This scene will review the various types of insect growth and metamorphosis.
Insects are covered by an exoskeleton that serves as both a skin and a skeleton.
The properties of the exoskeleton are a major factor for the success of insects.
Like a skin - the exoskeleton serves in an interface between the insect and its environment.
The exoskeleton serves as a barrier to environmental toxins, prevents infection
by parasites and microbes, and prevents injury and the loss of water and ions.
Like a skeleton - it gives the insect body form and serves as an attachment point for muscles.
However, a rigid insect exoskeleton constrains insect growth, and therefore,
insects must undergo periodic molting of the cuticle at which time a new, larger cuticle
is formed into which the immature insect is able to grow during the intermolt period.
Metamorphosis is the process of changing the structure of an organism from an immature larval
form into a reproductive adult form. Insects have three strategies for metamorphosis: ametabolous,
hemi-metabolous and holometabolous. Ametabolous insect species consist of the primitive,
wingless silverfish. Silverfish larvae look identical to adults, and only grow larger at
each molt with no morphological metamorphosis evident between the larval and adult forms.
Winged insect species are more advanced with hemimetabolous and holometabolous metamorphosis.
The larvae of hemimetabolous orders resemble adults, but with small wing pads and no genitalia.
Hemimetabolous larvae grow larger and have increasingly pronounced wing pads at each molt.
At the final larval-adult molt the resulting individual is full-sized, and the wings are fully
expanded. Holometabolous larvae usually have soft bodies with no resemblance to their winged adults.
At each larval-larval molt the larval instars enlarge.
The fifth instar larvae feed voraciously, and grow dramatically and molt to a quiescent pupa.
It is hypothesized that holometabolous insects have three sets of genes: larval, pupal and adult
- which set is active, depends on the endocrine state of the animal. At the larval-pupal molt,
the epidermal cells are re-programmed by changes in the endocrine balance and form the immobile
pupa. The pupa often shows the outer appearance of the adult: wings, mouthparts, legs and antennae.
During the pupal instar, metamorphosis occurs because of rebalancing of the endocrine activity,
and the set of adult genes is activated, The pupal cuticle cells undergo "apolysis" which
is degradation of the pupal endocuticle, and the adult forms within the pupal epi- and exocuticles.
This adult enclosed within the pupal cuticle is termed a "pharate adult".
Subsequently, the adult emerges from the residual pupal cuticle, expands and hardens its wings,
and becomes reproductively active. It is important to stress that there are many
species-specific variations on these events, and what is described here is the general situation.
Finally, what are the advantages and disadvantages of metamorphosis? The advantages of hemimetabolous
metamorphosis are: that both nymphs and adults are highly mobile; there is no vulnerable pupal stage;
and available adults may care for developing offspring. The disadvantages are: the nymphs
and adults compete for common resources; and they share common predators. In the
case of holometabolous insects, the advantages are: that adults and nymphs do not compete for
resources such as food and environment; and they do not share common predators.
The disadvantages are: that only the adults are highly mobile; and immatures require a long
period for metamorphosis to adults, whereas adults have a comparatively short life for reproduction.
In the next scene, we shall examine the endocrine changes that result in metamorphosis.