Lesson: Moulting and Metamorphosis in Insects. Lesson Author: Dr. Anjana Singha Naorem. College/Dept: Miranda House, University of Delhi,

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1 Lesson: Moulting and Metamorphosis in Insects Lesson Author: Dr. Anjana Singha Naorem College/Dept: Miranda House, University of Delhi, 1

2 Table of Contents Introduction Kinds of metamorphosis Ametamorphic/Ametabolous Gradual/paurometabolous Incomplete/hemimetabolous Complete/hemimetabolous Hormonal control of moulting and metamorphosis Background Interaction of hormones during development What stimulates secretion of PTTH Physiological changes during moulting Summary Exercise Glossary References/ Bibliography/ Further Reading 2

3 Introduction All living organisms in some or the other way undergo changes in their form, structure, physiology, habit, habitat, etc. These changes vary from slight to drastic changes. Insects are one group of organisms which exhibit a wide range of such changes, more aptly transformations in their life cycle. Insects have one kind of body when they are young and a different kind of body in the adult stage. Such transformations are known as metamorphosis. The term metamorphosis is derived from the Greek words, meta, change, and morphe, form, designating change in form. Metamorphosis is a biological process of transformation or abrupt changes in the animal's body structure through cell growth and differentiation, after birth or hatching into one or different forms before attaining the sexually mature adult form. Simply it can be said as the overall change in shape and body form during development of an insect/animal. As the insect s body is covered by exoskeleton which is a rigid framework, the growth of the insect becomes difficult. In order for the insect to grow, it must shed its exoskeleton through the process of moulting and re-grow a new one. Shedding of the exoskeleton is called ecdysis and the cast-off is called an exuviae. Moulting is one of the processes of growth during the development of insect by metamorphosis Value addition: Video Heading text: Let us very quickly look at moulting in Cicada, a bug. Body text: Cicada moulting Source: Value addition: Mystery!!! Heading text: How did insect metamorphosis evolve? Body text: Source: 3

4 As in all organisms, development in insects starts with egg. Eggs in insects are usually macro- and centro-lecithal and undergo superficial cleavage and elaborate embryological changes getting ready to hatch. Immatures vary in the degree of their development at hatching. The basic stages of development involve egg, juvenile and adult, though in some cases other stages may as well be present. These multiple stages of development is called instar and the final adult stage attained is called imago. Kinds of Metamorphosis Although all insects moult and change body shape through their life, some undergo minor changes while most undergo extreme changes in structure and function. Based on the degree of changes and stages involved, four types of metamorphoses (plural) are recognized: 1. Ametamorphic/ametabolous (Fig. 1): Some insects develop without metamorphosis and they just grow by a series of successive moults. This kind of development is called ametabolous. Juvenile-In this type of development the young one of the insect called juvenile or young looks identical to adult in body proportions and structures. Food and habits of the young and the adults are similar. Adult-Adults are bigger in size as compared to juveniles and possess fully developed reproductive organs. Very few insect species (apterygotes) are ametamorphic example- collembolan (spring tails), dipluran (campodeids), proturan, thysanuran (silverfish). There is apparently no metamorphosis in these insects. 4

5 Fig. 1. Life cycle of silverfish (ametabolous development) Source: ILLL in house Value addition: Knowing the Ametaboly insects Heading text: Representative insects in each order 5

6 Source: a. CC b. CC c. CC d. CC 2. Paurometabolous/gradual (Fig. 2): Paurometabolous insects also known to exhibit gradual development, the juveniles resemble an adult with some exceptions (given below). Nymph-They are smaller than adults, wings are either absent or reduced and are sexually immature. As in the case of hemimetabolous insects, the juveniles are known as nymphs. They develop wing buds in the late instar Usually there is a characteristic number of nymphal moults which ends after the attainment of adult stage. Adult- Adults are winged, sexually mature and live in the same habitat and feed on the same foods as nymphs so they possess similar mouthparts. This category of insect includes some of the very serious crop insect pests and both nymphs and adults generally have the same pest status. Paurometabolous insect includes: orthopteran (grasshoppers, crickets, cockroaches), dermapteran (earwigs), isopteran (termites), psocopteran (booklice), mallophaga and anopluran (lice), hemipteran (true bugs), and homopteran (cicadas, hoppers and aphids). 6

