- - Integumentary System Integumentary System of insectsof insects

CONTENTSCONTENTSAdvantages of an exoskeletonInsect growth and developmentStrategies for growthInstars, stadia, and hidden phasesStructure of the integumentModified features of the integumentChemistry of the cuticleThe molting processEndocrine control of moltingEndocrine control of metamorphosisMetamorphosis and the radically changing cuticle

Insect Integument Insect Integument Insect integument system – exoskeleton– like the skin of vertebrates - provide a barrier to

the environmentwater (*high surface-to-volume ratio)ionsparasitesenvironment chemicals, including pesticides

– as the skeleton system in insects - allow for the insertion of muscles to locomotion

– as food reservoir (???) / molting & starvation– mating recognition - responsible for releasing

particular behavioral sequences– many other functions

Insect IntegumentInsect IntegumentAdvantages– 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 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 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 InsectsThree Major Types of Metamorphosis in Insects

Instars Stadia and Hidden PhaseInstars Stadia and Hidden PhaseInstars: a term to describe an immature insect between ecdysesStadium: a term to describe the length of time spent between ecdysesPharate instar (adult): a term to describe an insect within the loosened, but not yet shed, cuticle

Structure of the IntegumentStructure of the IntegumentThe 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 IntegumentStructure of the IntegumentBasement membrane: a continuous sheet of mucopolysaccharide, as much as 0.5 m in thickness; initially secreted by hemocytesEpidermis: the only living portion of the integument; modifications of these cells produce dermal glands, sensory receptors and their support cells, and oenocytes.Cuticle: 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 ProcuticleThe ProcuticleThe 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 EpicuticleThe Epicuticle The epicuticle is a complex consisting of several layers that are produced by both the epidermal cells and dermal glands. (next slide)– Cement layer: consists mostly of lipoprotein secreted by

dermal glands.– Wax layer: 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 and the enzyme polyphenol oxidase, which involved in tanning the cuticle.

Fig. The relative water loss in two insects as a Fig. The relative water loss in two insects as a function of temperature. function of temperature.

Modified Features of the IntegumentModified Features of the Integument Arthrodial membrane: 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: areas of reduced exocuticle that they are programmed areas of weakness that serve as emergence points during ecdyses. (next slide)Pore canals: cytoplasmic extensions of the epidermal cells extend from the epidermis through the cuticle to its surface. (next slide)

The Generalized Insect IntegumentThe Generalized Insect Integument

The Generalized Insect IntegumentThe Generalized Insect Integument

Chemistry of the CuticleChemistry of the Cuticle The insect cuticle is composed largely of– Proteins

comprise more than half the dry weight of the insect cuticleprimarily located within the procuticlesynthesized mainly by epidermal cells

– Chitinconsisting 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

– Lipidsmainly located in the wax layer of epicuticlesynthesized 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 proteinsFlexible cuticle: acidic proteins (bind water)

R. prolixus, lower the pH of portions of cuticle to below 6 more plastic to expand when blood meal

The steps inchitin biosynthesis.

Chitosan

The Steps in the Synthesis of Cuticular The Steps in the Synthesis of Cuticular Tanning PrecursorsTanning Precursors

(NADA)

(NBAD)

(*less dark than NBAD)

More dark

Less dark

Fig. Differences between quinone sclerotization and -sclerotization in where the cross-linked proteins are attached.

Catecholamines phenolosidases

quinones

The Molting Process 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: the separation of the epidermal cells from

the old cuticle– Ecdysis: the casting off of the old cuticle

The Steps of Molting ProcessThe 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 ProcessThe 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 ProcessThe Steps of Molting Process

• Formation of the new epicuticle;• Absorption of the molting fluid;• Ecdysis: induced by eclosion hormone.

Eclosion Behavior and Eclosion Behavior and Its Endocrine RegulationIts 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 MoltingEndocrine Control of Molting

Control of PTTH releaseProthoracico tropic hormone

– nervous stimuli such as stretch receptors and critical size (or body mass)

– environmental stimuli such as photoperiod, temperature

Endocrine Control of Endocrine Control of MetamorphosisMetamorphosis

Two major hormones are involved in the metamorphosis– juvenile hormone– ecdysteroids

Fig. Hormonal regulation of insect metamorphosis.

Sclerotization Sclerotization Cuticular Cuticular sclerotizationsclerotization, also known as , also known as tanningtanning, stabilizes the protein matrix of the , stabilizes the protein matrix of the cuticle to make it stiffer and harder, more cuticle to make it stiffer and harder, more insoluble, and more resistant to degradation.insoluble, and more resistant to degradation.The process of sclerotization The process of sclerotization cross-links the cross-links the functional groups of cuticular proteins when functional groups of cuticular proteins when they react with they react with quinonesquinones..The amino acid The amino acid tyrosinetyrosine provides one of the provides one of the precursors (DOPA or NADA) for sclerotization.precursors (DOPA or NADA) for sclerotization.The precursors are oxidized by The precursors are oxidized by phenoloxidasesphenoloxidases to form reactive quinones. to form reactive quinones.

Hormonal Regulation of Hormonal Regulation of Sclerotization 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 Mechanism of JH ActionThe Mechanism of JH Action

Supernumerary Larvae of Supernumerary Larvae of Spodoptera lituraSpodoptera litura

The OenocytesThe Oenocytes The The oenocytesoenocytes are large polyploid cells are large polyploid cells associated with the basement membrane.associated with the basement membrane.– some oenocytes might be involved in the some oenocytes might be involved in the

production of production of cuticular lipidcuticular lipid that are deposited that are deposited in the epicuticle.in the epicuticle.

– other types of oenocytes may secrete other types of oenocytes may secrete ecdysteroid hormonesecdysteroid hormones..