Ecdysis & Metamorphosis

Question 1

Ecdysis

Answer 1

act of molting or shedding an outer cuticular layer

Question 2

Functions of ecdysis

Answer 2

1. allow insect to grow
2. replace mouthparts
3. replace/extend lining of trachea
4. restore waterproofing ability
5. shed pathogens

Question 3

disadvantages of ecdysis

Answer 3

1. process is slow - time away from feeding
2. dangerous - leaves the insect vulnerable to predation during the process and while cuticle is hardening
3. Energetically expensive - cuticle = made mostly from protein and sclerotised cuticle cannot be re-used

Question 4

The steps of ecdysis

Answer 4

1. Apolysis
2. Epicuticle formation
3. New procuticle deposition
4. Ecdysis
5. procuticle expansion
6. hardening and darkening
7. endocuticle deposition

Question 5

Insects are termed:

Answer 5

• Pharate
• Teneral

Question 6

Pharate

Answer 6

• when actively constructing new cuticle
• Adult form still in pupa

Question 7

Teneral

Answer 7

• Prior to hardening and darkening of cuticle, when soft and colourless

Question 8

1. Apolysis

Answer 8

• epidermal cells separate from inner surface of endocuticle
• inactive moulting fluid containing chitinases, proteases, lipases secreted into space

Question 9

2. Epicuticle formation

Answer 9

• new outer epicuticle laid down below moulting fluid + above epidermis
• new inner epicuticle deposited just inside outer epicuticle
• both layers chemically stabilised
• new outer epicuticle protects epidermal cells and new layers

Question 10

3. Activation of moulting fluid & new procuticle deposition

Answer 10

Chitin microfilaments formed beneath inner epicuticle

moulting fluid begins to digest original endocuticle

new procuticle = protected by new outer epicuticle

Question 11

3. New procuticle deposition

Answer 11

• digestion is completed + only exocuticle + epicuticle remains

Question 12

4. Ecdysis

Answer 12

• old cuticle splits along ecdysial lines
• exuvium formed from epicuticle + exocuticle

Question 13

5. Procuticle expansion (starts at part of ecdysis)

Answer 13

• muscular contractions develop pressures in range 1-30 kPa
• some insects support this by swallowing air (e.g. locusts) or water (e.g. dragonfly larvae)
• pressure splits cuticle along ecdysial line
• this inflates body + smoothes out surface and stretches procuticle

Question 14

6. hardening and darkening

Answer 14

• stabilisation of new procuticle

Question 15

7. Endocuticle deposition

Answer 15

• protein and chitin laid down

Question 16

Hormonal control of ecdysis
Hormonal steps

Answer 16

1. environmental cues trigger neurosecretory cells in brain to secrete Prothoracicotropic hormone (PTTH)
2. PTTH passes to corpora cardiaca for storage and is later releases into circulatory system
3. PTTH stimulates prothoracic glands to secrete Ecdysone (prohormone) into haemolymph + is transported to epidermis
4. Ecdysone converted to active form 20-Hydroxyecdysone (20-HED) by haem-containing oxidase in mitochondria and microsomes of peripheral tissues
5. 20-HED regulates gene controlling cuticle production; epidermal cells synthesise enzymes that digest & recycle components of cuticle - apolysis
6. ecdysis triggering hormone (ETH) from Inka cells initiates pre-ecdysis behaviour that loosen muscle attachments from old cuticle
7. ETH acts on brain to cause release of Eclosion hormone (EH) from brain neurosecretory cells
8. EH in haemolymph causes plasticisation of cuticle
9. release of bursicon, produced in neurosecretory cells in brain + stored in corpora cardiaca, is part of eclosion behaviour but subject to sensory control
10. Bursicon in haemolymph first plasticises cuticle then causes cuticle sclerotisation and deposition of endocuticle

Question 17

Metamorphosis

Answer 17

• transformation between larval + adult stages
• 10 holometabolic orders represent ~86% of all insect species
• life cycle compartmentalisation - larval form and function optimized for growth and feeding
• no compromise to adaptations of adult for dispersal + reproduction
• can exploit different environmental resources and occupy different habitats

Question 18

Hormonal control of metamorphosis
Role of Juvenile Hormone (JH)

Answer 18

• JH secreted from corpora allata prior to each moult
• inhibits genes that cause development into adult
• BUT at last larval instar corpora allata reduce activity
• at same time, there is increase in ability to degrade JH
• cause JH levels to drop below threshold & trigger release of PTTH
• causes release of ecdysone -> stimulated behavioural changes + moult to pupal development

Question 19

Role of JH in adult insects

Answer 19

• at onset of sexual maturity, brain neurosecretory cells release hormone
• reactivates corpora allata + renewed production of JH stimulated
• JH stimulates yolk production (females) or accessory glands to produce proteins needed for seminal fluid and spermatophore case (male)
• adult remain sexually sterile in absence of normal JH production

Question 20

Maria Sibylla Merian

Answer 20

• one of the earliest European naturalists to observe insects directly
• documented evidence on the process of metamorphosis + plant hosts of 186 European insect species
• until her work, it had been thought that insects were “born of mud” by spontaneous generation

Question 21

Sir Vincent Brian Wigglesworth

Answer 21

• hormonal control of insect metamorphosis was first shown by Wigglesworth (1934)
• Eminent insect physiologist
• respected for research into role of hormones in insect growth, metamorphosis and reproduction
• discovered PTTH

Question 22

Rhodnius prolixus

Answer 22

• blood-sucking hemipteran w/ 5 instars prior to striking metamorphosis
• 1st instar nymph decapitated + fused to moulting 5th instar nymph
• 1st instar developed cuticle, body structure and genitalia of adult
• showed that hormones in haemolymph responsible for induction of metamorphosis

removal of corpota allata from the normal 3rd instar nymph -> small premature adult

implant 4th instar corpora allata to normal 5th instar nymph -> 6th instar nymph

Question 23

Sir Vincent Brian Wigglesworth - corpora allata

Answer 23

• demonstrated that corpora allata produced hormone preventing metamorphosis
• remove corpora allata + 3rd instar nymph, transforms into a small adult at subsequent moult
• conversely, if corpora allata from 4th instar nymph implanted into 5th instar nymph, moult into very large 6th instar nymphs rather than into adults