7 Fig. 2 Life cycle of a Locust (paurometabolous development) Source: +H CC Value addition: Knowing the paurometabolous insects Heading text: Representative insects in each order 7

8 Source: a. Grasshopper_2.JPG CC b. CC c. CC d. CC e. CC f. written for permission g. CC h. written for permission 3. Hemimetabolous/incomplete (Fig. 3 & 4): In hemimetabolous insects juvenile look quite different from the adult and this kind of metamorphosis is also known as incomplete development. Nymph- They have wing pads that gradually increase in size and become functional in the adult stage only after the final moult. The juveniles are largely referred to as nymphs or 8

9 naiads. These nymphs are sexually immature and are aquatic in habitat and thus possess gills for breathing. Adult- They occupy separate ecological niche (aerial) from the nymphs and hence do not compete for resources. Adults have different mouthparts from that of the nymph. Just before moulting into the adult stage, the aquatic or subterranean juvenile crawls out of the pond, stream, or ground into a twig or stem and a winged adult emerges from its skin. Hemimetabolous insects include ephemeropteran (mayflies), odonata (dragonflies and damselflies) and plecopteran (stoneflies). Value addition: Greek origin of Naiad Heading text: Origin of the word naiad Body text: In Greek mythology, the Naiads were a type of nymph (female spirit) who presided over fountains, wells, springs, streams, brooks and other bodies of fresh water. A Naiad by John William Waterhouse, 1893; a water nymph approaches the sleeping Hylas. In classical mythology, Hylas was a youth who served as a companion of Roman Hercules. His abduction by water nymphs was a theme of ancient art, and has been an enduring subject for Western art in the classical tradition. Source: 9

10 Fig. 3 Life cycle of Mayfly (Hemimetabolous development) Source: Life cycle: mayfly. Art. Encyclopædia Britannica Online. Web. 22 May < Permitted for educational use Value addition: Do you know? Heading text: Adult moult Body text: Members of the order Ephemeroptera (mayflies) do not have a pupal stage, but they briefly pass through an extra winged stage called the subimago. Insects at this stage have functional wings but are not yet sexually mature. Source: 10

11 Fig. 4 Life cycle of Dragonfly (hemimetabolous development) Source: ILLL in house Value addition: Knowing the hemimetabolous insects Heading text: Representative insects in each order Body text: 11

12 Source: a. b. c %29.jpg d jpg 4. Holometabolous/complete (Fig. 5): It is a complete type of metamorphosis in which the insects pass through four stages of life: egg, larva, pupa, and adult. Larva- Immediately after hatching from egg, active, immature, voracious feeder wriggling stage called larva emerges. Larva undergoes several moults, each stage being called as instar. Pupa- After several moults larva transform into a resting, non-feeding and inactive stage called pupa. Pupa is sometimes also referred as chrysalis and is more commonly used for butterflies and moths. Pupa is encased in a firm case or cocoon for the protection of developing pupa. Inside the pupa, larva undergoes drastic changes in which 12

13 larval organs are hydrolysed and adult structures develop from clusters of embryonic reserve cells called imaginal discs. Adult- Finally the adult emerges which is completely different from the larvae in form, structure, feeding habit and habitat. Larval forms lack wing pad. Their food is different from the adults and the two often have different kind of mouthparts. Examples: coleopteran (beetles), dipteran (flies), hymenopteran (bees and ants), lepidopteran (butterflies), etc. Larva is differently referred in some insect orders like maggots (flies), caterpillar (butterflies and moths) and grubs (beetles).. Fig. 5 Life cycle of a butterfly (Holometabolous development) Source: Value addition: Knowing the Paurometabolous insects Heading text: Representative insects in each order Body text: 13

14 Source: a. CC b. _side_(aka).jpg CC c. CC d. CC Value addition: Types of Insect larvae Heading text: `Types of insect larvae Body text: Among endopterygotes the extent of variation between the larval and adult habits and structures are enormous. There are largely four types of larvae in insects:] a. Protopod larvae: Found in parasitic Hymenoptera Egg with little yolk Emerge in early embryonic phase but can survive as they develop immersed in a highly nutritive medium of the host During emergence, variable degree of development in different 14

