Metamorphosis in Insects by DR

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COMPILED AND CIRCULATED BY DR. POULAMI ADHIKARY MUKHERJEE, ASSISTANT PROFESSOR, DEPARTMENT OF ZOOLOGY, NARAJOLE RAJ COLLEGE Metamorphosis in insects BY DR. POULAMI ADHIKARY MUKHERJEE ASSISTANT PROFESSOR DEPARTMENT OF ZOOLOGY NARAJOLE RAJ COLLEGE

ZOOLOGY: SEM- II, PAPER- C3T: NON-CHORDATES II, UNIT 3: ARTHROPODA COMPILED AND CIRCULATED BY DR. POULAMI ADHIKARY MUKHERJEE, ASSISTANT PROFESSOR, DEPARTMENT OF ZOOLOGY, NARAJOLE RAJ COLLEGE MEANING OF METAMORPHOSIS ➢The word “metamorphosis” has been derived from the Greek word which means to transform.

➢Metamorphosis is the process of transformation of an immature larval individual into sexually mature reproducing adult.

➢Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation.

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➢Some insects, fish, amphibians, molluscs, crustaceans, cnidarians, echinoderms, and tunicates undergo metamorphosis, which is often accompanied by a change of nutrition source or behaviour.

➢The transformed adult is completely different from larvae in form, structure and habit. It is the way insects grow and mature. Their lives are divided into separate stages for resting, growing and reproducing.

➢Insects grow in stages and the cycle of stages is metamorphosis.

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✓ Most insects begin life as an egg and hatch within a few days of being laid.

✓ But there are some insects that will live through an entire season as an egg before hatching.

✓ The insects that stay in the egg longer need more time to grow and become strong enough to live outside of the egg.

✓ When the temperature becomes warm and comfortable these tiny insects will break out of their eggs and, depending on the species, will go through any of the said types of metamorphosis.

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Type 1. Ametabolic Metamorphosis (No Metamorphosis): ❖ This type of metamorphosis is also known as ametabolous development.

❖ In this type, the newly hatched creature looks like an adult except in size and differences in armature of spines and setae.

❖ In lower insects (Collembola, Thysanura) the young which hatches from an egg is a miniature of the adult and is called a nymph, it differs from the adult in having immature reproductive organs; by several moultings and growth it becomes an adult.

❖ These insects are primitively wingless, they are also called Apterygota, e.g., Lepisma, the change from young to adult is negligible, such insects are ametabolic because there is no metamorphosis.

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Type 1. Ametabolic Metamorphosis (No Metamorphosis): ❖ The most primitive insects, such as springtails, silverfish, and firebrats, undergo little or no true metamorphosis over the course of their life cycles.

❖ Entomologists refer to these insects as "ametabolous," from the Greek for "having no metamorphosis.“

❖ When they emerge from the egg, immature ametabolous insects look like tiny versions of their adult counterparts. They continue molting and growing until they reach sexual maturity.

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Examples of Ametabolic Metamorphosis Examples Orders Silverfish Collembola Springtail Collembola Chewing lice Mallophaga Sucking lice Anoplura

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Type 2. Gradual Metamorphosis :

❖ This type of metamorphosis is also known as paurometabolous development.

❖ In this type, the newly hatched young ones resemble the adult in general body form but lacks wings and external genital appendages.

❖ The young nymphs undergo several nymphal stages through successive moulting to transform into adult.

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Examples of Gradual Metamorphosis

Examples Orders Mayflies Ephemeroptera Dragonflies Odonata Stone-flies Plecoptera

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Type 3. Heterometabolic Metamorphosis : ❖ This type of metamorphosis is marked by three life stages: egg, nymph, and adult.

❖ Entomologists refer to insects that undergo gradual metamorphosis as "hemimetabolous," from "hemi," meaning "part," and may classify this type of transformation as incomplete metamorphosis.

❖ Growth for hemimetabolous insects occurs during the nymph stage.

❖ Nymphs resemble the adults in most ways, particularly in appearance, exhibit similar behaviors, and typically share the same habitat and food as the adults.

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Type 3. Heterometabolic Metamorphosis :

❖ In this type, the immature stages are called as nymphs or naiads.

❖ These immature stages are aquatic and they respire with the help of tracheal gills. On the other hand the adults are terrestrial and respire with the help of tracheae.

❖ In winged insects, nymphs develop external wings as they molt and grow.

❖ Functional, fully-formed wings mark their emergence into the adult stage of the life cycle.

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Type 3. Heterometabolic Metamorphosis : ❖ In winged insects the adult differs in several respects from the young, such insects are said to undergo metamorphosis in becoming adults.

❖ The nymph which hatches from the egg has a general resemblance to the adult in body form, type of mouth parts and possession of compound eyes, though these nymphs may have adaptations associated with their particular habits of being aquatic, swimming or burrowing.

❖ In these the change from nymphs to adults is a gradual process in which appendages, mouth parts, antennae and legs of the nymph grow directly into those of the adult.

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❖ The reproductive organs mature gradually.

