Insect Physiology - Integument Systems Department of Entomology National Chung Hsing University CONTENTS Advantages of an exoskeleton Insect growth and development Strategies for growth Origins of holometaboly Instars, stadia, and hidden phases Structure of the integument Modified features of the integument Chemistry of the cuticle The molting process Endocrine control of molting Endocrine control of metamorphosis Metamorphosis and the radically changing cuticle Insect Integument Insect integument system – exoskeleton – like the skin of vertebrates - provide a barrier to the environment water (*high surface-to-volume ratio) ions parasites environment chemicals, including pesticides – as the skeleton system in insects - allow for the insertion of muscles to locomotion – as food reservoir (???) / molting & stravation – mating recognition - responsible for releasing particular behavioral sequences – many other functions Insect Integument Advantages – significant mechanical strength over an endoskeleton of the same weight (next slide) Disadvantages – restrict insect growth - molt – molting is dangerous to insects – molting consumes time, energy, and metabolic resources Insect Growth and Development The growth and development of insects are largely a function of the growth and development of their integuments. – Molting – Metamorphosis Strategies for Growth Metamorphosis: the change that occurs as an insect develops from an immature to an adult; separates and early feeding stage from a later reproductive stage. – Ametabolous development - continue to molt as sexually mature adults and there is no real metamorphosis – Hemimetabolous (incomplete) development immatures lack wings and genitalia (exoptergotes) – Holometabolous (complete) development - a sometimes very radical change in form and ecological habits between immatures and adults (endopterygotes) Three Major Types of Metamorphosis in Insects Origins of Holometaboly (Berlese, 1913) (Hinton, 1963) (Truman and Riddiford, 1999) Instars Stadia and Hidden Phase Instars: a term to describe an immature insect between ecdyses Stadium: a term to describe the length of time spent between ecdyses Pharate instar (adult): a term to describe an insect within the loosened, but not yet shed, cuticle Structure of the Integument The outer covering of insects is referred to both as an exoskeleton and an integument. The integument consists of – basement membrane – epidermal cell layer – epidermis – nonliving cuticle Structure of the Integument Basement membrane基底膜: a continuous sheet of mucopolysaccharide, as much as 0.5 mm in thickness; initially secreted by hemocytes Epidermis上皮層: the only living portion of the integument; modifications of these cells produce dermal glands, sensory receptors and their support cells, and oenocytes. Cuticle表皮: secreted by epidermis; divided into two main regions – epicuticle: consists of cement layer, wax layer, outer epicuticle (cuticulin layer), and inner epicuticle – procuticle: consists of exocuticle, mesocuticle, and endocuticle, contains largely of chitin and protein The Procuticle原表皮 The procuticle is secreted by the epidermal cells and consists largely of chitin and protein. (next slide) – exocuticle: the proteins become heavily cross-linked and insoluble; are not broken down during the molting cycle; pigments deposited within it – endocuticle: synthesis continues after the old cuticle is shed, often in daily layers; cross-linking is reduced; completely broken down during molting process – mesocuticle: as a transitional layer in which the proteins are untanned like the endocuticle but impregnated with lipid and proteins like the exocuticle. The Epicuticle上表皮 The epicuticle is a complex consisting of several layers that are produced by both the epidermal cells and dermal glands. (next slide) – Cement layer固結層: consists mostly of lipoprotein secreted by dermal glands. – Wax layer腊層: are mixtures of hydrocarbons with 25-31 carbon atoms, alcohols of 24-34 carbon atoms, and esters of fatty acids; produced by the epidermal cells – Outer epicuticle (i.e. cuticulin): synthesized by epidermal cells; present in all insects; the first layer of the new cuticle to be synthesized – Inner epicuticle: contains both polyphenols多酚 and the enzyme polyphenol oxidase多酚氧化酵素, which involved in tanning the cuticle. 體壁硬化作用 水分散失測試 Fig. The relative water loss in two insects as a function of temperature. Modified Features of the Integument Arthrodial membrane節間膜: the flexible membranes between body segments where the exocuticle is absent; untanned endocuticle contains special acidic proteins and resilin (a flexible protein) to provide the flexibility in the region. (next slide) Ecdysial line脫皮線: areas of reduced exocuticle that they are programmed areas of weakness that serve as emergence points during ecdyses. (next slide) Pore canals孔道: cytoplasmic extensions of the epidermal cells extend from the epidermis through the cuticle to its surface. (next slide) Cuticle between Two Segments The Cuticle in Ecdysial Lines The Generalized Insect Integument Chemistry of the Cuticle The insect cuticle is composed largely of – Proteins comprise more than half the dry weight of the insect cuticle primarily located within the procuticle synthesized mainly by epidermal cells – Chitin consisting of 20-40% of the total dry weight of the cuticle (the other major component of