Cuticle Growth and Metamorphosis

Question 1

What layer of cells secretes the cuticle?

Answer 1

Hypodermis

Question 2

What is the purpose of the cuticle?

Answer 2

To protect the insect from the environment, contain the body and serve as a skeletal system for the muscles to act upon

Question 3

Define apodemes

Answer 3

endoskeletal rods that form internal strengthening for the exoskeleton or serve as attachments for muscles - they allow muscles to move structures that are remote to them

Question 4

What are the two layers of the cuticle?

Answer 4

The outer epicuticle and the inner procuticle

Question 5

What is the epicuticle composed of?

Answer 5

An outer layer of wax and an inner layer of cuticulin

Question 6

What is cuticulin made of and what is its function?

Answer 6

Lipoprotein and to bind the outer wax to the underlying procuticle

Question 7

What molecules make the cuticle greasy?

Answer 7

Straight-chain paraffins and esters of straight-chain FA and primary alcohols which are mixed with unsaturated molecules and reactive chemicals

Question 8

What is the purpose of the monolayer of wax?

Answer 8

To protect the insect from the environment as it helps slow the rate of evaporative water and prevents water from entering the insect

Question 9

What is the structure of the base wax layer?

Answer 9

The hydrophobic groups point outward to the wax and the hydrophilic groups point inward to the cuticulin

Question 10

Why is the epicuticle important?

Answer 10

Without it, insects would rapidly dehydrate in the dry air due to their large surface-area-to-volume ratio that results from their small size

Question 11

How did insects invade freshwater habitats?

Answer 11

The wax layer isolates the internal fluids of insects from the osmotically different external medium

Question 12

What is the function of the procuticle?

Answer 12

To provide mechanical protection and skeletal strength

Question 13

What is the procuticle made of?

Answer 13

Protein and chitin

Question 14

What are the 2 regions in the procuticle?

Answer 14

The outer exocuticle and the inner endocuticle

Question 15

Where does protein ‘tanning’ occur?

Answer 15

In the outer exocuticle

Question 16

What is protein tanning and what is its function?

Answer 16

When protein chains become cross-linked by quinones so they form gigantic molecules which restricts molecular movement and makes the exocuticle horny and thus protective

Question 17

Describe the structure of the endocuticle

Answer 17

The polypeptide chains have few links in the endocuticle so they are free to move or slide over each other and thus it is relatively soft and flexible

Question 18

What causes ‘hardness’ in insects bodies and how does it differ from crustaceans?

Answer 18

Crustaceans incorporate calcium salts into their cuticle - very few insects do this, instead they rely entirely on tanning

Question 19

Describe the orientation of the microfibrils in the procuticle and their effect

Answer 19

The microfibrils are all orientated in the same direction in a layer, and adjacent layers make an angle of approx. 60 degrees to each other. This produces different refractive indices which make the lamellae visible

Question 20

Describe the structure and function of the protein Resilin

Answer 20

Resilin has elastic properties and all its component polypeptide chains are joined end-to-end so there are a minimum of free end groups. There is also fewer tyrosine molecules. The end groups react with quinones and join together and the tyrosine residues cross-link the chains. This results in a structure of long, freely-rotating polypeptide chains held together at infrequent intervals by stable linkages. This allows resilin to recover from plastic deformation. It is very common in the hinge joints of wings where it aids in the conservation of energy during flight.

Question 21

Explain why the pore canals in the procuticle follow a cork-screw path

Answer 21

Because of their passing through successive lamellae of microfibrils which are orientated in different directions

Question 22

What is ecdysis?

Answer 22

The process of casting off the outer cuticle

Question 23

Where is wax secreted from?

Answer 23

The ends of the cytoplasmic filaments immediately prior to and during ecdysis

Question 24

What is eclosion?

Answer 24

Moulting

Question 25

What is the function of the cytoplasmic filaments?

Answer 25

To secrete wax and to convey the tanning chemicals or their precursors to the outer regions of the procuticle when the exocuticle is being formed

Question 26

Which amino acid are the tanning agents derived from?

Answer 26

Tyrosine

Question 27

Describe the process of sclerotization

Answer 27

Tyrosine accumulates in the blood before eclosion and moves into the cuticle at the time of hardening. Enzymatic reactions result in the formation of diphenols from tyrosine and then subsequently the production of quinones. The quinones react with free amine groups on the proteins which causes extensive cross-bonding. The result is a horny protein called sclerotin. The protein that remains unsclerotized is called arthropodin.

