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.

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).

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:

______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.

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:

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.

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

• Notes from the subject “Advanced Zoology” taken during my Biology studies at Universitat  Autònoma de Barcelona (UAB).
• Bellés X. (2009). “Origen y Evolución de la Metamorfosis de los Insectos”. Instituto de Biología Evolutiva (CSIC-UPF), Barcelona.
• Jordán Montés F. (2013). “El universo de los insectos”. Mundi-Prensa Libros, Madrid.
• Los Insectos. Reproducción y Metamorfosis (asturnatura.com).

Main picture by Steve Greer Photography.

What do insects tell us about the health of our rivers?

Nowadays, concern about the health of inland waters (rivers, lakes, etc.) is growing, mainly due to increased use (and abuse) of these for human consumption. A few years ago, an expansion of the use of biotic indices took place, which allow us to determine the health of aquatic ecosystems; these indices usually use data such as presence, absence or/and abundance of different organisms known as ‘bioindicators’, that is, species that can be used to monitor the health of an environment or ecosystem. Among these organisms, there are a lot of arthropods.

Along this article, I will briefly explain what bioindicators are, the main role of arthropods as bioindicators and also introduce some of the most used bioindication indices to monitor the quality of riverine ecosystems in the Iberian Peninsula.

What is a bioindicator?

The term ‘bioindicator’ is used to refer to those biological processes, species or/and communities of organisms which can be used to assess the quality of an ecosystem and also how this ecosystem evolves over time, which is especially useful when changes take place due to anthropogenic disturbances, such as pollution.

Thus, in accordance with the above, a bioindicator can be:

• A particular species, whose presence/absence or abundance rate informs us of the state of health of a studied ecosystem, or
• A population or a community composed of various organisms which varies functionally or structurally according to the conditions of  environment.

Example: Lecanora conizaeoides lichen is highly resistant to pollution. Its presence on the studied ecosystem, coupled with the disappearance of another lichens, is indicative of high air pollution.

Lecanora conizaeoides (Picture by James Lindsey).

What do we consider a ‘good bioindicator’?

Do all the organisms have the necessary traits to become bioindicator subjects? The answer is no. Even though there is not a bioindicator prototype (because all depends on the studied ecosystem), we can resume here some of the traits that scientists take into account to select good bioindicator organisms:

• They have to respond to disturbances that take place on their ecosystem to a greater or lesser degree. This response should be comparable to that emitted by the rest of the organisms of the same species, and this response also has to be well correlated with the studied environment disturbances.
• Their response have to be representative of all the community or population.
• They must be native of the studied ecosystem and also be ubiquitous (that is, to be present in almost all ecosystems of the same or similar characteristics).
• They have to be abundant (rare species aren’t optimum subjects).
• They must be relatively stable to moderate climate changes (i.e. a storm or a natural temperature change does not affect them more than normal).
• They should be easy to detect and, as possible, they have to be sedentary.
• They have to be well studied, both from an ecological point of view as taxonomic (to know, therefore, their tolerance to environmental disturbances).
• Finally, they should be easy to manipulate and monitor in the laboratory.

The use of bioindicators will be optimized if we use entire communities or populations instead of using a single or a couple species, because this allows us to cover a wide interval of environmental tolerances: from organisms with a narrow tolerance range (that is, stenotopic) and sensitive to pollution, to very tolerant organisms that can survive in very polluted environments.

Thus, we will be able to know if an ecosystem is highly altered if we find only a very tolerant species and none of the considered sensitive species.

Bioindicator animals from inland waters

Nowadays, scientists use a lot of animals as bioindicators: from microorganism and microinvertebrates to terrestrial and aquatic vertebrates (micromammals, birds, fishes, etc.). In inland waters, and especially in the context of studies of riverine water quality, scientists mostly use aquatic macroinvertebrates to assess the quality of these ecosystems. Next, let’s see what a macroinvertebrate is.

What are macroinvertebrates?

The term ‘macroinvertebrate’ does not correspond to any taxonomic classification, but with an artificial concept that includes different aquatic invertebrate organisms.

Generally, is said that macroinvertebrates are organisms that can be trapped by a net with holes about 250μm.

Collecting macroinvertebrates by using a kick seine (Picture by USFWS/Southeast , Creative Commons).

Macroinvertebrates are mainly benthic, that is, animals that inhabit the substrate of aquatic ecosystems at least during some stage of their life cycle (although there are some that swim freely in water column or on its surface).

We can find a lot of macroinvertebrate groups in rivers and lakes, which can be classified in two main groups:

Picture sources: (1) Luis Silva Margareto ©, (2) DPDx Image Library, (3) Oakley Originals, Creative Commons, (4) Ryan Hodnett, Creative Commons, (5) Will Thomas, Creative Commons, (6) Duncan Hull, Creative Commons.

Among these groups, there are both tolerant organisms to environment distrubances (i.e. leeches) and sensitive organisms (i.e. a lot of larvae insects).

