran (Picture from _________________________astrunatura.com)

In contrast with hemimetabolous insects, the appearance of wings in holometabolous organisms takes place inside their body and become visible only at the end of the pupal stage. For this reason, they are also known as Endopterygota (from Latin endo-= “inside” + pteron=”wings”).

Origin and function of insect metamorphosis

Origin: the fossil record

Insects are, as we discussed in previous articles, one of the animals with greater evolutionary success. Between 40%-60% of all insect species are holometabolous (complete metamorphosis), because of what we deduce that holometabolous metamorphosis was positively selected during the evolution of this group. In fact, fossil records suggest that this kind of metamorphosis appeared only once, so all holometabolous insects derive from the same ancestor.

According to these data, wingless insects or ancient Apterygota and early winged insects were ametabolous. Then, all winged insects started to develop some kind of hemimetabolous metamorphosis during the Carboniferous and the Permian (300 Ma). Finally, the first insects considered as holometabolous appeared during the Permian period (280 Ma).

What could be the reason of this positively selection?

In the latest paragraphs, we talked about the different feeding sources and habitats of both youth and adult. The fact that different life stages of the same animal exploit different resources could prevent the intraespecífic competition (i.e. competition for resources between organisms of the same species). This fact would mean a great advantage for these organisms, so that holometabolous development, which is characterized for being divided in very different stages, could have been more successful than the hemimetabolous or the ametabolous.

Thus, we can say the main functional sense of metamorphosis could be to minimize the intraespecífic competition for resources. But there is still more: the more specialized are the different stages of an insect, the greater would be the chance to exploit more and better the resources. E.g. in parasitic forms, the differences between different stages tend to be huge, because the difficult situations they have to face require a specific specialization in each moment of the life cycle.

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Larva and adult of Danaus plexippus (monarch butterfly) (sources: larva picture by Victor Korniyenko, Creative Commons; adult picture of public domain).

.        .         .

So, likewise the appearance of wings promoted the expansion and diversification of insects worldwide, the metamorphosis could have acted as a diversifying engine by increasing the capacity to exploit more and better resources.

REFERENCES

Main picture by Steve Greer Photography.

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