Bombi Natura

Metamorphosis is a change, more or less abrupt, that transforms the form and structure of some animals after embryonic development.

Although this article focuses on insect metamorphosis, this process also occurs in marine invertebrates, fish and amphibians.

Origin of metamorphosis in insects

The evolutionary origin of metamorphosis is believed to lie in the related to the appearance of wings in insects, approximately 400 million years. Some insects of primitive lineages, such as silverfish, are wingless and do not metamorphose. These insects, called ametoboles, grow in size and moult periodically throughout their lives, even when they are adults and able to reproduce. Moulting is the process by which an insect changes its exoskeleton in order to continue growing.

Types of metamorphosis

Among the insects that do undergo metamorphosis, there are two main groups:

• Hemimetabolous or with incomplete metamorphosis: in these insects, the immature stages are called nymphs, and have a appearance similar to that of adults. The wings and genitalia of nymphs develop gradually, until they reach the adult stage with the last moult. Examples of hemimetabolous insects are cockroaches, grasshoppers and dragonflies.

• Holometabolous or with complete metamorphosis: in these cases, the immature stages, known as the larvaehave a very different from that of adults, and have no genitalia or external wings. The larvae grow progressively and also moult. To carry out metamorphosis, the larvae pass through the stage of pupa stage,during which the insect transforms its structures. In some cases, as in flies, most of their tissues are destroyed to form the adult structures. Once this stage is completed, the adult individual, with developed wings and genitalia, molts no more. Examples of holometabolous insects are butterflies, bees and beetles.

Regulation of metamorphosis

The process of metamorphosis is regulated by two main hormones with opposing functions: the juvenile hormone, which maintains the insect in its current state of development, and the ecdysone, which induces moulting and metamorphosis. When the insect reaches a determined weightThe hormonal balance shifts towards ecdysone, triggering metamorphosis. Although the number of immature stages and moults before reaching the adult form is usually fixed, factors such as poor nutrition, damage or disease can increase or decrease the number of moults.

Benefits of metamorphosis

During the pupal stage, the insect will does not feed and remains immobile, vulnerable to a multitude of predators and parasites. In fact, when the pupa develops in exposed environments, as in the case of butterflies, it usually has a hardened and coloured surface layer that helps to camouflage it and protect it from other arthropods. Despite the risk and the high cost in both time and energy, complete metamorphosis has two major advantages:

-It has allowed the larvae and the adults, specialise in very different functions to better exploit resources: the larvae are adapted to a non-motile lifestyle, efficient in the exploitation of food resources and growth, while the adults have a body adapted to dispersal, mating and reproduction.

-It has helped the insects to specialise in different food resources during immature and adult stages, avoiding competition for food between individuals of the same species.

Over 60 % of all animal species described are holometabolous insects, suggesting that the emergence of complete metamorphosis was a key factor in the evolutionary success and the expansion of insects.

References

Belles, X. (2019). The innovation of the final moult and the origin of insect metamorphosis. Philosophical Transactions of the Royal Society B, 374(1783), 20180415.

Konopová, B. (2024). Evolution of insect metamorphosis-an update. Current Opinion in Insect Science, 101289.

Lowe, W. H., Martin, T. E., et al. (2021). Metamorphosis in an era of increasing climate variability. Trends in Ecology & Evolution, 36(4), 360-375.

McMahon, D. P., & Hayward, A. (2016). Why grow up? A perspective on insect strategies to avoid metamorphosis. Ecological entomology, 41(5), 505-515.

Ten Brink, H., de Roos, A. M., et al. (2019). The evolutionary ecology of metamorphosis. The American Naturalist, 193(5), E116-E131.

Truman, J. W. (2019). The evolution of insect metamorphosis. Current Biology, 29(23), R1252-R1268.