Insect Metamorphosis
Candice Hawkinson (MG ’06) & Linda Steber (MG '91)
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Metamorphosis refers to the process through which insects develop, grow and change form. Metamorphosis actually infers "change." The term is of Latin and Greek origins: meta means “change” and morphe means “form.” Metamorphosis describes the series of changes through which an insect passes in its growth from the egg stage (some insects, such as aphids, may produce eggs and/or give birth to live young) through the immature stages (ex., nymph, larva or pupa) to the adult stage.
For nearly three centuries, scientists have abided by a strict body of international rules when naming plants and animals (scientific names). This help to avoid confusion and redundancy when discussing or citing a specific plant or animal.
It appears that scientists are not ready to make a similar leap when it comes to the concept of insect metamorphosis. It can be confusing to amateur entomologists and gardeners alike when different naming conventions are provided by different entomologists for the same insect—at least when insect metamorphosis is concerned. Entomology departments within different universities across the USA do not seem to have a consensus for defining types of insect metamorphous.
We elected to use the definitions presented by Dr. Bastiaan M. Drees (Professor & Extension Entomologist, The Texas A&M University System) and Dr. John A. Jackman (Professor & Extension Entomologist, The Texas A&M University System) in their book entitled A Field Guide to Common Texas Insects (Gulf Publishing: Field Guide Series; ISBN 0-87719-263-4).
They describe two forms of insect metamorphosis (Simple Metamorphosis and Complete Metamorphosis):
I. Simple Metamorphosis (Egg > nymph > adult)
Insects with simple metamorphosis have three life stages: egg, nymph and adult.
Adults and nymphs of these insects usually feed on the same foods. These insects
may have wings, which if present, develop externally. Young are known as nymphs
and there is no resting stage (pupa).
Note: Some naming conventions provide sub-categories for simple
metamorphosis as follows:
1) insects with no metamorphosis—or no apparent metamorphosis
(such as Springtails, Order: Collembola);
2) insects with incomplete metamorphosis (such as Dragonflies, Order: Odonata);
and
3) insects with gradual metamorphosis (such as True Bugs, Order: Hemiptera).
II. Complete Metamorphosis (Egg > Larva (more
precisely: larval instars) > pupa > adult )
Insects with complete metamorphosis have four (4) life stages: egg,
larva, pupa
and adult. Larvae are very different in form from the adult. The active immature
stages are known generally as larvae.
These insects also have a resting stage known as a pupa. The pupal stage is a transition stage, when the larva transforms into the adult. Wings, if present, develop internally within a pupa. The pupa molts to the adult form.
Insects with complete metamorphosis include lacewings (Order Neuroptera), beetles (Order Coleoptera), butterflies and moths (Order Lepidoptera), true flies (Order Diptera) and wasps and bees (Order Hymenoptera). In some insect orders, larvae are referred to by other names, such as maggots (flies), caterpillars (butterflies and moths) or grubs (beetles). Larvae of insects with complete metamorphosis usually have chewing mouthparts; many are pests of various crops.
There several additional naming conventions utilized by various university entomologists to describe insect metamorphosis. Examples include:
• Without Metamorphosis, Gradual Metamorphosis, Incomplete Metamorphosis &
Complete Metamorphosis
• Simple Metamorphosis & Complete Metamorphosis
• Incomplete Metamorphosis & Complete Metamorphosis
• Simple Metamorphosis, Gradual Metamorphosis & Complete Metamorphosis
Yes, there is a confusing (and/or perplexing) variety of naming conventions used by different scientists to describe insect metamorphosis. However, each approach seeks to address the process of metamorphosis in varying detail and precision.
Instars
Another
term commonly encountered in reading about insects is "instar." As an immature
larva or nymph feeds and grows, it enlarges by shedding its outer skin known as
an exoskeleton. Each time it does this it changes to a new stage called an
instar. The first instar emerges from the egg and develops to the first molt.
The last instar develops into the adult in simple metamorphosis, or into the
pupa in complete metamorphosis. Most common insects have 3 to 6 instars.
The photo to the left show various instars of the tobacco hornworm (Manduca sexta). There normally are five instars , but occasionally six are observed and gardeners are mostly likely to encounter these caterpillars during their fifth or sixth instar when they become conspicuous even to the casual observer.
The important concept that amateur entomologists and gardeners should understand is that an awareness of an insect’s metamorphosis provides important information on the life cycle of an insect. Whether an insect is considered a pest or beneficial, understanding its life cycle can help us learn how to attract/maintain beneficial insects and control/manage insect pests!
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