Metamorphosis is a temporal process of development involving the interaction of hormones triggered at particular stages of growth. Metamorphosis of greater or lesser degree is found in most organisms where there is a developmental transition over time in body form between the egg and adult. Insects undergo a particularly noticeable metamorphosis involving distinct stages of development that often occur in different habitats or utilize different food resources. Developmental transitions occur between different juvenile stages and are terminated when the adult stage is reached.
With a relatively inflexible outer integument or exoskeleton, insect growth is only possible through periodic shedding or molting of cuticle between each instar followed by a rapid expansion of a soft, new cuticle until it hardens. This expansion facilitates further growth during each developmental stage or instar. At emergence from the egg, most insects are structurally different from their adult counterparts. This difference may be slight or pronounced. Juvenile stages are usually characterized by feeding, growth, and development of external and internal structures such as wings and reproductive organs that are not fully developed until the final molt into the adult. In many insects metamorphosis is confined to a series of instars during a single season or year for those species with an annual life cycle.
Metamorphosis between instars may be confined to a matter of days in species with rapid life cycles (such as insects feeding on ephemeral fungal fruiting bodies) or spread out over many years in long-lived species with an extended juvenile growth period. There can also be considerable variation within a single species. Juvenile development in the woodboring ghost moth Aenetus virescens, for example, may vary from as little as 9 months between egg and adult to as long as 4 years within a single population. The number of instars is also variable between species, and sometimes within species. Many insects, especially those that develop through their life cycle each season, have relatively few instars, with four to five stages being common.
There are several distinct patterns of insect metamorphosis with contrasting developmental patterns that contribute to the structural diversity of a group of organisms that may otherwise have had a more homogenous appearance. Insects that never evolved wings, where juveniles resemble adults and adults also continue to molt, are called ametabolous (or aptergota = without wings). This development pattern occurs in five primitive insect orders that include springtails and the common silverfish. Most insect orders are hemimetabolous (also called exop- terygota for their externally visible wing development) with a dimorphic life history divided into a series of nymphs that molt through several instars and adults that do not molt. In these insects, wings develop gradually as external wing pads in the older juveniles, and only the adult has fully functional wings (with the unique exception of mayflies, where the final juvenile instar has fully developed wings). Hemimetabolous insects include grasshoppers and their close relatives, such as stoneflies, and true bugs.
Insects with the most distinctive stages of metamorphosis are holometabolous, where there are three main stages of development: larva, a pupa, and adult. In this developmental sequence the larva is structurally and behaviorally different from the adults. Compound eyes are usually absent in the larval and pupal stages (where the eyes are otherwise absent limited to several single lenses). The holometabola are also referred to as endop- terygotes, because the wings and other features develop internally until the pupal stage, when they become everted and visible externally, although they are not fully expanded to the adult structure. Evolution of holometabolous development is widely regarded as a key evolutionary innovation contributing to the comparatively diverse speciation within 11 orders that compose about 75% of all insect species.
Transitions between different stages during metamorphosis involve a sequential web of interacting genetic and biochemical factors and the balancing effects of hormones that effect molting with those that maintain the juvenile stage (juvenile hormone) by preventing the epidermis from depositing adult cuticle in response to the presence of molting hormone. The specific developmental triggers for the secretion of brain hormone are generally not understood. In some cases, metamorphosis is triggered by internal indicators of body growth, such as stretch receptors that indicate a particular level of body expansion has been reached, or the attainment of a critical body weight.
Molting proceeds through three major complementary processes:
- Old cuticle is separated from the underlying epidermis, and 80% to 90% of this cuticle is reabsorbed through the action of enzymes while a new cuticle is secreted by the epidermis. This process is initiated by release of prothoracicotropic hormone in the corpora cardiaca (or corpora allata in Lepidoptera) of the brain, and this hormone stimulates production of ecdysome in the prothoracic gland. In turn, ecdysome results in the production of the molecule 20-hydroxyecdysome, which regulates the genes that produce new cuticle.
- Molting is controlled by release of a molting trigger from the epitracheal glands. This hormone stimulates the brain to produce molting hormone and behavioral changes in the insect as a prelude to molting. The molting hormone also produces a positive feedback loop between the brain and epi- tracheal glands resulting in a massive release of molting hormone that in turn results in the release of crustacean cardioactive hormone by the ventral ganglia. This regulates the transition from premolting behavior, such as body movements that help separate the overlying cuticle, and molting behavior, which comprises waves of body contractions that continue until the molt is complete.
- Following the molt, the body undergoes expansion and hardening of the cuticle. Wing expansion in the new adult is facilitated through abdominal contractions forcing blood into the wings, and this activity also stimulates release of bursicon, which further increases the flexibility of wing cuticle and then initiates hardening of the cuticle.
In holometabolous insects, brain and juvenile hormones are both produced during the immature stages until the last immature instar, when juvenile hormone production is either terminated or decreases below a threshold level where metamorphosis into the pupal and adult stage occurs as adult characteristics are no longer inhibited. The adults of most insects do not molt, because the prothoracic glands degenerate either before or after adult emergence, and there is no longer the secretion of molting hormone. In hemimetabolous insects, the transformation between the immature and adult stages is more gradual. Brain hormone includes a number of steroids that act on genes through a receptor-mediated process that determines which genes are activated at a given time and consequently which enzymes and structural proteins are synthesized.
The transition between instars usually takes place over a few minutes as the old cuticle breaks open along the dorsal midline of the thorax and the insect extrudes is body through this opening, with the appendages such as legs and antennae along with the tracheal tubes being the last to pull away. This process is preceded by a separation of epidermal cells from the old cuticle and the secretion of a new cuticle as well as enzymes that digest 80% to 90% of the old cuticle. The insect pumps air into the body, which expands its volume, resulting in the breakage of old cuticle along the dorsal line. The body is extruded out of this break, and the insect pulls itself away from the old cuticle. This is followed by a period of resting as the new cuticle is hardened. It is during this process that the size of the body is expanded so the new instar is larger than the previous instar. The process of metamorphosis begins with hatching from the egg when all insects are small, sexually immature, and lack wings. As the juvenile and adult stages have often diverged evolutionarily in form and function, the juvenile is more efficient at feeding and growth, while the adult is more specialized with respect to dispersal and reproduction.
The size at which molting occurs is not absolute and depends on the size of the insect at the beginning of the instar. In some species and where food is insufficient, the molt may result in a smaller instar or in the retention of a juvenile stage rather than a subsequent instar such as the pupa. Environmental conditions may also modify the amounts or timing of hormone secretion, and where these factors are predictable components of development, they will result in characteristic differences between individuals composing different castes in social insects. Juvenile honey bees will develop into queen bees when fed a diet based on secretions of the nurse bee’s mandibular glands, and they will develop into workers when fed higher proportions of hypopharyngeal gland secretions from worker bees. Ants will develop into minor or major workers or soldiers according to the quantity of food that will result in larger juveniles. These size differences affect the quantity of juvenile hormones, which is higher in the final instars of larvae developing into queen bees or major and soldier ants. In aphids, metamorphosis into the final adult form is affected by day length. Under long photoperiods, the largest embryos will develop into parthenogenetic forms, whereas under short photoperiods the embryos will give rise to sexually reproducing forms. The development of parthenogenetic forms is stimulated by the secretion of hormones from cells in the brain that respond to the amount of light passing directly through the cuticle of the head.
John R. Grehan
See also Evolution, Organic; Photosynthesis
Chapman, R. F. (1998). The insects: Structure and function. Cambridge, UK: Cambridge University Press.
Heming, B. S. (2003). Insect development and evolution. London: Comstock.