15 insects Fig a. Steel-blue Sawfly larvae Photographer:Melissa Murray Rights: Melissa Murray b. Polypod larvae: Found in Lepidoptera, saw-flies and scorpion flies Also called eruciform larvae Well defined segmentation Poorly developed thoracic legs and antennae Presence of abdominal limbs or prolegs Fig b. Lepidoptera: Papilio machaon caterpillar with 5 prolegs c. Oligopod larvae: Well-developed thoracic legs Absence of abdominal appendages Head capsule and appendages well developed Two types exist: Campodeiform & Scarabaeiform Campodeiform larvae has elongated, sclerotised and depressed body with prognathous head. They are active predators. Example-Neuroptera, some Coleoptera 15

16 Fig c. An active, campodeiform larva (Carabidae). Fig d. A c-shaped larva, or grub (Scarabaeidae). Scarabaeiform larvae has stout subcylindrical C-shaped body with shorter thoracic legs and soft fleshy body and no caudal processes. Example Scarabaeiodea family of Coleoptera d. Apodous larvae: Trunk appendages completely suppresses. It is derived from the oligopod larva. Depending on the degree of development of head, they are of three types: Eucephalous, hemicephalous & acephalous larva Eucephalous larvae have more or less scelrotized head capsule with relatively little reduction of the cephalic appendages. Example Nematocera Hemicephalous larvae have reduced head capsule and its appendages accompanied by marked retraction of the head into the thorax. Example Tipulidae Acephalous larvae have no head capsule or appendages. Example Cyclorrhapan Diptera 16

17 Fig e. An apodous, or legless, larva (Curculionidae) Photo by Gerald J. Lenhard, Louisiana State University, Bugwood.org Text Source: IMMS General textbook of Entomology, Image Source: a. b. c. Gyorgy Csoka, Hungary Forest Research Institute, Bugwood.org Node Affiliation: University of Georgia = &CFID= &CFTOKEN=17e139be155b8df3-D0CD4C0B-B903-7D34-E95C4E143A560D6C#sthash.sMnRn59V.dpuf CC d. Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org Node Affiliation: University of Georgia - See more at: = &CFTOKEN=17e139be155b8df3-D0CD4C0B-B903-7D34- E95C4E143A560D6C#sthash.gEIimvHS.dpuf e. Gerald J. Lenhard, Louisiana State University, Bugwood.org Node Affiliation: University of Georgia = &CFTOKEN=17e139be155b8df3-D0CD4C0B-B903-7D34- E95C4E143A560D6C#sthash.X5pKCjCC.dpuf CC Value addition: Different larval forms in the same insect. Is that possible??? Heading text: Heteromorphosis/Hypermetamorphosis Body text: In most endopterygotes the larval instars are more or less like. However, in some species of Neuroptera, Coleoptera, Diptera, Hymenoptera, and in all Strepsiptera, a larva undergoes characteristic changes in habit and morphology as it grows,a phenomenon known as heteromorphosis (hypermetamorphosis). In such species several of the larval types described above may develop successively. For example, blister 17

18 beetles (Meloidae) hatch as free-living campodeiform larvae (planidia, triungulins) that actively search for food. At this stage the larvae can survive for periods of several weeks without food. Larvae that locate food soon moult to the second stage, a caterpillar like (eruciform) larva the insect then passes through two or more additional larval instars, which may remain eruciform or become scarabaeiform. Some species overwinter in a modified larval form known as the pseudopupa or coarctate larva, so called because the larva remains within the cuticle of the previous instar. The pseudopupal stage is followed the next spring by a further larval feeding stage, which then moults into a pupa. Text Source: Entomology by Cedric Gillott Image Source: wriiten for permission Value addition: Types of Pupa Heading text: Types of Pupa Body text: Types of Pupa Pupa is a non feeding and inactive stage of insect between the larva and adult with complete metamorphosis. The insect pupae are classified into two types on the basis of mode of emergence of adults from the pupal case. 1. Decticous Pupa: In this of pupa, more or less fully formed adult, within the pupal case has relatively powerful sclerotized mandibles by means of which it comes out from the pupa. This type of pupa is always execrated (free) type e.g. Lace wing, Scorpion flies. 2. Adecticous Pupa: In this of pupa, the adult developed within pupal case, often possess reduced and non articulating mandibles which are not utilized for escaping from the pupa. Two main types of adecticous pupae are: a. Exarate Adecticous Pupa: In this type of pupae the appendages are free of any 18

