Ecdysis
What is ecdysis?
What occurs during pre-ecdysis?
What hormones control ecdysis in insects?
Ecdysis, the shedding of the outer body layer in some animals for growth or change in shape and body form. The term is derived from Greek ekdýein, which means “to strip off.” Ecdysis can refer to the process whereby a member of the animal group Ecdysozoa discards its exoskeleton, which is an external covering secreted by the animal’s epidermis. The term can also refer to the periodic shedding of the skin in lizards, snakes, and related reptiles. This article discusses ecdysis in arthropods, members of Ecdysozoa in which the process has been most comprehensively studied.
Process
Although the term is often used synonymously with molting in arthropods, particularly insects, ecdysis refers only to the final step of molting. Ecdysis, as well as the events preceding (pre-ecdysis) and following (post-ecdysis) it, is characterized by distinct molecular, chemical, and behavioral changes in the animal that are regulated by specific hormones and neuropeptides.
The exoskeleton of arthropods is a nonliving, organic, jointed external body covering made from materials secreted by the cells of the animal’s epidermis (outermost portion of the skin). It is composed of an outer, water-resistant layer of wax and polymerized lipid overlying an inner cuticle. The cuticle (a term often used interchangeably with exoskeleton) comprises an inner, mostly chitinous procuticle and an outer, protein-containing epicuticle. The procuticle is further divided into an inner, flexible endocuticle and an outer, stiff exocuticle. The epicuticle has two layers as well: the inner epicuticle and the outer epicuticle (also called the cuticulin layer). During ecdysis much of the endocuticle is digested and reabsorbed, whereas the undigested exocuticle and epicuticle are discarded as the sloughed-off exoskeleton.
Pre-ecdysis
During pre-ecdysis the old cuticle pulls away from the epidermis in a process called apolysis. As this happens, the epidermal cells and dermal glands release molting fluid into the space under the old cuticle. This fluid contains enzymes such as chitinase and β-N-acetylglucosaminidase, which break down the chitin in the old cuticle, and peptidases, which break down cuticle proteins into smaller parts that can be reused. The endocuticle (the basal layer of the old cuticle) is digested by this fluid, but the tougher exocuticle and epicuticle are not. These outer layers stay intact and are shed later, during ecdysis.
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The molting fluid becomes active only after a new epicuticle forms. Epidermal cells make this new epicuticle during apolysis, starting with the cuticulin layer. This epicuticle protects the epidermal cells from the enzymes in the molting fluid. After the new epicuticle is in place, the insect forms a new exocuticle and then a new endocuticle. In some insects the new endocuticle is completed only after ecdysis is completed; indeed, it may take several weeks to finish. Most of the materials used to build the new cuticle come from the digested parts of the old one.
During pre-ecdysis an arthropod may contract its body parts (head, thorax, and abdomen) to facilitate the separation of its old cuticle. To prepare for ecdysis, it may cease feeding and become virtually inactive.
Ecdysis
Once the new cuticle forms and the remains of the old cuticle (called the exuvia, which comprises the old exocuticle and epicuticle) dry up, involuntary muscle contractions in the arthropod push its circulatory fluid (hemolymph) toward the front of the body. The animal may also suck air or water to increase hemolymph volume and internal pressure. This causes the exuvia to split along preformed rupture lines or points of weakness. In insects the split occurs dorsally, along the middle of the back of the thorax, whereas in crustaceans such as lobsters the split develops between the carapace and the abdomen. The animal breaks free from the old cuticle through physical maneuvering, such as wriggling and pulsating motions.