procuticle) a polymer of N-acetyl-D-glucosamine (-galactosamine) synthesized by epidermal cells – Lipids mainly located in the wax layer of epicuticle synthesized largely by the oenocytes and the fat body Families of protein in insect Class C proteins Class BD proteins Class H proteins Class T proteins Kinds of cuticular proteins Varies Heavily sclerotized: hydrophobic, positively charged proteins Flexible cuticle: acidic proteins (bind water) R. prolixus, lower the pH of portions of cuticle to below 6 more plastic to expand when blood meal Fig. The mechanism of ootheca production in the cockroach. A Portion of the Chitin Chain 1-4 b-linkage N-acetyl-Dglucosamine glucosamine The Orientation of the Chitin Chains in the Cuticle • cross-linked by hydrogen bonds (A) The orientation of a chitin, the most common form in insects; (B) The orientation of b chain; (C) Two possible orientation of g chitin; (D) The location of the chains of a chitin in a single chitin microfibril. Fig. The helicoidal arrangement of the chitin layers as they are rotated by a constant angle during their synthesis. The steps in chitin biosynthesis. Chitosan Sclerotization 骨化作用 Cuticular sclerotization, also known as tanning, stabilizes the protein matrix of the cuticle to make it stiffer and harder, more insoluble, and more resistant to degradation. The process of sclerotization cross-links the functional groups of cuticular proteins when they react with quinones. The amino acid tyrosine provides one of the precursors (DOPA or NADA) for sclerotization. The precursors are oxidized by phenoloxidases to form reactive quinones. The Steps in the Synthesis of Cuticular Tanning Precursors (NADA) (*less dark than NBAD) Less dark More dark (NBAD) Catecholamines phenolosidases quinones Fig. Differences between quinone sclerotization and bsclerotization in where the cross-linked proteins are attached. Hormonal Regulation of Sclerotization At least two hormone are involved in the regulation of sclerotization – Ecdysteroids: induce the epidermal cells to synthesize the dopa decarboxylase (to synthesize NADA) – Bursicon: induced by declining ecdysteroid titers; increase the permeability of epidermal cells to tyrosine and to hemolymph catecholamines. The Molting Process The molting process involves an elaborate sequence of events that produces a new cuticle capable of significant expansion before the old one is discarded. The molting process begins with apolysis and ends with ecdysis. – Apolysis剝離作用: the separation of the epidermal cells from the old cuticle – Ecdysis脫皮作用: the casting off of the old cuticle The Steps of Molting Process Exuvial space: the area between the cuticle and epidermis; fills with a molting gel that contains inactive enzymes including a chitinase and proteases for digesting the old cuticle. The Steps of Molting Process • The epidermal cells secrete a new outer epicuticle (lipoprotein: cuticulin); • The activation of the enzyme in the molting gel, now called the molting fluid; • The molting fluid begin the digestion of the old unsclerotized endocuticle; • The epidermal cells begin to secrete the new procuticle; • The Steps of Molting Process • Formation of the new epicuticle; • Absorption of the molting fluid; • Ecdysis: induced by eclosion hormone. Eclosion Behavior and Its Endocrine Regulation Behavior of ecdysis are divided into two phases: (control by central nervous system) – Pre-ecdysis behavior: loosen the old cuticle through rotational movements of the abdomen – Ecdysis behavior: shed the old cuticle by means of peristaltic contractions A cascade of neurohormones is responsible for eliciting eclosion behavior – Ecdysis-triggering hormone: from epitracheal glands – Eclosion hormone: from CC – Crustacean cardioactive peptide (CCAP): from the ventral ganglion Endocrine Control of Molting Control of PTTH release – nervous stimuli such as stretch receptors and critical size (or body mass) – environmental stimuli such as photoperiod, temperature Mode of action – via a second messenger, cAMP Correction of Cellular Events during A Molting Cycle with the Ecdysteroid Titer Endocrine Control of Metamorphosis Insect metamorphosis is a function of gene expression by epidermal cells and the temporal pattern of their protein synthesis. Two major hormones are involved in the metamorphosis – juvenile hormone – ecdysteroids Fig. Hormonal regulation of insect metamorphosis. Fig. The correlation of hormone levels with developmental events of Drosophila melanogaster. Fig. The relationship between the size of the Manduca larva and its tendency to pupate and undergo metamorphosis. Supernumerary Larvae of Spodoptera litura The Mechanism of JH Action Imaginal Discs Imaginal discs are derived from ectoderm and are small groups of embryonic cells that persist in larvae of the Holometabola. When the insect pupates, the imaginal discs provide the cells to make adult structures. Fig. The imaginal discs of a larval Drosophila and the corresponding structures in the adult to which they give rise. Fig. The evagination of a leg disc during Drosophila development. Two Cross Section of Two Appendages The Oenocytes The oenocytes are large polyploid cells associated with the basement membrane. – some oenocytes might be involved in the production of cuticular lipid that are deposited in the epicuticle. – other types of oenocytes may secrete ecdysteroid hormones.