Insect metamorphosis
Complete and incomplete metamorphosis, larval and pupal types, the hormones that control moulting, diapause, insect growth regulators and degree-day sums.
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A caterpillar that chews cabbage leaves and the moth that sips nectar are the same animal, yet they look, live and feed completely differently. A grasshopper nymph, by contrast, looks like a small, wingless version of its parents. These differences in how insects grow up, called metamorphosis, are not just biological curiosities: they decide which stage damages the crop, which stage can be reached by a spray or a natural enemy, and when control should be timed.
Types of metamorphosis, larval and pupal forms, and the hormones behind them are standard exam topics in entomology.
Why insects moult
An insect's hard exoskeleton cannot stretch, so the insect grows in steps by moulting (ecdysis): it forms a new, soft cuticle under the old one, sheds the old one and expands before the new one hardens. The stage between two moults is an instar; the time spent in it is a stadium. Most insects pass through four to eight larval or nymphal instars.
The three patterns
No metamorphosis (ametabolous)
Young hatch looking like small adults and simply grow, moulting even as adults. This occurs only in primitively wingless insects, such as silverfish.
Incomplete or gradual metamorphosis (hemimetabolous)
Egg → nymph → adult. Nymphs resemble adults, feed on the same food, and develop wings externally in wing pads that grow with each moult. Examples: grasshoppers and locusts (Orthoptera), bugs, aphids, whiteflies and hoppers (Hemiptera), termites and cockroaches. Aquatic young of dragonflies and mayflies, called naiads, differ more from the adults. Some texts use paurometabolous for insects whose nymphs share the adults' habitat. Whiteflies, thrips and scale insects show intermediate forms with an inactive stage resembling a pupa.
Because nymphs and adults usually feed together on the same plants, both stages are targets for control.
Complete metamorphosis (holometabolous)
Egg → larva → pupa → adult. Larvae look and live completely differently from adults; wings develop internally and appear only in the pupa. Examples: beetles, moths and butterflies, flies, bees, wasps, ants, lacewings and fleas. Over 80 per cent of insect species develop this way.
Separating the feeding larva from the dispersing, reproducing adult means the two stages do not compete for food, which may explain why this group is so diverse. For agriculture it often means the larva is the pest (caterpillars, grubs, maggots, wireworms) while the adult may be harmless or even a pollinator.
Types of larvae
| Type | Description | Examples |
|---|---|---|
| Eruciform | caterpillar: head, three pairs of thoracic legs, fleshy abdominal prolegs | moths, butterflies, sawflies |
| Scarabaeiform | C-shaped, fleshy grub with short legs | white grubs (chafers), dung beetles |
| Campodeiform | elongated, flattened, active, long legs | lacewings, ground and ladybird beetles |
| Elateriform | slender, hard-bodied, short legs | wireworms (click beetles) |
| Vermiform | legless, worm-like, often without a distinct head | maggots of flies; weevil grubs |
Types of pupae
- Exarate: appendages free, not glued to the body, as in beetles, bees and wasps.
- Obtect: appendages glued to the body in a hard case, as in moths and butterflies; a butterfly pupa is a chrysalis, and many moth pupae sit in a silk cocoon.
- Coarctate: the pupa forms inside the hardened skin of the last larval instar, a puparium, as in house flies and fruit flies.
Many pests pupate in the soil, which is why deep ploughing or flooding can kill fruit fly, armyworm and bollworm pupae.
Hormonal control
Three hormones coordinate moulting and metamorphosis:
- Prothoracicotropic hormone (PTTH), from the brain, triggers moulting.
- Ecdysone (converted to its active form, 20-hydroxyecdysone), from the prothoracic glands, starts each moult.
- Juvenile hormone (JH), from the corpora allata, decides the type of moult. High JH means the next instar is still juvenile; when JH falls in the last larval instar, the moult leads to the pupa and then the adult.
These hormones are the basis of insect growth regulators (IGRs):
- Juvenile hormone mimics (pyriproxyfen, methoprene, fenoxycarb) prevent normal metamorphosis.
- Ecdysone agonists (tebufenozide, methoxyfenozide) force a premature, lethal moult in caterpillars.
- Chitin synthesis inhibitors (diflubenzuron, lufenuron, novaluron, buprofezin) prevent formation of a new cuticle.
IGRs act slowly, mostly on immature stages, and are relatively safe for vertebrates, which lack these hormones.
Diapause
Diapause is a hormonally controlled pause in development that lets insects survive cold winters, dry seasons or the absence of their host. It is triggered mainly by day length and temperature, often well before the harsh season arrives, and can occur in eggs, larvae, pupae or adults. Pink bollworm larvae, for example, enter diapause in cotton seeds and carry the pest from one season to the next.
Development and temperature
Insects are cold-blooded, so their development speeds up with temperature between a lower and an upper threshold. Entomologists predict life stages with degree-days, just as agronomists do for crops.
Predicting a pest's generations with degree-days
A moth pest develops from egg to adult above a lower threshold of 10 °C and needs 550 degree-days to complete a generation.
- At a mean temperature of 25 °C: each day gives 25 − 10 = 15 degree-days, so a generation takes 550 ÷ 15 ≈ 37 days.
- At a mean of 20 °C: 10 degree-days per day, so 55 days.
A warm season delivering 2,200 degree-days above 10 °C allows about 2,200 ÷ 550 = 4 generations. Warmer climates mean more generations a year, faster build-up of populations and often faster evolution of insecticide resistance. Degree-day models are used to time sprays at egg hatch, when young larvae are most vulnerable.
Key points
- Insects grow by moulting; each stage between moults is an instar.
- Ametabolous: no change; hemimetabolous: egg → nymph → adult; holometabolous: egg → larva → pupa → adult.
- Larval types: eruciform, scarabaeiform, campodeiform, elateriform, vermiform; pupal types: exarate, obtect, coarctate.
- PTTH, ecdysone and juvenile hormone control moulting; IGRs mimic or disrupt them.
- Diapause lets insects survive unfavourable seasons; degree-days predict development.
What the exams ask
- School-leaving and entrance papers: types of metamorphosis with examples, and life cycles of common pests.
- Degree and research-fellowship entrances: larval and pupal types, hormonal control of moulting, IGRs, diapause, and degree-day calculations.
- Professional examinations: timing control measures to vulnerable stages, using degree-day models, and choosing IGRs.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Which stage of a moth does the damage?
Almost always the caterpillar, which chews leaves, stems, fruits or seeds. Adults only feed on nectar or not at all, but they lay the eggs, so traps that catch adults warn of coming larvae.
Why are young larvae easier to kill?
They are small, have a thin cuticle, and usually feed exposed on the surface before boring into fruit or stems. Older larvae eat much more and are often protected.
Do IGRs kill adult insects?
Mostly not. They act on moulting and metamorphosis, so they affect immature stages, although some reduce the fertility of adults or the hatching of their eggs.
Sources
- Gullan, P. J. and Cranston, P. S. The Insects: An Outline of Entomology, 5th edition. Wiley-Blackwell, 2014. Chapter 6.
- Chapman, R. F. The Insects: Structure and Function, 5th edition, eds Simpson, S. J. and Douglas, A. E. Cambridge University Press, 2013.
- Truman, J. W. and Riddiford, L. M. "The origins of insect metamorphosis." Nature 401, 447–452 (1999).
- Dhadialla, T. S., Carlson, G. R. and Le, D. P. "New insecticides with ecdysteroidal and juvenile hormone activity." Annual Review of Entomology 43, 545–569 (1998).
- Pruess, K. P. "Day-degree methods for pest management." Environmental Entomology 12, 613–619 (1983).
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