Morphogenesis

 

Once an insect hatches from the egg it is usually able to survive on its own, but it is small, wingless, and sexually immature.  Its primary role in life is to eat and grow.  If it survives, it will periodically outgrow and replace its exoskeleton (a process known as molting).  In many species, there are other physical changes that also occur as the insect gets older (growth of wings and development of external genitalia, for example).  Collectively, all changes that involve growth, molting, and maturation are known as morphogenesis.

 

Molting

The molting process is triggered by hormones released when an insect’s growth reaches the physical limits of its exoskeleton.  Each molt represents the end of one growth stage (instar) and the beginning of another (Figure 1).  In some insect species the number of instars is constant (typically from 3 to 15), but in others it may vary in response to temperature, food availability, or other environmental factors.  An insect is known as an imago (adult) when it becomes sexually mature.  At this point, molting stops and energy for growth is channeled into production of eggs or sperm.

An insect cannot survive without the support and protection of its exoskeleton, so a new, larger replacement must be constructed inside the old one — much like putting an overcoat under a sweater!  The molting process begins when epidermal cells respond to hormonal changes by increasing their rate of protein synthesis.  This quickly leads to apolysis — physical separation of the epidermis from the old endocuticle.  Epidermal cells fill the resulting gap with an inactive molting fluid and then secrete a special lipoprotein (the cuticulin layer) that insulates and protects them from the molting fluid’s digestive action.  This cuticulin layer becomes part of the new exoskeleton’s epicuticle.

After formation of the cuticulin layer, molting fluid becomes activated and chemically “digests” the endocuticle of the old exoskeleton.  Break-down products (amino acids and chitin microfibrils) pass through the cuticulin layer where they are recycled by the epidermal cells and secreted under the cuticulin layer as new procuticle (soft and wrinkled).  Pore canals within the procuticle allow movement of lipids and proteins toward the new epicuticle where wax and cement layers form.

When the new exoskeleton is ready, muscular contractions and intake of air cause the insect’s body to swell until the old exoskeleton splits open along lines of weakness (ecdysial sutures).  The insect sheds its old exoskeleton (ecdysis) and continues to fully expand the new one.  Over the next few hours, sclerites will harden and darken as quinone cross-linkages form within the exocuticle.  This process (called sclerotization or tanning) gives the exoskeleton its final texture and appearance.

After formation of the cuticulin layer, molting fluid becomes activated and chemically “digests” the endocuticle of the old exoskeleton.  Break-down products (amino acids and chitin microfibrils) pass through the cuticulin layer where they are recycled by the epidermal cells and secreted under the cuticulin layer as new procuticle (soft and wrinkled).  Pore canals within the procuticle allow movement of lipids and proteins toward the new epicuticle where wax and cement layers form.

When the new exoskeleton is ready, muscular contractions and intake of air cause the insect’s body to swell until the old exoskeleton splits open along lines of weakness (ecdysial sutures).  The insect sheds its old exoskeleton (ecdysis) and continues to fully expand the new one.  Over the next few hours, sclerites will harden and darken as quinone cross-linkages form within the exocuticle.  This process (called sclerotization or tanning) gives the exoskeleton its final texture and appearance.

An insect that is actively constructing new exoskeleton is said to be in a pharate condition.  During the days or weeks of this process there may be very little evidence of change.  Ecdysis, however, occurs quickly (in minutes to hours).  A newly molted insect is soft and largely unpigmented (white or ivory).  It is said to be in a teneral condition until the process of tanning is completed (usually a day or two).

Metamorphosis

Each time an insect molts, it gets a little larger.  It may also change physically in other ways — depending on its type of metamorphosis:  ametabola, hemimetabola, or holometabola.

Ametabolous insects undergo little or no structural change as they grow older.  Immatures are called young; they are physically similar to adults in every way except size and sexual maturity.  Other than size, there is no external manifestation of their age or reproductive state.

Hemimetabolous insects exhibit gradual changes in body form during morphogenesis.  Immatures are called nymphs or, if aquatic, naiads.  Maturation of wings, external genitalia, and other adult structures occurs in small steps from molt to molt.  Wings may be completely absent during the first instar, appear in the second or third instar as short wing buds, and grow with each molt until they are fully developed and functional in the adult stage.  Developmental changes that occur during gradual metamorphosis are usually visible externally as the insect grows, but adults retain the same organs and appendages as nymphs (eyes, legs, mouthparts, etc.).

Holometabolous insects have immature forms (larvae) that are very different from adults.  Larvae are “feeding machines”, adapted mostly for consuming food and growing in size.  They become larger at each molt but do not acquire any adult-like characteristics.  When fully grown, larvae molt to an immobile pupal stage and undergo a complete transformation.  Larval organs and appendages are broken down (digested internally) and replaced with new adult structures that grow from imaginal discs, clusters of undifferentiated (embryonic) tissue that form during embryogenesis but remain dormant throughout the larval instars.  The adult stage, which usually bears wings, is mainly adapted for dispersal and reproduction.

Larval Types

Most larvae can be grouped into one of five categories based on physical appearance:

Eruciform (Caterpillars)

Body cylindrical with short thoracic legs and 2-10 pairs of fleshy abdominal prolegs.

Examples:  Butterflies and moths

Campodeiform (Crawlers)

Elongated, flattened body with prominent antennae and/or cerci. Thoracic legs adapted for running

Examples:  Lady beetles and lacewings

Scarabaeiform (White grubs)

Body robust and “C”-shaped with no abdominal prolegs and short thoracic legs

Examples:  June beetles and dung beetles

Elateriform (Wireworms)

Body long, smooth, and cylindrical with hard exoskeleton and very short thoracic legs

Examples:  Click beetles and flour beetles

Vermiform (Maggots)

Body fleshy, worm-like.  No head capsule or walking legs

Examples:  House flies and flesh flies

Pupal Types

Most pupae can be grouped into one of three categories based on physical appearance:

Obtect (Chrysalis)

Developing appendages (antennae, wings, legs, etc.) held tightly against the body by a shell-like casing. Often found enclosed within a silken cocoon.

Examples: Butterflies and moths

Exarate (No common name)

All developing appendages free and visible externally.

Examples: Beetles and lacewings

Coarctate (Puparium)

Body encased within the hard exoskeleton of the next-to-last larval instar

Examples: Flies