Most inland aquatic macroinvertebrates (≃80%) are arthropods (of which I will discuss in the next section), among which there are many insects and, especially, their larvae (which are generally benthic), whose study and observation play an essential role on calculating indices of water quality.

Importance of insects in bioindication

As I’ve said above, about 80% of macroinvertebrates of inland waters are arthropods and, mostly, different orders of insects in its larval or nymphal form. Let’s see some of the most common groups we can find in rivers and lakes:

Trichoptera (or caddisflies)

They are insects closely related to the Lepidoptera order (butterflies and moths). Their aquatic nymphs can build a shelter around their bodies made of substrate materials. We can distinguish them from other aquatic insect larvae because they have a couple of anal filaments provided with strong hoofs. They usually inhabit clear and clean waters with a lot of currents.

Trichoptera nymph (inside its shelter, left) and adult (right). Picture of the nymph by Matt Reinbold (Creative Commons) and picture of the adult by Donald Hobern (Creative Commons).

Ephemeroptera (or mayflies)

One of the most ancient orders of flying insects. Their aquatic nymphs, which usually inhabit rivers, are characterized for having three long anal filaments. Adults, which fly over the water surface, are very fragile and have a short life cycle in comparison with nymphs (the name Ephemeroptera is derived from Greek ‘ephemera’ meaning sort-lived, and ‘ptera’ meaning wings).

Ephemeroptera nymph (left) and adult (right). Picture of the nymph by Keisotyo (Creative Commons) and picture of the adult by Mick Talbot (Creative Commons).

Plecoptera (or stoneflies)

Flying insects very similar to Ephemeroptera order. Like these, they have anal filaments, but they differentiate from them because they have two apical hooks in each leg. They usually inhabit lakes and streams.

Plecoptera nymph (left) and adult (right). Picture of the nymph by Böhringer (Creative Commons) and picture of the adult by gailhampshire (Creative Commons).

Other groups of insects with aquatic larvae or nymphs

Among the most common insects inhabiting rivers and lakes we can also find species of Odonata order (dragonflies and damselflies), Coleoptera (beetles), Diptera (mosquitoes and flies), etc.

Among all the organisms mentioned above, there are very tolerant species to pollution (i.e. some Diptera larvae; this is the case of some species of Chironomidae family, which are very tolerant to organic and inorganic pollution due to the presence of heavy metals in their environment) and also very sensitive species (i.e. some species of Trichoptera order).

Depending on their tolerance to environment disturbances, scientists group these organisms (plus the rest of macroinvertebrates) into different categories that are assigned a value. This values, at the end, allow us to calculate water quality indices.

Biotic indices for riverine waters

The different pollution tolerance degrees among macroinvertebrates of a community allow us to classify them and to assign them a qualitative value (the bigger the number is, more sensitive are organisms to pollution). Thanks to these values, we can calculate different biotic indices, which are no more than qualitative values assigned to a community in order to classify it according to its quality: the greater the value is, better is the water quality.

One of the most used indices on the assessment of ecological state of rivers from the Iberian Peninsula is the IBMWP index (Iberian Bio-Monitoring Working Party), an adaptation by Alba Tercedor (1988) of the British index BMWP. In rough outlines, the greater the value is, better is the water quality. On this website you will find more details about this index, and also the pre-established values assigned to each macroinvertebrate (available in Spanish only).

In additions, there is also used the IASPT index, a complementary index which is the result of divide the IBMWP value by the number of identified taxa. This index give us information about the dominant community in the studied location. You can see more details on this website (available in Spanish only).

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As you probably have seen while reading of this article, macroinvertebrates, and insects especially, play an important role in the study of inland water quality. Furthermore, their presence or absence is extremely important for the rest of the organisms of their ecosystem, because of what we must become aware of the problems deriving from the reduction of their number or diversity.

REFERENCES

• Bioindicadors com a detectors de la contaminació ambiental. Diari Apícola Ecolluita (Associació Col·lectiu Abellaire).
• Bioindicadors. GESNA (Estudis Ambientals S.L.).
• Estudio de determinación de índices bióticos en 87 puntos de los ríos de Navarra (Informe Final, 2005). Ekolur, Asesoría Ambiental.
• Holt, E. A. & Miller, S. W. (2010) Bioindicators: Using Organisms to Measure Environmental Impacts. Nature Education Knowledge 3(10):8.
• MEDPACS (MEDiterranian Predictions And Classifictions System).
• Prat N., Ríos B., Acosta R., Rieradevall M. (2009). Los macroinvertebrados como indicadores de calidad de las aguas.
• Protocolo de cálculo del índice IBMWP. Ministerio de Agricultura, Alimentación y Medio Ambiente (Gobierno de España).
• http://www.cricyt.edu.ar/enciclopedia/terminos/Bioindic.htm
• https://ilbca.wordpress.com/bioindicadores/
• http://xtec.cat/cda-delta/pdf/rv_invertebrats.pdf

Head photography by U.S. Fish and Wildlife Service Southeast Region.