19 secondary attachment to the body e.g. Honey bee, wasp, white grub etc. b. Object Adecticous Pupa: In this type, the appendages are family pressed against its body and are soldered to it e.g. Gram pod borer, lemon butter fly etc. 3. Coarctate Pupa: In this type, the appendages are not visible. The pupa is enclosed in a puparium, formed from the last larval skin. This is clearly adecticous exarate pupa e.g. House fly, fruit fly etc. Significance of Pupal Stage: 1. Being non feeding stage it avoids or reduces the competition for food. 2. Helps in re-modeling and re-structuring or the body to exploit many habitats. 3. Chances of survival of insects are increased by entering in inactive stages. Source Hormonal control of moulting and metamorphosis As we now know that all insects during the stage of their development secrete a new more flexible exoskeleton under the old one and finally moult or ecdyse by shedding their exuviae you must be pondering what triggers such changes!!! The answer is HORMONES. Both the process of moulting and metamorphosis are under the control of hormones which in turn is under the control of brain. These periodic moulting from larval-larval, larval-pupal, or pupal-adult have been found to be under endocrinological control. Background Stefan Kopeč, a Polish biologist and a pioneer of insect endocrinology, was the first to establish the role of insect brain in metamorphosis. He established the importance of brain for successful pupation and the concept of critical period during brain hormone secretion. For the normal process of metamorphosis the presence of the brain, at least up to a certain moment, is indispensable... Stefan Kopeč (1917) Kopec's study indicated the importance of secretions (brain hormone) from the neurosecretory cells of the insect brains for the regulation of insect metamorphosis. This 19

20 brain hormone is presently known as prothoracicotropic hormone (PTTH). The interesting observation was the functioning of the nervous tissue like an endocrine gland. This astonishing discovery stirred research in this area leading to the establishment of a new field of science known as neuroendocrinology. Much later, experiments by Wigglesworth (1934) on the reduviid bug, Rhodinus prolixus, established the hormonal control of insect metamorphosis. He identified that the secretion of juvenile hormone which he initially called as inhibitory hormone, from glands in the insect head modifies the type of moult. In 1940s, experiments by Fukuda on Bombyx mori led to the concept of involvement of secretion from the prothoracic region important for insect pupation. Finally the dichotomy surrounding the roles of the brain and prothoracic glands was resolved by Carroll Williams at Harvard University in a series of experiments performed on pupae of a native silkmoth Hyalophora cecropia. Williams (1947) established that both brain and prothoracic glands were essential for development and the brain hormone activates the prothoracic glands to secrete moulting hormone. Value addition: A great Insect Physiologist Heading text: Sir Vincent Brian Wigglesworth Body text: Sir Vincent Brian Wigglesworth (17 April February 1994) was a British entomologist who made significant contributions to the field of insect physiology. In particular, he studied metamorphosis. His most significant contribution was the discovery that neurosecretory cells in the brain of the South American kissing bug, Rhodnius prolixus, secrete a crucial growth hormone, prothoracicotropic hormone (PTTH), which regulates the process of metamorphosis. This was the first experimental confirmation of the function of neurosecretory cells. He went on to discover another hormone, called the juvenile hormone, which prevented the development of adult characteristics in R. Prolixus until the insect had reached the appropriate larval stage. Wigglesworth was able to distort the developmental phases of the insect by controlling levels of this hormone. From these observations, Wigglesworth was able to develop a coherent theory of how an insect's genome can selectively activate hormones which determine its development and morphology. Source: Interaction of hormones during development The entire process of insect development is largely controlled by three main hormones (Fig. 6) 20