❖ Insects showing this slight change from nymph to adult are known as heterometabolic (gradual), they include Dictyoptera, Orthoptera, Isoptera, Hemiptera and Anoplura.

❖ Some hemimetabolous insects include grasshoppers, mantids, cockroaches, termites, dragonflies, and all true bugs.

❖ Though nymphs of dragon flies, may flies, etc., are quite different from the adult in having special nymphal adaptations because their nymphs are aquatic, while the adults are aerial, the nymphal adaptations are shed in changing into adults, such forms with slightly greater changes are called hemimetabolic (incomplete), they include Odonata, Plecoptera and Ephemeroptera.

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Examples of Heterometabolic Metamorphosis Examples Orders Grasshoppers Orthoptera Termites Isoptera Booklice Corrodentia Thrips Thysanoptera Truebugs Hemiptera Aphids Homoptera Earwigs Darmaptera

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Type 4. Holometabolic Metamorphosis : ❖ Most insects undergo complete metamorphosis over the course of a lifetime.

❖ This type of metamorphosis is also known as complete metamorphosis.

❖ In this type, four metamorphic stages are included namely egg, larva, pupa and adult.

❖ Each stage of the life cycle—egg, larva, pupa, and adult— is marked by a distinctly different appearance.

❖ Entomologists call insects that undergo complete metamorphosis "holometabolous," from "holo," meaning "total." ZOOLOGY: SEM- II, PAPER- C3T: NON-CHORDATES II, UNIT 3: ARTHROPODA COMPILED AND CIRCULATED BY DR. POULAMI ADHIKARY MUKHERJEE, ASSISTANT PROFESSOR, DEPARTMENT OF ZOOLOGY, NARAJOLE RAJ COLLEGE

Type 4. Holometabolic Metamorphosis :

❖ After hatching larva moults several times to become fully grown one. ❖ It later becomes a pupa within a secreted case called as puparium. ❖ Inside the puparium, the pupa differentiates into adult and then breaks open the case to emerge out.

❖ The larvae of holometabolous insects bear no resemblance to their adult counterparts. ❖ Their habitats and food sources may be entirely different from the adults as well.

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Type 4. Holometabolic Metamorphosis :

❖ Larvae grow and molt, usually multiple times. Some insect orders have unique names for their larval forms: butterfly and moth larvae are caterpillars; fly larvae are maggots, and beetle larvae are grubs.

❖ When the larva molts for the final time, it transforms into a pupa.

❖ The pupal stage is usually considered a resting phase, although many active changes are occurring internally, hidden from view.

❖ The larval tissues and organs break down entirely, then reorganize into the adult form. After the reorganization is complete, the pupa molts to reveal a mature adult with functional wings.

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Type 4. Holometabolic Metamorphosis :

❖ In Lepidoptera, Coleoptera, Hymenoptera, Diptera, Siphonoptea, etc., the young which hatches from the egg is called a larva, the larva is very different from the adult in structure, body form, mouth parts, legs and in its mode of life, the larva has lateral ocelli in place of compound eyes, it feeds voraciously, grows, moves about and undergoes ecdyses.

❖ The larva is so different from the adult that it first changes into a resting, quiscent instar called a pupa which is often enclosed in a cocoon secreted by the labial glands of the larva.

❖ Great transformation occurs in this instar, wings develop internally from pockets of the hypodermis, and they are not visible from outside.

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Type 4. Holometabolic Metamorphosis :

❖ Because wings develop from internal imaginal discs these insects are also called endopterygota.

❖ Appendages are formed, muscles, tracheae and parts of the alimentary canal are replaced by corresponding organs of the imago.

❖ Such vast changes are called holometabolic metamorphosis.

❖ Most of the world's insect species—including butterflies, moths, true flies, ants, bees, and beetles—are holometabolous.

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Examples of Holometabolic Metamorphosis Examples Orders Lacewings Neuroptera Beetles Coleoptera Scorpion-flies Mecoptera Coddid-flies Trichoptera Moths, Butterflies Lepidoptera Flies Diptera Fleas Siphonoptera Wasps, bees Hymenoptera

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➢ In holometabolic insects there is an internal reconstruction during late larval and pupal instars.

➢ Larval organs, with the exception of central nervous system and developing reproductive organs, are disrupted, their breaking down is called histolysis, this is brought about by phagocytes which feed on the organs, and products of their digestion are then used for building new structures.

➢ The building of new structures is brought about by growth centres called imaginal buds or discs.

➢ Imaginal discs are groups of formative cells which are set aside in the larva, they are the rudiments of future organs of the imago, they form legs, mouth parts, internal organs and wings.

➢ This process of formation of organs of an imago from imaginal discs inside the pupa is known as histogenesis and it results in the formation of the imago. ZOOLOGY: SEM- II, PAPER- C3T: NON-CHORDATES II, UNIT 3: ARTHROPODA COMPILED AND CIRCULATED BY DR. POULAMI ADHIKARY MUKHERJEE, ASSISTANT PROFESSOR, DEPARTMENT OF ZOOLOGY, NARAJOLE RAJ COLLEGE

➢ Thus, two postembryonic processes occur in all insects, the first is growth in the young and the second is metamorphosis, in both of which moulting takes place; both processes are controlled by hormones of endocrine glands.

