One of the general characteristics that defines the phylum Arthropoda (which includes insects, spiders, and crustaceans) is an external skeleton, also called an exoskeleton . The arthropod exoskeleton completely covers the outside of the body and the muscles inside adhere to it. Exoskeletons are hard and protect the body. Because the exoskeleton is hard and rigid, an arthropod cannot grow unless it sheds its old exoskeleton and secretes a new one. This process is called molting.
Arthropod growth is limited to molting, so growth happens in steps rather than continuously. The stages between moltings are called instars. The extent of an individual's growth between molts and the length of time between molts are related to the temperature as well as to the amount of food and water an individual gets.
Warmer temperatures and more food and water can shorten the instar length and make the individual bigger. Other cues, such as the length of the day, are used to determine the timing of molting. Most insects have a specific end point to their growth, and after their final molt they are sexually mature adults. Most insects with wings acquire wings only in their adult stage. For example, except for the absence of wings, baby crickets are born looking exactly like adult crickets but are tiny in size. Crickets undergo several molts and live through several instars as they grow bigger. In their final molt, they become sexually mature and gain wings for flight.
Beneath the exoskeleton is an underlying cell layer called the epidermis, which secretes the exoskeleton, also called the cuticle. The exoskeleton is noncellular and made of chitin and proteins, which give the exoskeleton its rigid and protective properties. The exoskeleton and the epidermis together form the integument of an arthropod.
The molting process is a series of steps. It is controlled by the hormone ecdysone . Ecdysone is secreted from glands behind the brain. Once it is released the molting process begins. The arthropod builds a new exoskeleton underneath the old one. The epidermis pulls away from the existing exoskeleton. This creates a space between the epidermis and the exoskeleton. This space is filled with a gel that promotes shedding of the old exoskeleton.
Under this gel, the epidermis secretes a new cuticle. This requires a lot of energy. The new cuticle is secreted in various layers, and many biochemical processes change the newly excreted cuticle from cellular secretions into the insoluble form of the new exoskeleton. At this point, the new exoskeleton is still soft and pliable.
The gel that is between the new and old cuticles contains digestive enzymes . These enzymes start to break down the old exoskeleton once the new cuticle has been made insoluble and can resist being damaged by these enzymes. These digestive enzymes dissolve the inside of the old exoskeleton and the products are reabsorbed by the individual and used in making the new cuticle. This recycling of material reduces the amount of energy needed for molting. Only the inside of the old exoskeleton can be reused. The outside of the exoskeleton is shed in a process called ecdysis .
Ecdysis consists of splitting the exoskeleton, usually along the back of the arthropod, and crawling out of the old exoskeleton. Old exoskeletons of insects can be found in nature. They look just like the insect did but are hollow inside.
When an individual first emerges from the old exoskeleton following ecdysis, it is very vulnerable because the new exoskeleton is quite soft. Newly emerged individuals are wrinkly and whitish. The swallowing of air by the individual expands the cuticle. This pulls out the wrinkles and makes the individual become larger during molting. After expansion of the cuticle, another biochemical process takes place that hardens and darkens the exoskeleton. This biochemical process is a reaction to oxygen in the air. It can take several hours for an individual to undergo expansion and hardening.
Even though molting happens only occasionally, most arthropods continue to add layers to the inside of the exoskeleton all the time. Some insects do this every twenty-four hours and form growth rings similar to those of trees.
see also Skeletons.
Laura A. Higgins
Campbell, Neil A., Jane B. Reece, and Lawrence G. Mitchell. Biology, 5th ed. Menlo Park, CA: Addison Wesley Longman, 1999.
Evans, Howard E. Insect Biology. Menlo Park, CA: Addison Wesley Longman, 1984.
molt / mōlt/ (Brit. moult) • v. [intr.] (of an animal) shed old feathers, hair, or skin, or an old shell, to make way for a new growth: the adult birds were already molting into their winter shades of gray | [tr.] the snake molts its skin. ∎ (of hair or feathers) fall out to make way for new growth: the last of his juvenile plumage had molted.• n. a loss of plumage, skin, or hair, esp. as a regular feature of an animal's life cycle.