Metamorphosis: a remarkable change

Metamorphosis refers to a major change of form or structure during development. One of the most dramatic forms of metamorphosis is the change from the immature insect into the adult form.

Most of the major insect orders have a typical life cycle which consists of an egg, which hatches into a larva which feeds, moults and grows larger, pupates, then emerges as an adult insect that looks very different from the larva. These insects are often called 'holometabolous', meaning they undergo a complete (holo = total) change (metabolous = metamorphosis or change). Those which have immature stages similar in shape to the adult minus the wings are called 'hemimetabolous', meaning they undergo partial or incomplete (hemi = part) change.

Typical holometabolous insect groups are the Coleoptera (Beetles), Lepidoptera (moths, butterflies and skippers) and Hymenoptera (sawflies, wasps, ants and bees) and Diptera (flies). All of these groups have a life cycle where the egg hatches into a larva (e.g. a caterpillar, grub, maggot) which goes through an inactive, pupa stage (e.g. wrapped up like a cocoon) before emerging as an adult (e.g. a butterfly, beetle, wasp).

Typical hemimetabolous insects are the Hemiptera (Scales, Aphids, Whitefly, Cicadas, Leafhoppers and True Bugs), Orthoptera (Grasshoppers and Crickets), Mantodea (Praying Mantids), Blattodea (Cockroaches), Dermaptera (Earwigs) and Odonata (Dragonflies and Damselflies). These groups go through gradual changes as they turn into adults. Immature forms of these insects are called nymphs and these gradually increase in size and change form. As the insect grows, it sheds its skin (called moulting). After each moult, the nymph looks a bit different or a bit bigger. After a final moult, the full adult form emerges.

Metamorphosis is one of the key elements that makes insects so successful. Many insects have immature stages with completely different habitats from the adults. This means that insects can often exploit valuable food resources while still being able to disperse into new habitats as winged adults. The potential for adaptation and evolution is greatly enhanced by metamorphosis.

Growth and maturity

There is an important feature to note regarding metamorphosis. Insects are not able to mate and reproduce until they undergo their final moult or emerge from a pupa as a winged adult. Wings do not appear until the final moult (the one exception to this is the Ephemeroptera, or Mayflies). When you see an insect with wings, it is fully grown. This means that small flies do not become larger flies, they are as big as they will get.

Holometabolous larvae are larvae that pupate before emerging as adult insects, and include many of the most familiar insects.

Holometabolous larva in general are little more than tubular, efficient eating machines. They do not have to lay eggs, or find a mate. Apart from eating, they are mainly concerned with avoiding being eaten themselves. This means that they may have good camouflage, or hide in shelters or holes, or they may taste dreadful to any prospective predators.

The major insect orders have larvae with different common names. For instance, moths, butterflies and skippers have larvae which are usually called caterpillars. Fly larvae are nearly always called maggots. Beetle larvae are often referred to as grubs.

Caterpillars

Moth, butterfly and skipper (Lepidoptera) caterpillars have pairs of prolegs on their abdomen in addition to the three pairs of jointed walking legs on the thorax. Prolegs differ from the usual insect legs in that they are not jointed. Each proleg has a set of tiny hooks, which are arranged in rings or series around the tip of the proleg. These are called crochets, and only occur in the insect order Lepidoptera. Although there are some caterpillar-like larvae from other insect orders, such as sawfly larvae (Order Hymenoptera, Suborder Symphyta) and leaf beetle larvae (Order Coleoptera, Family Chrysomelidae), they can be distinguished from lepidopteran larvae by the absence of prolegs with crochets. Lepidopteran larvae have chewing mouthparts, and the majority of species are adapted to eating plant material.

Maggots

Fly larvae (Diptera) lack any segmented legs on the thorax, and are often highly specialised for living in wet environments. Very few are adapted to dry conditions. Quite a few species are internal parasites of other animals, where legs would be of no use. Unlike the larvae of Lepidoptera there is no one character that can be used to separate fly maggots from other large orders such as the Hymenoptera (Wasps, Bees, Ants, and Sawflies), as the immature stages of many species in these orders also lack segmented legs. Fly maggots live on a huge range of foods - from human flesh through to kelp on the seashore.

Grubs

Beetle larvae (Coleoptera) are highly diverse in their shapes. The majority live in concealed habitats, such as underground, or inside trees. There are many aquatic species, and a few which resemble caterpillars and feed openly on leaves. Many retain segmented legs, although weevil grubs nearly always lack legs. Most legless beetle grubs have robust chewing mouthparts and can be distinguished from fly maggots, which often have modified mouth 'hooks'.

The larvae of sawflies, wasps, bees and ants (Hymenoptera) are diverse in form. Many sawfly larvae are similar to lepidopteran caterpillars, and feed externally on plant material. The social Hymenoptera, which includes some wasps, some bees, and all ants have larvae with very few external features, as they do not have to forage for food. In these species food is brought to them by the adult nest mates. The parasitic Hymenoptera are similar in that they spend their larval period inside hosts or well-stocked nests. They do not need camouflage or legs in these habitats.

Hemimetabolous insects do not have a pupal stage. The general appearance of the immature stages is somewhat similar to that of adults, although there may be some dramatic differences in lifestyle. Only adult insects are able to reproduce, and only adult insects have functional wings (in those species that have wings).

The immature stages of these insects are generally called nymphs rather than larvae. Some have common names such as 'hoppers' (immature grasshoppers, Order Orthoptera), 'crawlers' (immature scale insects, Order Hemiptera) and 'mudeyes' (immature dragonflies, Order Odonata).

Examples of hemimetabolous insects include cockroaches (Order Blattodea), crickets and grasshoppers (Order Orthoptera), stick insects (Order Phasmatodea), praying mantids (Order Mantodea), termites (Order Isoptera), dragonflies and damselflies (Order Odonata), earwigs (Order Dermaptera), sucking bugs (Order Hemiptera), wood and book lice (Order Psocoptera), and parasitic lice (Order Phithaptera).

The feeding habits of hemimetabolous insects commonly mirror those of the adults, but often with a significant twist. Dragonfly nymphs are aquatic predators, but the adults are active flying insects, which hunt other flying insects. Stick insect nymphs can resemble ants, while later stage nymphs blend with the food plants. All stages of stick insects feed on plant material. The final moult between mature nymph and adult is usually accompanied by changes in colour, and in the shape of the body, but there is never the dramatic difference between larvae and adult as observed in holometabolous insects.