– Holometabolous insects are insects that undergo complete metamorphosis, which means they have four distinct life stages: egg, larva, pupa, and adult.

– The larva and the adult are very different in morphology, physiology, and behavior. The pupa is a resting stage in which the larva transforms into the adult. Holometabolous insects are also called endopterygotes, because their wings develop internally and are not visible until the adult stage. Holometabolous insects make up about 80% of all insect species and include butterflies, moths, flies, bees, wasps, ants, beetles, and fleas.

– Hemimetabolous insects are insects that undergo incomplete metamorphosis, which means they have three life stages: egg, nymph, and adult.

– The nymph and the adult are similar in morphology, physiology, and behavior, except that the nymph is smaller and lacks wings and reproductive organs. The nymph grows and develops through several molts until it reaches the adult stage. Hemimetabolous insects are also called exopterygotes, because their wings develop externally and are visible from the early nymphal stages. Hemimetabolous insects make up about 20% of all insect species and include grasshoppers, crickets, cockroaches, termites, dragonflies, damselflies, mayflies, stoneflies, true bugs, and lice.

METAMORPHOSIS

– The metamorphosis of holometabolous and hemimetabolous insects involves hormonal regulation, gene expression, tissue remodeling, and behavioral changes. The main hormones involved are juvenile hormone (JH) and ecdysone (E). JH maintains the juvenile characteristics of the insect and inhibits metamorphosis. E stimulates molting and metamorphosis. The balance between JH and E determines the timing and outcome of each developmental stage.

– In holometabolous insects, the egg hatches into a larva that feeds voraciously and grows rapidly. The larva has a simple body plan with a head capsule, a segmented thorax and abdomen, and simple eyes. The larva may have legs or be legless depending on the species. The larva molts several times as it increases in size. Each larval stage is called an instar. After the final larval instar, the insect enters the pupal stage.

– The pupa is usually immobile and protected by a hard case called a puparium or a cocoon made of silk or other materials. The pupa undergoes drastic changes in its body structure as it reorganizes its tissues and organs to form the adult features. This process is called histolysis (destruction of old tissues) and histogenesis (formation of new tissues). The pupa also develops compound eyes, antennae, mouthparts, legs, wings, and reproductive organs. After the pupal stage is completed, the adult insect emerges from the pupal case.

The adult is usually winged and sexually mature. The adult has a complex body plan with a head, thorax, and abdomen. The adult may feed or not depending on the species. The adult's main functions are dispersal, mating, and egg laying.

– In hemimetabolous insects, the egg hatches into a nymph that resembles a miniature adult but lacks wings and reproductive organs. The nymph feeds on similar food sources as the adult and lives in similar habitats. The nymph has a complex body plan with a head, thorax, and abdomen. The nymph also has compound eyes, antennae, mouthparts, and legs. The nymph molts several times as it grows in size. Each nymphal stage is called an instar. After each molt, the nymph develops wing buds that grow larger until they become fully developed wings in the final instar.

– The final instar is also called a subimago in some aquatic insects such as mayflies that have an additional molt after emerging from water. After the final nymphal instar or subimago stage is completed, the adult insect emerges from the exoskeleton. The adult is usually winged and sexually mature. The adult's main functions are dispersal, mating, and egg laying.

▶️ How does the metamorphosis of holometabolous and hemimetabolous insects differ in terms of hormonal regulation?

– The main hormones involved in insect metamorphosis are juvenile hormone (JH) and ecdysone(E). JH maintains the juvenile characteristics of the insect and inhibits metamorphosis. E stimulates molting and metamorphosis. The balance between JH and E determines the timing and outcome of each developmental stage.

– In holometabolous insects, the egg hatches into a larva that has a simple body plan and feeds voraciously. The larva molts several times as it grows in size. Each larval stage is called an instar. During the larval instars, JH is high and E is low, which prevents the larva from undergoing metamorphosis. After the final larval instar, JH drops and E rises, which triggers the pupal stage.

– The pupa is a resting stage in which the larva transforms into the adult. The pupa undergoes drastic changes in its body structure as it reorganizes its tissues and organs to form the adult features. This process is called histolysis and histogenesis. After the pupal stage is completed, E drops and the adult insect emerges from the pupal case. The adult is usually winged and sexually mature.

– In hemimetabolous insects, the egg hatches into a nymph that resembles a miniature adult but lacks wings and reproductive organs. The nymph feeds on similar food sources as the adult and lives in similar habitats. The nymph molts several times as it grows in size. Each nymphal stage is called an instar. During the nymphal instars, JH is high and E is low, which allows the nymph to grow and develop without undergoing metamorphosis. After each molt, the nymph develops wing buds that grow larger until they become fully developed wings in the final instar. The final instar is also called a subimago in some aquatic insects such as mayflies that have an additional molt after emerging from water. After the final nymphal instar or subimago stage is completed, JH drops and E rises, which triggers the adult stage. The adult is usually winged and sexually mature.

– Therefore, the main difference between holometabolous and hemimetabolous metamorphosis is that holometabolous insects have a pupal stage that separates the larval and adult stages, while hemimetabolous insects do not have a pupal stage and gradually transition from nymph to adult stages.

– Another difference is that holometabolous insects have a high degree of morphological and physiological difference between the larval and adult stages, while hemimetabolous insects have a high degree of similarity between the nymph and adult stages.