What makes the blue colour of male blue butterflies?

Tiny structures in the wing scales

Pea Blue

Lampides boeticus

The pea blue (Lampides boeticus), also called the long-tailed blue, is a small butterfly in the gossamer-winged family Lycaenidae and one of the most widely distributed butterflies in the world, found from Britain to Japan and across Africa, southern Asia and Australia. Males are violet-blue above and females mostly brown, while both have a grey-brown underside with wavy lines, thin tails and two eyespots that form a “false head”. Its caterpillars feed in the flowers and pods of peas, beans and other legumes, and are often visited by ants.

Also known as Long-tailed blue, Long-tailed pea-blue, Pulse blue

Pea blue abilities

5 false-head features scientists scoreStill debated
The back of the wings looks like a head, which may trick a predator into grabbing the wrong end.
- The tails stand in for antennae, the black eyespots for eyes, and wavy lines lead towards the fake face.
- A study of 928 lycaenid butterflies found that these features tend to evolve together, building a more convincing fake head.
- Losing a scrap of hindwing is far less dangerous than a bite to the real head, so scientists think the trick buys time to escape.
- The evidence is still thin: a few small experiments support it, and researchers say more tests are needed.
Sources [1, 2, 3]
2 hindwings moved to wiggle the tailsStill debated
Resting on a flower, the pea blue moves its hindwings so the thin tails twitch like antennae.
- UK butterfly experts describe the hindwings rotating while the butterfly rests, which makes the tails move.
- Scientists think moving tails make the fake antennae more convincing to a hunter.
- But in one experiment, hairstreak butterflies with their tails clipped were caught by mantises no more often than those with tails, so the tails’ job is still debated.
Sources [3, 7]
8–9.5 days adult life in a lab study
Adults drink sugary nectar from flowers through a long tube that uncoils from under the head.
- The proboscis unrolls into the flower and coils back up after drinking.
- Butterflies can drink a steady column of liquid, or a “bubble train” that draws up thin films of nectar.
- In a lab study on field pea, males lived about 8 days and females about 9.5 days as adults.
- In Britain, long-tailed blues are seen mostly around garden flowers.
Sources [6, 7, 11]
0.5 mm egg width
A female lays her eggs one at a time on flower buds of peas, beans and their relatives, so each caterpillar hatches next to food.
- Each egg is a flattened, pale bluish-green to white disc, about 0.5 mm across.
- Eggs hatch in 2–3 days in warm southern India and in about a week in an English summer.
- In one lab study on field pea, each female laid about 14 eggs, on twigs, flowers and pods.
- The new caterpillar eats a round hole in the top of its egg and soon bores into a bud.
Sources [1, 7, 11, 12]
15–35 days egg to butterfly in warm countries
In warm places a pea blue can go from egg to butterfly in about 2 to 5 weeks, so there are many generations a year.
- In Indian studies: egg 2–3 days, caterpillar about 8–13 days, a short pre-pupa, then a chrysalis of 3–8 days.
- The caterpillar has four stages (instars), with a moult between each.
- In cooler England it is slower: about a week as an egg, 3–4 weeks as a caterpillar and about 10 days as a chrysalis.
- On the continent it breeds continuously, one generation after another, but it cannot survive British winters.
Days in each stage on cowpea (India)
Caterpillar11.8 days
Chrysalis5.2 days
Egg3.2 days
Pre-pupa1.4 days
Averages from one lab study; in cooler places each stage takes longer.
Sources [7, 10, 11, 12, 13]
≈3/4 of lycaenid species live with ants
Pea blue caterpillars are often visited by ants, which drink sugary droplets from them and may protect them in return.
- About three-quarters of lycaenid butterflies associate with ants as caterpillars or pupae.
- The pea blue does not depend on ants, but many kinds visit it, including carpenter ants, Iridomyrmex ants, the ghost ant and the invasive Argentine ant.
- Lycaenid caterpillars reward ants with food from special glands and use chemical and sound signals to manage them.
- The deal varies: in a related blue, one ant species helped female caterpillars grow into heavier adults, while others made no difference.
Sources [9, 14, 15, 16]
60 adults that crossed to England in 2015
A strong migrant: most summers a few long-tailed blues fly north from the Mediterranean to Britain, and in some years many more cross the Channel.
- It was first recorded in Britain in August 1859, at Brighton and at Christchurch.
- In 2015, about 60 adults crossed the Channel in August and laid thousands of eggs in gardens and allotments on the south coast.
- Their young can emerge the same autumn, as they did in 2013, but they cannot survive the British winter.
- Experts think a warming climate is behind more frequent arrivals, and models predict its range will spread north.
Long-tailed blue sightings in Britain
1940–198885 sightings
1945 alone38 sightings
1859–193936 sightings
1945 is part of the 1940–1988 total.
Sources [7, 8, 17]
6+ plant families on the caterpillar menu
The caterpillar feeds hidden inside flower buds and pods, which makes it a pest of pea and bean crops in many warm countries.
- Young caterpillars bore into buds through the petals, leaving only a tiny hole.
- Older ones move into a pod and eat the seeds inside.
- In India it attacks cowpea, pigeon pea, green gram, black gram and sunhemp. In southern Europe it can damage pea crops.
- Cold winters limit where it can live, so warmer climates may let it spread further north.
Sources [7, 9, 10, 17]

Tap a pin on the model or a part below.

