Insects have amazed mankind since time immemorial. They are fascinating organisms that surprise us not only by their great diversity of shapes, sizes, and colors but also by the morphological and physiological adaptations they present to establish themselves in different habitats and cope with environmental changes. This "plasticity" also allows them to survive thanks to the use of different strategies, such as feeding and reproductive habits, among others.
An excellent strategy adopted by insects to better adapt to different environments is metamorphosis, which is the change of form through different stages during the life of organisms. The stages through which the individual passes during metamorphosis are egg, larva, pupa, and adult.
Sometimes the changes are very small and the young specimens (juvenile stage) are very similar in shape to the adults (adult stage), the change is mainly in size. This phenomenon is known as simple metamorphosis and occurs, among others, in bedbugs and plant aphids.
In other cases, young and adult individuals are very different, both in shape, size, and habits. This is known as complete metamorphosis and can be observed in butterflies.
Insects that undergo this phenomenon have young individuals -called nymphs- very similar to adults. If they are winged organisms, the wings develop externally during the immature stages; there is no pupal stage before the last molt, in which the individual reaches its final size. There are several types of simple metamorphosis:
Ametabolous ("without" metamorphosis). Insects with this type of development are wingless in their adult stage and the only difference between the nymph and the adult is size. Examples of organisms with this type of metamorphosis are the protura, collembola, and tisanurus.
Hemimetabola (with "incomplete" metamorphosis). Nymphs are aquatic or live in the gills of plants. They differ considerably from adults. Organisms of this type are dragonflies, whose nymphs are aquatic and adult individuals are winged.
Parametabola (with "gradual" metamorphosis). Adult individuals are winged and both nymphs and adults live in the same habitat and the main change is in size. The stick insect and the praying mantis have this type of development.
Insects with this phenomenon have a pupal stage before the last molt, in which the individual does not move and has a very considerable change towards the adult form. If wings are present, they develop internally during the immature stages. They have larval or immature stages that are completely different from the adult stage and most of the time live in different habitats, possessing different habits. These organisms are known as holometabolous and, as an example, we can point to butterflies.
This mechanism makes it possible to overcome adverse conditions such as winter or drought, among others and involves the interaction of different phenomena and changes in individuals. To better understand how metamorphosis occurs, let's see what happens in the case of the monarch butterfly.
This one (whose scientific name is Danaus plexipus) has been greatly admired for the migration phenomenon in which it participates since one cannot imagine how such a small and fragile insect can endure such a long journey; many studies have been carried out on this subject. However, all the physiological and morphological changes that individuals of this species undergo during their lifetime are also surprising.
Like all butterflies, the monarch butterfly goes through all the stages of complete metamorphosis: egg, larva, pupa, and adult.
The monarch egg is grayish-white to cream-white and is barrel-shaped. It lives in the egg stage for approximately 7 days. This stage cannot be considered a molt because the larva is inside the egg and grows until it hatches and eats the eggshell. In the early stages, the animal is worm-shaped and goes through five larval stages in which it increases in size.
The larvae have black, yellow, and white stripes across the larvae and are mainly engaged in eating. The animal lives in this juvenile stage for approximately three weeks. In each molt (of the five molts it goes through as a larva) it forms a new soft exoskeleton which expands under blood pressure and then hardens by chemical action. At each molt, the old exoskeleton breaks down and the larva emerges in the next instar. To prepare to pupate, the larva stops eating and eliminates whatever food remains in its digestive tract. The pupa leaves the last old larval exoskeleton and remains immobile.
The animal has a very complex endocrine system and metamorphosis is controlled mainly by three hormones. The first is the brain hormone, produced precisely by the neurosecretory cells of the brain, which stimulates the molting glands. These glands in turn secrete ecdysone, the second hormone, which promotes larval growth. Together with these two acts the juvenile hormone, the third hormone, whose job is to inhibit metamorphosis. Once the butterfly has reached the last larval stage, the production of this hormone stops, allowing ecdysone to promote the formation of the pupa.
The pupa is inactive and does not feed. However, although it has no visible activity, it is when the animal performs more physiological activity and considerable changes take place. At this time, histolysis, the process in which larval structures are transformed into the material to be used in the development of adult structures, and histogenesis, the process in which adult structures develop, take place.
The main sources of material for histogenesis are hemolymph (which is the equivalent of human blood), the fat body (energy source organ in insects), and histolyzed tissue (such as larval muscles). The wings and legs develop from the cuticle (the hardened insect skin, or exoskeleton, whose composition is chitin) of the larva. In the last larval stage, these tissues are devoted to building adult structures that only become apparent when the pupa builds its chrysalis. The rest of the organs can be retained from the larva or can be reconstructed from regenerative cells.
The change of the internal organs during metamorphosis depends on their activity during the different stages. Thus, the heart, the nervous system, and the tracheal system change very little. Others, which are present in a rudimentary form in the larva or which do not exist, develop in the pupa to appear in adult individuals; such is the case of the reproductive apparatus.
The monarch butterfly has a pupa that, because of its coloration and structure, is called a chrysalis and is generally found attached upside down to the trunks and leaves of plants. It adheres to the surface of these using the cremaster, a thick thread based on the silk it produces and which is found at the end of the abdomen.
The chrysalis is thick, pale green to bluish-green, with gold and black spots, and oval. When the time approaches for the adult to emerge, it darkens and its cover allows the butterfly to be seen, and the orange color of its developing wings can be perceived. The pupal stage lasts approximately 15 days.
The newly emerged adult is usually pale in color, its wings are soft and folded. After a period, which in the monarch is approximately 40 minutes, the wings expand, harden and the coloration has acquired its final shade.
The life of the adult butterfly depends on its fate, i.e., whether it will migrate, whether it is the victim of a predator or weather conditions, etc. But, despite these variations, it can be calculated that the duration of the complete life cycle of the monarch is approximately 5 to 7 weeks.
Advantage of metamorphosis
This phenomenon allows the animal to live in completely different environments and, in a way, colonize diverse habitats. Larvae have limited movement because their job is to satiate their appetite, which allows them to accumulate energy. Adults, on the other hand, have a very wide distribution because their wings allow them to move and move over a much larger area.
Also, as already mentioned, metamorphosis allows the animal to evade adverse situations, although in the monarch there is no diapause activity, that is, extending the time in a certain stage, which is usually the pupa stage, to survive until conditions are suitable.
Author: Gabriela Jiménez Casas, Source: Correo del Maestro No. 35.