21 Hormone Prothoracicotropic hormone Source Brain neurosecretory cells (NSC) (PTTH) Ecdysteroids Juvenile hormone (JH) Prothoracic gland Corpora allata When the larval or nymphal stage of an insect has grown sufficiently big, it is said to have attained a critical weight and it requires a larger exoskeleton to accommodate its growing body. At this stage interplay of the hormonal titers regulate the metamorphosis (Fig. 7). Growing and the distended body of the insect sends a sensory signal to the brain resulting in activation of clusters of neurosecretory cells in the brain which in turn triggers the production of PTTH. PTTH is not released directly from the brain into the body but it passes down into the neurohemal organ, the bilaterally paired corpora cardiaca (CC) to release the stored PTTH into the circulatory system. But in the case of probably all lepidopterans PTTH is released from corpora allata (CA), another neurohemal organ. In any case, the sudden pulse of PTTH stimulates the prothoracic glands to secrete ecdysteroids/ecdysone, the moulting hormone. Ecdysone is not stored in the prothoracic gland but is secreted into the hemolymph as it is produced. Ecdysone, however, is not an active hormone, but a prohormone that must be converted into an active form 20-hydroxyecdysone. This conversion is accomplished by a hemecontaining oxidase (20-monooxygenase enzyme requiring cytochrome P450) in the mitochondria and microsomes of peripheral tissues such as the fat body. This active form of moulting hormone stimulates a series of physiological events (discussed in the next section) leading to the synthesis of a new exoskeleton by the process of apolysis. Depending on the type of insect species, the duration of apolysis varies which usually ranges from days to weeks. Once new exoskeleton is formed, the insect prepares to shed its old exoskeleton. At this stage when insect body is covered by two layers of exoskeleton, the insect is said to be pharate. Both PTTH and ecdysone together trigger every moult that is larva-to-larva and spupa-to-adult. The third hormone juvenile hormone is responsible for maintenance of the young (larval or nymphal) state of the insect and is synthesized and secreted by a neurohemal organ, corpora allata. This implies that neurohemal organ not only serve as a hormone release site but also can synthesize hormones. JH modifies the expression of the moult and acts in conjunction with ecdysone as it can exert its effect only after the moulting 21

22 process has been initiated. JH titers tend to be high at the early part of an instar (larvallarval moult) and falls (very low in hemimetabolous insects and moderate level in holometabolous insects) when the instar approaches its full growth (larva-pupa) and when its titer is negligible or absent, a pupal-adult moult ensues (Fig. 7). Thus, the JH is considered a modifying agent that favours the synthesis of larval structures and opposes adult differentiation. Fig. 6 Hormonal control of insect development Source: +Rita+Zeng 1. NSC in the brain produce PTTH and stored in corpora cardiac for future release 2. PTTH send signals to prothoracic gland to produce ecdysone 3. Ecdysone release results in insect moult when a specific instar is fully grown 4. JH released from corpora allata is a decisive hormone as its titer helps in determining which state would be maintained next in metamorphosis 22

23 Fig. 7 Schematic graph showing hormone fluxes in the hemolymph of a growing larva Just before every moult, juvenile hormone (JH) production decreases by attainment of critical weight which leads to prothoracicotropic hormone (PTTH) production, ecdysone secretion and the cessation of feeding. A large fraction of total growth occurs in the interval to cessation of growth (ICG). Displayed for permission Source: Value addition: Hormones for insect population control!!! Heading text: Insect Growth Regulators Body text: Carroll Williams Carroll Milton Williams (December 2, 1916 in Oregon Hill, Richmond, Virginia - October 11, 1991 in Watertown, Massachusetts) was an American zoologist known for his work in entomology and developmental biology -- in particular, metamorphosis in insects, for which he won the George Ledlie Prize. He performed groundbreaking surgical experiments on larvae and pupae, and developed multiple new techniques, including the use of carbon dioxide as an anesthetic. His impact on entomology has been 23

