FunNature & Animalmetamorphosis in insects

metamorphosis in insects

The life cycle of butterflies is well known, especially to those who have reared silkworms. A larva emerges from the egg, the caterpillar , which grows and sheds its skin until, at a certain point, it forms a silken cocoon inside which it encloses itself. There it undergoes a metamorphosis, during the phase called pupa or chrysalis , and some time later, the butterfly emerges from the cocoon, which after reproduction, will lay the eggs of the next generation.

But what happens inside the cocoon is not so well known. How does this metamorphosis happen? How does that caterpillar that disappears into its silken wrapper go on to become such a different animal as the adult butterfly?

how insects grow

Insects have a rigid and articulated exoskeleton of chitin, a kind of external armor that prevents them from growing gradually, like the rest of the vertebrates; hence as they gain weight they need to shed their exoskeleton in order to grow in volume.

This process is called molting or ecdysis , and in the time that elapses from when they are freed from their previous covering until the new exoskeleton finishes forming, it is when they can grow. Also at this time they are more vulnerable, losing the resistant protection of their armor, so this process is usually very short in time.

The different types of metamorphosis

Not all insects have a complete metamorphosis like butterflies or holometabola. In some cases, the process is much simpler, more like a gradual transition, molt after molt, from the infantile to the adult form, or hemimetabolous metamorphosis .

In insects with hemimetabolous metamorphosis, a juvenile form is also born from the egg, but with the appearance of a miniature adult, without wings or formed genitalia, but with all its legs, head, antennae, mouthparts and, basically, all adult anatomy. This juvenile phase is called the nymph . As it sheds, it grows and develops. Upon reaching the last molt of the nymph, when it becomes an adult, the wings -if it has them- and the genitals are formed.

They are hemimetabolous insects, among others, grasshoppers, cockroaches, praying mantises, dragonflies, mayflies or stone flies.

Insects with holometabolic metamorphosis form their own monophyletic evolutionary group —they descend from the same common ancestor—. They are called endopterygotes. Among them are the most successful organisms in the animal kingdom, both in diversity and in number of species.

In holometabolous, the larva passes to the pupal state and from there to the adult . Although in the silkworm butterfly, the pupa is enclosed in a cocoon, in many other insects, however, the pupa is left exposed, only covered by its rather thickened and very hard exoskeleton.

Unlike hemimetabolous insects, in which the nymph and the adult generally feed on the same thing, the holometabolous, larvae and adult have different diets, and occupy different ecological niches, so that the juveniles and adults do not compete. each. Some researchers point to this fact as one of the key factors in its success .

Among the insects with this type of complete metamorphosis, apart from butterflies and moths, there are flies, ants, bees and wasps, beetles or fleas, among others.

The pupa, key phase of the complete metamorphosis

As has been exposed, this metamorphosis that represents the passage from juvenile —larva— to adult occurs through an intermediate phase, called pupa. It acquires a very different form from that of the larva, and from that of the final adult, it is totally sessile and does not feed .

During the pupal stage, the insect undergoes serious transformations. The elongated, sausage-shaped midgut of the larva shrinks day by day to a much shorter but more complex tube—except in some species, which is atrophied and will never feed again. The anterior part of the digestive apparatus disappears. In addition, in the ventral part a small air bag is formed between the cuticle of the pupa and the body that develops inside it, which will gradually become larger, separating the skin of the adult from the exoskeleton and facilitating that, When the time comes, it can easily emerge. Wings, malpighian tubes—the excretory system of insects—and genitalia also form inside the pupa.

The exoskeleton of the adult does not form during the pupal stage. It begins to develop once the adult has emerged. It leaves behind not only the exoskeleton of the chrysalis, but also a good part of the larval structures.


Aldaz, S. et al. 2010. Imaginal discs. Current Biology, 20(10), R429-R431. DOI: 10.1016/j.cub.2010.03.010

Borror, DJ et al. 1976. An introduction to the study of insects (4th ed.). Holt, Rinehart and Winston.

Lowe, T. et al. 2013. Metamorphosis revealed: time-lapse three-dimensional imaging inside a living chrysalis. Journal of The Royal Society Interface, 10(84), 20130304. DOI: 10.1098/rsif.2013.0304

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