Lecture 5 - The Integument, Molting, and Endocrine control Flashcards
Major Components of an insect cuticle (2)
a. Chitin
b. Protein
Chitin (4)
Makes up 25-40% of the insect weight
It consists of 50-1000 polymer chain of N-acetylglucosamine
These chains are bounded by a protein to form sheets or lamellae (and lamellae are laid down at different angles every half and hour so that the cuticles are non stretchable)
Approximately 85% of chitin is reabsorbed b/t molts so they are reused
Proteins (2)
Eg
Make up about 50% of the insect weight
It is found in the joints of the insect appendages and the intersegmental membrane of the abdominal
Eg: A special protein is known as resilin which is a natural elastomer that can stretch about 30x its length
Layers of the insect integument (4)
a. Epidermis
b. Endocuticle
c. Exocuticle
d. Epicuticle
Epidermis (2)
The epidermis is the living layer that is the first layer of unicellular units
It give rises to all other layers (Endocuticle, Exocuticle, Epicuticle) and below this basement of the epidermis, it is the bloodstream
Endocuticle (2)
This is the innermost form of the cuticle that is reabsorbed during the process of molting
It is not sclerotized
Exocuticle
This is the hardened layer that is sclerotized
Epicuticle (4)
Is the waterproof layer
It is waterproof because the wax is secreted from the epidermal cells which provides waterproofing to help insects retain water in a terrestrial environment
Since insects have a small body therefore consisting of a small surface area to ratio thus terrestrial insects will be most prone to losing water to desiccation if they did not have wax around them
Additionally, at high temperature the wax could melt down and therefore causing the insect to die quickly
3 stages of Molting
a. Apolysis
b. Ecdysis
c. Sclerotization
Apolysis
This is the first stage where there is separation of the epidermis from the cuticle
Ecdysis (3)
This is the second stage where there is a casting off (escape from the residual cuticle of the previous stage)
A fully formed insect inside the old cuticle prior to the ecdysis is called a pharate
The cast cuticle is called exuvium
Sclerotization (2)
This is the last stage where there is chemical process that causes the hardening of the outer section of the cuticle
Once sclerotization is complete after the final molt, the insect is referred to as teneral adult (which is pale in color)
Sclerotization of the Cuticle (3)
This is the process that chemically hardens the exocuticle through crosslinking of the cuticular proteins w/ oxidized phenols
The process can take up to 6 hours and occurs from the outside in
It requires the control of a hormone known as bursicon
Unsclerotized area
(2)
The unsclerotized areas consists of the intersegmental membranes and the joints of the appendages
In the holometabolous larvae, the body remains unsclerotized expect for the head capsule, mandibles, and true legs
What does ecdysis depends on? (2)
a. Blood Pressure
b. AIr pressure
2 ways of undergoing ecdysis
For air pressure, insects would need to contract the abdomen to force blood into the head and the thorax
This can also be assisted by swallowing air into their foregut
- However, if the foregut of a cockroach nymph is punctured therefore it unsuccessfully undergo ecdysis
Process of Ecdysis (2)
Ecdysis is often aided by gravity meaning that the insect will hang upside down by securing the claws of the hind tarsi
If there was rupture of the ecdysial cleavage line on the head capsule this will allow the insect to “slide out” of the exuvium
Example of Process of Ecdysis
Muscid flies have spinalized structure on the head known as ptilinum which is an inflatable bladder that breaks open the puparium to allow the adult fly to emerge
Physical aspect of molting
(4)
After ecdysis, the insects often have to again swallow air or contract specific muscles to facilitate expansion of the new cuticle prior to sclerotization
This means that blood will pump into the wings to provide them with full expansion
In some species, muscles of the abdominal wall must remain in a constant state of contraction through sclerotization
The need to remain motionless during sclerotization combined with the weak state of the cuticle makes the insect vulnerable to predators
5 Hormones for endocrine control of molting
a. Juvenile hormone
b. Ecdysone
c. Prothoracicotropic hormone (PTTH)
d. Eclosion hormone
e. Bursicon
Juvenile hormone
This hormone determines what stage the insect will molt through
Ecdysone
This is stimulates the epidermal cells to undergo the molting process
Prothoracicotropic hormone (PTTH)
This stimulate the release of ecdysone
Eclosion hormone
This triggers the molting behaviors
Bursicon
This stimulate the sclerotization of the cuticle
The process of endocrine control of molting and development
(3) + Description
- PTTH stimulates the release of Ecdysone (E)
- Ecdysone goes through the bloodstream and enter the epidermis - Ecdysone will trigger the molting
- JH titre will determine the outcome of the molt
-JH comes from the base of the brain and the travel through the insect bloodstream to the epidermal cell
Juvenile (immature stage)
(4)
The ecdysone is released from the PPTH and the amount of JH in the bloodstream will tell the epidermal cells what to produce
If there is a lot of JH in the bloodstream therefore it will produce a larva cuticle
If there is less JH that means the epidermal cells will produce the pupa
In the absence of JH in the bloodstream, epidermal cells will produce the adult cuticles
Juvenile Hormone
(5)
Juvenile Hormones are made up of C16- C18 Fatty acids methyl esters
There are 3 forms of JH (JH-III) which is most common outside of the Lepidoptera
These hormones are synthesized by the corpora allata glands which act directly on the epidermis and the internal tissues
It is release during the larval stages by gradually decline in levels as it is thought to prevent from overshooting the pupa stage and producing adult structure prematurely
This hormone will disappear during the final instar expect for a brief peak just prior to pupation and reappears in the adult stage to regulate reproduction
Ecdysone (6)
This is a type of molting hormone that stimulate apolysis and ecdysis
It is a steroid hormone that is synthesized from cholesterol or plant sterols and obtained from the diet
It is synthesized by the prothoracic gland (PT), which is a loose collection of cells in the prothorax
The PT gland secrete 3-dehydro-ecdysone that is converted to alpha ecdysone in the hemolymph and then oxidized to beta ecdysone in target tissues
Prothoracicotropic hormone (PTTH)
(3)
This is a polypeptide that has a molecular weight of 5000 daltons
Its function is to stimulate the prothoracic gland to synthesize and secrete ecdysone
It is produced by a specific groups of medical neurosecretory cells in the protocerebrum (brain) and transported by the nerve axons to the corpora cardiaca glands where it is stored
Eclosion hormone
(4)
This is a type of protein that has a molecular weight of 7500 mW
It is produced in the brain and abdominal ganglia
This type of hormone triggers the molting behavior, which includes stereotype sequence of behaviors involved in adult emergence from the pupa
This type of hormone is used to stimulate the release of bursicon and is released by the circadian clock
Bursicon
(2)
This is a type of protein that has a molecular weight of 40K mw from the perivisceral organ
It stimulate sclerotization of the new cuticle after each molt and increases the permeability of the epidermal cells to release dopamine (stimulate the conversion of tyrosine to dopa)
