Skip Navigation

CUTICLE & MOULTING

The insect exoskeleton gives

The outer exoskeleton is called the cuticle, integument or skin.

STRUCTURE

The insect cuticle consists of both living and non-living layers.

The living layer consists of a row of epithelial cells lying resting on connective tissue. These cells secrete the non-living layers of the cuticle during moulting. Other cells, interspersed in the epithelial cells, form sensory structures such as setae and hairs.

The non-living component consists of 2 main layers - the thin, outer epicuticle and a thicker, inner procuticle.

The epicuticle in turn comprises 3 layers -

The procuticle in turn consists of 2 layers

The insect cuticle is essentially chitin, a polysaccharide. It combines with proteins (esspecially in the endocuticle) into long, flexible chains.

For rigidity, the cuticle is modified by sclerotization (also called hardening or tanning). This is a complex process resulting in cross bonding  and a hard, plastic-like material. Fully tanned, cuticle is extremely hard e.g. mandibles of beetles.

Many larvae have a soft thin cuticle over most of their body . This cuticle has little or no exocuticle.

When combined with other proteins, the cuticle can be elastic and is used in the hinges of wing bases.

Abdomens can be expandable e.g. in gravid females, by having an unsclerotized cuticle, especially in the intersegmental regions, and by having the epicuticle folded over and unfolding it as needed.

TOP

MOULTING

STEPS INVOVLED IN MOULTING

  1. Epidermal cells divide mitotically
  2. Space develops between epidermis and cuticle
  3. Moulting fluid, containing proteinases and chitinase, secreted into space. This fluid will digest the endocuticle but the enzymes are inactive when first secreted.
  4. Secretion of cuticulin for epicuticle
  5. When cuticulin complete, epidermal cells begin to lay down procuticle
  6. Moulting fluid activated. Enzymes digest endocuticle.
  7. Ecdysial lines (lines of little or no exocuticle, e.g. Y shaped line on head of grasshopper and down dorsal mid line of thorax) become discontinuities in cuticle.
  8. Insect removes itself from old cuticle (epicuticle and exocuticle).

SHEDDING OF OLD CUTICLE

The insect swallows air or water and muscles force blood into the head and thorax. This increased pressure causes the cuticle to split along the ecdysial lines. The insect suspends itself on a support and helped by gravity, draws itself out, head and thorax first.

The new cuticle is soft and the insect expands it by swallowing more air or water.

Then hardening of the cuticle occurs, after which, no further expansion of the cuticle can occur. As the cuticle hardens, it also darkens.

Old cuticle is called exuviae and often includes old tracheae which is continuous with the cuticle.

DANGERS OF MOULTING

During moulting the body lacks skeletal support and is maintained by muscle tone and haemolymph pressure.

Disturbance during the hardening process can deform the cuticle and is usually fatal.

The insect is also susceptible to dessication at his time so insects usually moult in sheltered places at night when there are fewer predators and there is less risk of dessication.

Because of dangers, insects with fewer moults in their life cycle should have an advantage over those with more moults.

Most insects moult 5-10 times but mayflies 30 and silverfish up to 60.

REGENERATION

Insects have limited regenerative powers. If a leg is removed from a young juvenile, a new leg will appear at the next moult. Although fully functional, it will be much smaller than the other legs. The regenerated leg will increase in size with each subsequent moult but will never quite make the full size of the other legs.

HORMONAL CONTROL

Moulting is controlled by neurosecretory cells in the brain which in turn stimulate

Ecdysteroids stimulate the epithelial cells in the cuticle to begin the moulting process.

The outcome of a moult is determined by the level of juvenile hormone. Juvenile hormone suppresses adult characters.

Large amounts of juvenile hormone = larva -> larva

Small amounts of juvenile hormone = larva -> pupa

No juvenile hormone = pupa -> adult

If the corpora allata are removed from a small nymph and transplanted to a large nymph, the small nymph will moult into a tiny adult (since its source of JH has been removed), and the large nymph will moult into a monster nymph (since it has excess JH which will suppress adult features).

Synthetic juvenile hormones can be used for insect pest control by disrupting their development.

TOP

Copyright © University of Sydney. Last updated February, 2004. Site construction and maintenance: eResources Unit. Email us here with your comments and feedback.