What is metamorphosis?

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It is a developmental phase of several zoological groups (including arthropods, insects, etc.), characterized by irreversible morpho-anatomical and physiological changes of a larva, leading to the emergence of a fully developed adult known as the imago, or “image of the species.”

Bees are holometabolous insects, meaning they undergo complete metamorphosis: the adult bee at day 21 is completely different from the larva just before cell capping. At a temperature of 34 °C, development proceeds through the stages of egg, larva (after hatching from the egg), nymph (after cell capping), and finally the definitive adult. These developmental phases are highly sensitive to chemical pollutants, particularly the (~1,500) pesticides with endocrine-disrupting effects.

Three hormones, regulated by neurosecretory cells in the brain (a mechanism reminiscent of the pituitary function in humans), control metamorphosis:

• a growth hormone, activating, which allows larvae to grow;
• a molting hormone (ecdysone), activating, which triggers nymphosis;
• a juvenile hormone, inhibitory, which maintains the larval state and opposes nymphosis. Some pesticides act as analogues of this juvenile hormone and therefore prevent metamorphosis.

Environmental factors (temperature) also play a role by modulating the production of activating hormones, and worker bees therefore maintain the capped brood constantly at 34 °C.

During its growth, the larva undergoes five molts. The average duration of the nymphal stage of a bee depends on its caste. At the end of its growth, the larva changes position: it elongates, directs its head toward the surface of the comb, undergoes its final molt, and spins a silk cocoon with its mouth. Meanwhile, worker bees gradually cap the cell. The day after capping, the larva becomes immobile: it becomes a nymph and begins metamorphosis. Its organs undergo a profound reorganization. The body takes on a new form in which the three characteristic insect body regions soon become distinguishable: head, thorax, and abdomen. Legs, wings, and antennae develop. Of all organs, the eyes are the first to gain color. The skin yellows, then browns. The gut is completely remodeled to adapt to a dietary change (the larva requires proteins, whereas the imago mainly consumes sugars or carbohydrates). There is therefore no food competition between larva and adult insect. Twelve days after capping, i.e. 21 days after the queen lays the egg, the young worker moves, cuts open the cell cap with her mandibles, and exits the cell: this is emergence. The nymphosis of the future queen lasts on average 6½ days, while that of the male lasts about 14½ days.

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