➢ Insects have two such endocrine glands, they are corpora allata and prothoracic glands.

➢ The juvenile hormone of corpora allata controls growth and moulting up to the end of the larval period. So long as the juvenile hormone of corpora allata is produced the final moulting into a pupa or into an adult cannot take place.

➢ The prothoracic glands are a pair of small glands in the first thoracic segment, they produce a hormone called ecdyson which brings about moulting and development of imaginal discs and reproductive organs.

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➢ When both hormones are secreted, then moulting of the larva only will take place. ➢ The result of the two hormones is supression of adult characters from appearing during larval and pupal instars. ➢ When only ecdyson is secreted, and the juvenile hormone is not produced, then the larva will moult into a pupa, and the pupa into an imago. ➢ Thus, it is seen that ecdyson is essential for each moulting, but its action is modified as long as the juvenile hormone is present. ➢ Removal of the old cuticle in ecdysis is brought about by an enzyme secreted by the hypodermis, the enzyme erodes the lower surface of the cuticle, then the hypodermis secretes a new cuticle below the old one.

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➢ The role of hormones in the physiology of moulting was first described by V. B. Wigglesworth in the 1930's.

➢ When an immature insect has grown sufficiently, it requires a larger exoskeleton then the sensory input from the body activates certain neurosecretory cells in the brain.

➢ These neurosecretory cells in an insect's brain respond by secreting brain hormone (BH) which in turn triggers the corpora cardiaca to release prothoracicotropic hormone (PTTH) into the circulatory system.

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➢ The prothoracicotropic hormone (PTTH) that activates prothoracic glands secretes a second hormone, usually ecdysone (an ecdysteroid), that induces ecdysis.

➢ PTTH also stimulates the corpora allata, a retrocerebral organ, to produce juvenile hormone, which prevents the development of adult characteristics during ecdysis.

➢ In holometabolous insects, molts between larval instars have a high level of juvenile hormone, the moult to the pupal stage has a low level of juvenile hormone, and the final, or imaginal, molt has no juvenile hormone present at all.

➢ This sudden release of PTTH stimulates the prothoracic glands to secrete molting hormone/Prothoracic gland hormone (PGH).

➢ These PGHs are ecdysteroids which trigger moulting process. ZOOLOGY: SEM- II, PAPER- C3T: NON-CHORDATES II, UNIT 3: ARTHROPODA COMPILED AND CIRCULATED BY DR. POULAMI ADHIKARY MUKHERJEE, ASSISTANT PROFESSOR, DEPARTMENT OF ZOOLOGY, NARAJOLE RAJ COLLEGE

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➢ PGH affects many cells throughout the body, but its principle function is to stimulate a series of physiological events also known as apolysis.

➢ Apolysis leads to synthesis of a new exoskeleton.

➢ During this process, the new exoskeleton forms as a soft, wrinkled layer underneath the hard parts of the old exoskeleton.

➢ The duration of apolysis ranges from days to weeks, depending on the species and its characteristic growth rate.

➢ Once new exoskeleton has formed, the insect is ready to shed off its old exoskeleton.

➢ At this stage, the insect body is covered by two layers of exoskeleton and it is called as pharate.

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➢ Toward the end of apolysis, ecdysteroid concentration falls, and neurosecretory cells in the ventral ganglia begin secreting eclosion hormone.

➢ This hormone triggers ecdysis, the physical process of shedding the old exoskeleton.

➢ In addition, a rising concentration of eclosion hormone stimulates other neurosecretory cells in the ventral ganglia to secrete bursicon, a hormone that causes hardening and darkening of the integument due to the formation of quinone cross-linkages in the exocuticle (sclerotization).

➢ In immature insects, juvenile hormone (JH) is secreted by the corpora allata prior to each moult.

➢ This hormone inhibits the genes that promote development of adult characteristics causing the insect to remain in immature state.

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➢ Corpora allata becomes atrophied during the last larval stage and stops producing juvenile hormone.

➢ This releases inhibition on development of adult structures and causes the insect to molt into an adult.

➢ At the approach of sexual maturity in the adult stage, brain neurosecretory cells release a brain hormone that "reactivates" the corpora allata, stimulating renewed production of juvenile hormone.

➢ In adult females, juvenile hormone stimulates production of yolk for the eggs.

➢ In adult males, it stimulates the accessory glands to produce proteins needed for seminal fluid and the case of the spermatophore.

➢ In the absence of normal juvenile hormone production, the adults remain sexually sterile.

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The following table contains the details of the hormones involved in the process of metamorphosis

Hormone Secreted by Chemical nature Function

Brain Hormone Neurosensory Lipids Activates corpora (BH) cells cardiaca

Prothoracicotrop Corpora cardiaca Ecdysteroids Stimulates prothoracic hic hormone glands (PTTH) Prothoracic gland Prothoraacic Ecysone triggers moulting hormone (PGH) glancs

Juvenile hormone Corpora allata Lipids Regulates (JH) morphogenesis and promotes metamorphosis

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