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.