THE ANIMAL FILES

Old World swallowtail

Papilio machaon

A swallowtail caterpillar changes disguise as it grows.

Old World swallowtails have yellow-and-black wings with tails on the hindwings. Larvae begin with a bird-dropping-like appearance before becoming green and boldly patterned. Host plants and seasonal development vary across the species’ wide range.[1]

Follow the evidence ↓
Old World swallowtail
Image: Michael Wittwer · Commons · CC BY-SA 4.0
Adult wings
Yellow and black with hindwing tails[1]
Larval defence
Retractable scent-releasing osmeterium[1]
Main hosts
Often plants of the carrot family[1]

A disguise that changes

Small caterpillars resemble bird droppings. Larger larvae become green with black and orange markings. They also possess a retractable fleshy organ behind the head, the osmeterium, which releases an unpleasant smell when disturbed. Colour pattern and chemical defence thus operate through different structures and stages.[1]

Choosing a nursery plant

Females lay single eggs on suitable host plants. Many populations use members of the carrot family, but host breadth differs regionally; the British form is especially associated with milk parsley. A plant used in one country should not automatically be treated as the only host throughout the species’ range.[1]

When the next adult appears

Brood number varies with climate and latitude. Some pupae emerge in the same year, while others remain through winter. Adults visit flowers for nectar, and males can gather on hilltops. A butterfly resting on a summit and a pupa waiting through winter represent different uses of season and place within one life cycle.[1]

A COMMON MIX-UP

Does every caterpillar keep the same colour throughout growth?

No. Early bird-dropping-like larvae become green with contrasting marks as they grow.[1]

TRY A LITTLE REASONING

Why is one fixed date a poor prediction for adult emergence across the whole range?

Reveal the explanation

Climate, latitude and whether pupae overwinter change the timing and number of generations.[1]

Sources & editorial notes

Adapted and translated from the cited Wikipedia revisions, with source-linked reasoning. Image credit and licence are listed separately above.

Adapted and translated from Wikipedia contributors; exact revisions are linked below. CC BY-SA 4.0

  1. Wikipedia contributors — Papilio machaon (adapted and translated; exact revision)
  2. NCBI Taxonomy — Papilio machaon (identity and game lineage)

Last fact check:

One discovery leads to another

Mountain ApolloAdults visit flowers; caterpillars need a different set of plants.Parnassius apollo ↗Large white butterflyA cabbage leaf and a flower feed different stages.Pieris brassicae ↗Monarch butterflyOne migration route can take more than one generation to complete.Danaus plexippus ↗