Lecture 5 - The Integument, Molting, and Endocrine control

Question 1

Major Components of an insect cuticle (2)

Answer 1

a. Chitin

b. Protein

Question 2

Chitin (4)

Answer 2

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

Question 3

Proteins (2)

Eg

Answer 3

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

Question 4

Layers of the insect integument (4)

Answer 4

a. Epidermis

b. Endocuticle

c. Exocuticle

d. Epicuticle

Question 5

Epidermis (2)

Answer 5

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

Question 6

Endocuticle (2)

Answer 6

This is the innermost form of the cuticle that is reabsorbed during the process of molting

It is not sclerotized

Question 7

Exocuticle

Answer 7

This is the hardened layer that is sclerotized

Question 8

Epicuticle (4)

Answer 8

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

Question 9

3 stages of Molting

Answer 9

a. Apolysis

b. Ecdysis

c. Sclerotization

Question 10

Apolysis

Answer 10

This is the first stage where there is separation of the epidermis from the cuticle

Question 11

Ecdysis (3)

Answer 11

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

Question 11

Sclerotization (2)

Answer 11

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)

Question 12

Sclerotization of the Cuticle (3)

Answer 12

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

Question 13

Unsclerotized area

(2)

Answer 13

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

Question 14

What does ecdysis depends on? (2)

Answer 14

a. Blood Pressure

b. AIr pressure

Question 15

2 ways of undergoing ecdysis

Answer 15

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

Question 16

Process of Ecdysis (2)

Answer 16

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

Question 17

Example of Process of Ecdysis

Answer 17

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

Question 18

Physical aspect of molting

(4)

Answer 18

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

Question 19

5 Hormones for endocrine control of molting

Answer 19

a. Juvenile hormone

b. Ecdysone

c. Prothoracicotropic hormone (PTTH)

d. Eclosion hormone

e. Bursicon

Question 20

Juvenile hormone

Answer 20

This hormone determines what stage the insect will molt through

Question 21

Ecdysone

Answer 21

This is stimulates the epidermal cells to undergo the molting process

Question 22

Prothoracicotropic hormone (PTTH)

Answer 22

This stimulate the release of ecdysone

Question 23

Eclosion hormone

Answer 23

This triggers the molting behaviors

Question 24

Bursicon

Answer 24

This stimulate the sclerotization of the cuticle

Question 25

The process of endocrine control of molting and development

(3) + Description

Answer 25

1. PTTH stimulates the release of Ecdysone (E)
   - Ecdysone goes through the bloodstream and enter the epidermis
2. Ecdysone will trigger the molting
3. 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

Question 26

Juvenile (immature stage)

(4)

Answer 26

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

Question 27

Juvenile Hormone

(5)

Answer 27

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

Question 28

Ecdysone (6)

Answer 28

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

Question 29

Prothoracicotropic hormone (PTTH)

(3)

Answer 29

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

Question 30

Eclosion hormone

(4)

Answer 30

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

Question 31

Bursicon

(2)

Answer 31

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)