24 compared to that of Vincent Wigglesworth. Williams was the first to isolate juvenile hormone and ecdysone, and discovered cocoonase and cytochrome b5, as well as the "paper factor". He subsequently proposed that hormonal analogues could be used as pesticides by disrupting the developmental cycles of insects. Source: What stimulates secretion of PTTH? Having understood the hormonal interplay and regulation of moulting and metamorphosis, one very relevant and important question comes to one s mind what stimulates the brain to secrete PTTH at the first place? Complete answer is yet not clear. Research is underway to unravel the mystery but so far some of the stimuli known to initiate the PTTH secretion and hence the moulting are: a) Stretching of the abdomen in response to a large blood meal, b) Attainment of certain size, critical size, and c) Environmental stimulus, including exposure to cold Value addition: Insect Integument Heading text: Structure of Insect Integument Body text: Source: Author Fig. Diagrammatic view of insect cuticle Physiological changes during moulting The process of moulting is important for the growth of larval stages in successive instars which prepares the insect for transition from larva to adult stage. Moulting is initiated by the hormone ecdysone. During moulting insect undergoes several changes including some major histological changes (Fig. 7) which are as follows: The instar becomes less active 24

25 Increase in size and number of the epidermal cells by mitosis becoming more and more closely packed Retraction of the epidermis results in cuticular detachment forming a subcuticular space. The process is called apolysis The space beneath the old cuticle is filled with moulting gel which is secreted by the epidermal cells Initially, moulting gel contains inactive chitinase and protease enzymes that allows laying down of the first layer, the new epicuticle After the completion of formation of epicuticle the moulting gel becomes fluid that contains active enzymes and start hydrolysing/digesting the inner layer of old cuticle (endocuticle only is digested) The products of this digestion is resorbed Simultaneously, the epidermal cells begin to deposit the new cuticle which is undifferentiated initially called procuticle Exocuticle remains undigested by these active enzymes in the moulting fluid owing to it sclerotization The old cuticular sheath is finally ruptured which occurs along definite lines of weakness (ecdysial lines) in the head capsule and along the median anterodorsal part of the body. Ecdysial lines are areas formed in the cuticle that do not have the hardened exocuticle, so the moulting fluid digests away most of the cuticle along these lines The pressure needed to rupture the cuticle is generated by swallowing air or water by muscular contractions resulting in accompanied hemolymph pressure change. Finally the new instar wriggles out, usually head and thorax first followed by the rest of the body. All the cuticular parts including the lining of foregut and hindgut and lining of the major tracheae are shed In addition, after every moult hardening (sclerotization) and darkening of the integument (tanning) takes place by the secretion of bursicon hormone which is stimulated by the rising concentration of eclosion hormone Quinone cross-linkages in the exocuticle is responsible for the cuticular thickening and sclerotization 25

26 Fig. 7 Physiology of ecdysis Source: CC Value addition: Video Heading text: Cuticle degradation and re-formation during insect moulting Body text: Cuticle degradation and re-formation during insect moulting Source: Summary For growth and development of insect, moulting and metamorphosis is an necessary event Every young form (larva/nymph) grows to its fullest and being limited by its exoskeleton has to undergo the process of moulting/ecdysis to metamorphose into the next stage which can be again the young stage or pupa or adult, 26

27 Owing to the diversity of insects in terms of habit, habitat, ecology behaviour and so on.., different insects have developed different ways to transform or metamorphose. Four types of metamorphosis are- ametabolous, hemimetabolous, paurometabolous and holometabolous The process of moulting and metamorphosis is under the hormonal control When the larva/nymph is sufficiently big, NSC of the brain sends signals to the corpora cardiaca to release PTTH which in turn stimulate the release of ecdysone for insect moulting. Another hormone JH released from corpora allata, regulates the larval/nymphal cycle by regulating its titer becoming almost negligible at the adult moult. Interplay of these three hormones largely regulate the process of moulting and metamorphosis These hormonal interaction results in lot of physiological changes in the insect body, prime among it is the cuticular changes Cuticular changes begins with increase in number of epidermal cells followed by apolysis, laying down of new cuticle, lysis of old endocuticle and their resorption, finally exuvial shedding Newly formed exuviae are then tanned under the influence of bursicon hormone and thickening of the cuticle ensues. Exercise 1. Identify and write the type of metamorphosis. Give reasons: 27

28 a. b. c. d. e. 2. Match the following a. Pupa i. Corpora allata b. Grub ii. Damselfly c. Juvenile hormone iii. Cuticular thickening/tanning d. Ecdysone iv. Beetle e. Maggots v. Prothoracic gland f. Naiad vi. Mayflies g. Sub-imago vii. Complete metamorphosis h. Bursicon viii. Flies 28

