Insect Physiology - Molting

Molting

As an insect grows it needs to replace the rigid exoskeleton regularly (McGavin 2001; Triplehorn & Johnson, 2005,). Molting may occur up to three or four times or, in some insects, fifty times or more during its life (McGavin, 2001). A complex process controlled by hormones, it includes the cuticle of the body wall, the cuticular lining of the tracheae, foregut, hindgut and endoskeletal structures (McGavin 2001; Triplehorn & Johnson, 2005).

The stages of molting:

1. Apolysis—molting hormones are released into the haemolymph and the old cuticle separates from the underlying epidermal cells. The epidermis increases in size due to mitosis and then the new cuticle is produced. Enzymes secreted by the epidermal cells digest the old endocuticle, not affecting the old sclerotised exocuticle.
2. Ecdysis—this begins with the splitting of the old cuticle, usually starting in the midline of the thorax’s dorsal side. The rupturing force is mostly from haemolymph pressure that has been forced into thorax by abdominal muscle contractions caused by the insect swallowing air or water. After this the insect wriggles out of the old cuticle.
3. Sclerotisation—after emergence the new cuticle is soft and this a particularly vulnerable time for the insect as its hard protective coating is missing. After an hour or two the exocuticle hardens and darkens. The wings expand by the force of haemolymph into the wing veins (McGavin, 2001; Triplehorn & Johnson, 2005).