Question 28

Why are newly moulted insects light coloured and why do they darken as sclerotisation proceeds?

Answer 28

Because diphenols polymerise to form pigments like melanin which become incorporated into the exocuticle by cross reaction with the protein and therefore they darken or blacken it.

Question 29

What are the methods of colouration in insects?

Answer 29

Interference effects, accumulations of pigments and the production of own colours independently

Question 30

In what ways can insects change colour?

Answer 30

Either through long-term alterations in pigment metabolism or short term migration of pigments in mesodermal chromatocytes

Question 31

What are pheromones?

Answer 31

Chemicals used by individual insects to influence the behaviour or metabolism of other individuals in the same species

Question 32

What is the function of dermal glands?

Answer 32

To produce chemicals related to those secreted by the general hypodermic. Other dermal glands associated with the mouth are often digestive but may have other functions such as the production of silk or pheromones or for defensive/offensive purposes.

Question 33

What are ecdyses?

Answer 33

A series of moults

Question 34

What are instars and stadia?

Answer 34

Instars are the stages/forms of an insect between ecdyses and stadia are the intervals between ecdyses

Question 35

What is an exuviae?

Answer 35

An old cuticle that is shed at each ecdysis

Question 36

What are the three types of metamorphosis development?

Answer 36

Hemimetabolous, holometabolous and ametabolous

Question 37

Describe the life history Hemimetabolous

Answer 37

This occurs in insects known as exopterygotes. They go through series of juvenile instars and then undergo one metamorphosis to the adult. The juveniles resemble the adult except they lack full formed wings and reproductive systems. Their metamorphosis is relatively slight.

Question 38

Describe the life history Holometabolous

Answer 38

This occurs in pterygotes. The juvenile stages do not resemble the adults at all (caterpillars for example). The juveniles go through two metamorphoses: first to a pupal instar and second to the adult. The wing buds develop internally hence these insects have complete metamorphosis and are termed endopterygotes.

Question 39

Describe the life history Ametabolous

Answer 39

This occurs only in apterygote insects. These have wingless adults so the nymphs closely resemble them. Metamorphosis is so slight as to be considered lacking. Apterygotes continue to moult at intervals once they are adults unlike pterygotes.

Question 40

Describe the process that occurs during the moult stage

Answer 40

The hypodermis forms a thin layer under the cuticle. The hypodermis cells grow to become cuboidal. The hypodermis separates and pulls away from the cuticle to form the new instar. The cells divide mitotically. The refashioned hypodermis becomes highly folded to accommodate the form of the new instar within the old cuticle. The cuticulin layer is secreted and this is followed by layers of procuticle. A moulting fluid or gel is secreted between the old and new cuticle. The enzymes in this gel digest the old endocuticle and recycle the nutrients to contribute to the formation of the new cuticle. The epicuticle and endocuticle are not enzymatically eroded and are left intact. Once the old endocuticle is dissolved, the moulting fluid is absorbed and the actual moult occurs.

Question 41

What is meant by the term apolysis?

Answer 41

Separation from the cuticle

Question 42

What is meant by the term eclosion?

Answer 42

Moulting

Question 43

How does moulting start?

Answer 43

Usually when the insect swallows air (or water if aquatic) and swells until the old cuticle ruptures along preformed lines of weakness. These are located along the dorsal median line of the thorax and the Y-shaped epicranial suture of the head. Insects frequently hang upside down so gravity helps pull them out the exuviae. In soft bodied larvae, the exuviae is passed backward by peristaltic waves of contraction.

Question 44

What is the importance of pharate stages?

Answer 44

They allow the new form to develop within the old form and this reduces the period of time in which insects are vulnerable.

Question 45

What is an isometric growth pattern?

Answer 45

The increase in linear dimensions of sclerotised parts show at ecdyses a constant proportional increase through the instars. They produce linear graphs when the logarithm of the dimension is plotted against the instar number.

Question 46

What is an allometric growth pattern?

Answer 46

The insect or part of an insect develops at an increasing proportional rate through the instars. When these measurements are plotted, they still produce upward curves so they have to be plotted on a log-log graph to produce a straight line.