29 4. Differentiate between the following: a. Larva and nymph b. Ametabolous and paurometabolus development c. Hemimetabolous and holometabolous development d. Juvenile hormone and ecdysone 5 What triggers the initiation of moulting in insects? 6. Define metamorphosis and describe the different types of metamorphosis 7. Illustrate the interplay of hormones during insect development. 8. Describe briefly the physiological changes during insect moulting. Learning Activity List the insects that you see around you and think what kind of metamorphosis they exhibit and reason out. Glossary Ametabolous (no metamorphosis) Most simplest form of metamorphosis. The young ones resemble the adults but differ in size and sexual maturity. Present in apterygotes. Apolysis Separation of old exoskeleton from underlying epidermal at the beginning of moulting under the influence of ecdysteroids 29

30 Caterpillar The larva of a butterfly or moth, which has a segmented body, three pairs of true (jointed) legs and several pairs of leg-like appendages in the abdomen Cocoon A silky case spun by the larvae of many insects for protection as pupae Cuticle The outer covering layer of an insect made of chitin, a horny proteinaceous substance Ecdysis The process of shedding the old cuticle Endocuticle The innermost, thickest and laminated layer of insect cuticle and is made of chitin only Epicuticle The outermost extremely thin layer of insect cuticle that is without chitin and has waxy coating on the outer surface and has protein and lipids on the inner surface Exocuticle The middle layer of insect cuticle between epicuticle and endocuticle and is made up of chitin and melanin pigments. Exoskeleton The hard covering on the outside of an insect that provides structural support and protection Grub The larval form of a beetle Hemimetabolous (incomplete metamorphosis) Immature forms (naiads) differ largely in habitat, feeding habit and body structure and are sexually immature. Last instar exhibits wing buds. Holometabolous (complete metamorphosis) Immature forms are worm-like caterpillars, with a fixed number of larval instars and involving resting, inactive and nonfeeding state (state). Life cycle involves four stages egg, larva, pupa and adult Imaginal discs Clusters of undifferentiated embryonic cells in holometabolous insects that proliferate during larval stages, then differentiate during the pupal stage upon induction by ecdysteroids in the absence of juvenile hormone Imago The fully developed adult insect Instar A phase between two periods of moulting in the development of an insect Intermoult the stage between the moulting of an insect which begins with ecdysis from previous stage and ends with apolysis Juvenile Young one of an insect Larva The active immature form of an insect, especially one that differs greatly from the adult and forms the stage between egg and pupa, e.g. a caterpillar or grub Maggots The larval form of a fly Metamorphosis The process of transformation from an immature form to an adult form in two or more distinct stages Neurohemal organ: the general name for a structure from which neurosecretory hormones are released into the circulating hemolymph 30

31 Nymph An immature form of an insect that does not change greatly as it grows and in some insects resembles adult except for the wings and attainment of sexual maturity Procuticle Exocuticle and endocuticle together make up procuticle Pupa/ chrysalis The stage in holometabolous insect between larva and adult form. It is an inactive state with regard to feeding and locomotion unlike larval and adult form Teneral a state of the imago of an insect immediately after moulting during which it is soft and immature in coloring References/Suggested reading 1. Frost, S.W. (1942). Metamorphosis. In Insect life and insect natural history. Dover publications,inc. New York.PP Imms, A.D., Richards, O.W. and Davies, R.G (1964). Post embryonic development. In IMMS General Textbook of Entomology. Springer. Pp

32 3. Gilbert, L.J. (1964). Physiology of growth and development: Endocrine aspects. In The Physiology of Insecta. Morris Rockstein (ed).vol. I. Academic Press Inc. (London). Pp Nation, J.L. (2002). Hormones and Development. In Insect Physiology and Biochemistry. CRC Press. Pp Gillott, C. (2005). Post embryonic development. In Entomology 3 rd Ed.Springer. Pp Ruppert, E.E.; Fox, R.S. and Barnes, R.D. (2006). Hexapoda. In Invertebrate Zoology. A functional evolutionary approach (7ed). Thomson Brooks/Cole. Websites /molting.html