The Questions of Developmental Biology

Metamorphosis in Insects
Types of insect metamorphosis
Whereas amphibian metamorphosis is characterized by the remodeling of existing
tissues, insect metamorphosis often involves the destruction of larval tissues and their
replacement by an entirely different population of cells. Insects grow by molting shedding their
cuticle and growing new cuticle as their size increases. There are three major patterns of insect
development. A few insects, such as springtails and mayflies, have no larval stage and undergo
direct development. These are called the ametabolous insects (Figure 18.11A). These insects
have a pronymph stage immediately after hatching, bearing the structures that have enabled it to
get out of the egg.
But after this transitory stage, the insect begins to look like a small adult; after each molt,
they are bigger, but unchanged in form (Truman and Riddiford 1999). Other insects, notably
grasshoppers and bugs, undergo a gradual, hemimetabolous metamorphosis (Figure 18.11B).
After spending a very brief period of time as a pronymph (whose cuticle is often shed as the
insect hatches), the insect looks like an immature adult. This immature stage is called a nymph.
The rudiments of the wings, genital organs, and other adult structures are present, and these
structures become more mature with each molt. At the last molt, the emerging insect is a winged
and sexually mature adult.In the holometabolous insects (Figure 18.11C: flies, beetles, moths,
and butterflies), there is no pronymph stage. The juvenile form that hatches from the egg is called
a larva. The larva (caterpillar, grub, maggot) undergoes a series of molts as it becomes larger.