Post-ecdysis
- Apolysis: The separation of the old cuticle from the underlying epidermis; occurs during pre-ecdysis
- Ecdysis: The specific act of shedding the old cuticle at the end of a molt
- Instar: The developmental stage between two successive molts
- Intermolt period: The time between two molts, when the insect feeds and grows
- Larva/nymph: An immature stage of an insect that undergoes repeated molts
- Metamorphosis: The process of major body reorganization across molts
- Molting: The periodic shedding and replacement of the exoskeleton that allows an insect to grow and metamorphose
- Pharate stage: The developmental stage when an insect is actively secreting a new exoskeleton while the old exoskeleton is still in place
- Sclerotization: The chemical hardening and darkening of the new cuticle after ecdysis
The newly formed cuticle of an arthropod is soft and pale. An animal at this stage is said to be in a teneral condition. During this period it distends the body and expands the cuticle, often by taking in air or water and increasing hemolymph pressure. This is followed by cuticle hardening and sclerotization, when quinones in the cuticle form cross-links with cuticular proteins and chitin, stabilizing and stiffening the cuticle and sometimes imparting color. In some insects, such as beetles, cuticular hardening can take several weeks and may involve the deposition of additional layers of proteins and chitin.
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Triggers
In arthropods the rigid exoskeleton limits body growth, so it must be shed and replaced periodically. But what tells an arthropod’s body that it is time to molt? Two main ideas have been proposed. One suggests that stretch receptors in the abdomen detect when the body becomes too distended, which initiates molting. The other suggests that as arthropods such as insects grow, the hardened tracheal system fails to supply enough oxygen. When oxygen levels in the tissues fall below a certain threshold, this hypoxia may trigger molting and ecdysis. Other factors that have been suggested to trigger molting include the animal’s nutrition and changes in day length, which affect the animal’s circadian rhythm (the body’s natural 24-hour clock) and hormones that control molting and ecdysis.
Secreted by a pair of endocrine glands called the corpora allata, juvenile hormone is a sesquiterpenoid compound named for its role in maintaining juvenile characteristics in an insect during molting. When present at high concentrations at the beginning of each molt, this hormone causes an insect to retain its immature (larval or nymphal) features. In contrast, a decline in juvenile hormone levels to undetectable concentrations in the hemolymph prior to or during the final larval or nymphal stage triggers metamorphosis and ultimately leads to the emergence of the adult insect.
Control
The skin of reptiles does not grow along with the animal and therefore must be shed periodically. This process, known as ecdysis, usually results in the skin being shed either as a single piece, as in snakes, or in patches, as in lizards. In reptiles, as in insects, ecdysis is hormonally controlled and influenced by several factors, including the age and physical condition of the animal, as well as environmental conditions, such as temperature and humidity. One key difference between insects and reptiles is that ecdysis, as the final step of molting, occurs only during the juvenile stages of most insects, whereas reptiles continue to shed their skin throughout their lives.
Three main hormone groups control different steps in the development of arthropods: ecdysteroids, neuropeptides, and sesquiterpenoids. Among these, ecdysteroids and certain neuropeptides regulate the steps involved in ecdysis. Although different arthropod classes may share the same hormones regulating molting and ecdysis, the substances’ modes of action and ultimate effects may vary. In insects—the arthropod class in which ecdysis is best studied—molting and ecdysis are regulated by prothoracicotropic hormone (PTTH), 20-hydroxyecdysone (20E), ecdysis-triggering hormone (ETH), eclosion hormone (EH), crustacean cardioactive peptide (CCAP), and bursicon.
PTTH is a small neuropeptide stored in and released from the corpora cardiaca–corpora allata complex, which comprises neurohemal organs that store brain hormones in insects. At the beginning of ecdysis, PTTH enters the hemolymph and stimulates the prothoracic glands (paired thoracic glands associated with tracheal tubes) to produce the prohormone ⍺-ecdysone. This ecdysteroid is converted in peripheral tissues to its active form, the molting hormone 20E (β-ecdysone). Under the influence of 20E, the epidermis begins producing a new cuticle, and the new epicuticle is deposited when 20E levels are the highest.
The hormone 20E induces the production of ETH in Inka cells located on epitracheal glands along the tracheal surface. Under the influence of the neuropeptides corazonine and EH, and as 20E levels decline, ETH is released from the Inka cells. The release of this hormone and related peptides, including pre-ecdysis-triggering hormone (PETH), into the hemolymph initiates ecdysis-specific behaviors through the action of the peptide CCAP. Finally, the neuropeptide bursicon controls the hardening and tanning of the new cuticle in the freshly molted insect.