- The real antennae grow from the head. They are black, ringed with white.
- The thin tails at the back of the hindwings are thought to copy them, so a predator may mix up the front and the back.
Sources [1, 2, 3]
360 nm ultraviolet seen by a copper relative
- Blue butterflies (lycaenids) have an extra blue-sensitive pigment in their eyes, which may help them tell different blues apart.
- In a related copper butterfly, the eyes hold four visual pigments, including one for ultraviolet light at 360 nm, which people cannot see.
- In that species, males and females even have differently built eyes, matched to their different lives.
Sources [4, 5]
- Adults drink nectar through a long tube called a proboscis, which stays coiled under the head when it is not in use.
- Butterflies with a coiling proboscis can drink a steady column of liquid, or a chain of tiny bubbles that pulls up thin films of nectar.
- In Britain, long-tailed blues are most often seen around garden flowers, especially plants their caterpillars can eat.
Sources [6, 7]
32–42 mm wingspan in UK guides
- Males are violet-blue above with brown edges. Females are mostly brown, with shining blue scales near the wing bases.
- In related blue butterflies, the male’s blue is a structural colour: tiny structures in the scales reflect blue light, rather than a blue pigment.
- Those structures are so species-specific that a computer told nine blue species apart with more than 90% accuracy.
- UK guides give a wingspan of 32–34 mm for males and 36–42 mm for females. Other sources give 24–34 mm.
Sources [1, 4, 7, 8]
- Underneath, both sexes are grey-brown with many wavy brown lines edged with white.
- The hindwing alone carries eight or nine wavy brown bands at even distances apart.
- Lines that lead towards the tail end are one of the five features of a butterfly “false head”.
Sources [1, 2]
2 black eyespots by each tail
- Each hindwing ends in a thin black tail tipped with white.
- Beside each tail sit two small black eyespots with orange markings.
- Together they look like a second head, with eyes and antennae, at the back of the butterfly.
- At rest, the butterfly moves its hindwings, so the tails twitch.
Sources [1, 2, 7]
6+ plant families eaten by caterpillars
- Females lay their eggs one at a time on the flower buds of peas, beans and their relatives.
- Garden peas and broad beans are among the favourites. In Britain it also uses everlasting-peas, broom and bladder-senna.
- Caterpillars have been found on plants from at least 6 families, but the pea family is by far the main one.
Sources [1, 8, 9]
19 mm longest when fully grown
- The flat, woodlouse-shaped caterpillar first eats flowers, then bores into a pod and eats the seeds inside.
- It grows to about 16–19 mm and turns a dull lilac-pink just before it becomes a chrysalis.
- After each moult except the first, it eats its old skin.
- Ants often visit older caterpillars for sugary droplets.
Sources [7, 9, 10]

Real pea blues

Photos from Wikimedia Commons: males, a female, a caterpillar and Lang’s short-tailed blue for comparison.

Long-tailed blue in Greece. The tails and eyespots at the back form the false head.Photo: Charles J. Sharp, CC BY-SA 4.0
The wavy brown and white underside, photographed in the gardens of the Royal Palace of Caserta, Italy.Photo: Charles J. Sharp, CC BY-SA 4.0
MaleViolet-blue upperside with brown edges and fine whitish hairs; grey-brown wavy underside; thin tails with eyespots.The form most people noticeA male basking with open wings, showing the violet-blue upperside.. Photo: Bias Chakraborty, CC BY-SA 4.0
MaleViolet-blue upperside with brown edges and fine whitish hairs; grey-brown wavy underside; thin tails with eyespots.The form most people noticeMale upperside, Genova, Italy.. Photo: Hectonichus, CC BY-SA 4.0
FemaleMostly brown above, with shining blue scales near the wing bases and dark spots by the tails. The underside matches the male’s.Lays eggs one at a time on flower budsFemale in Andalucia, Spain: brown above with blue near the wing bases.. Photo: gailhampshire from Cradley, Malvern, U.K, CC BY 2.0
Caterpillar with antA flat, woodlouse-shaped caterpillar up to about 19 mm long, often visited by ants. It turns dull lilac-pink just before it pupates.Feeds in the flowers and pods of legumesA pea blue caterpillar in Maharashtra, India. Its flat, woodlouse-like shape is typical of blue butterflies.. Photo: Milind Bhakare, CC BY-SA 4.0
Sipping nectar with wings closed.Photo: B Savit, CC BY-SA 4.0
A pea blue in Osaka, Japan, at the far end of its range from Britain.Photo: Laitche, Public domain
Lang’s short-tailed blueSmaller (26–34 mm), with two small tails and a striped underside that gives it its other name, common zebra blue.Least Concern; a migrant across southern Europe and AfricaLang’s short-tailed blue, Portugal: smaller, with a striped underside and short tails.. Photo: Charles J. Sharp, CC BY-SA 4.0

Pea blue by the numbers

Measured by scientists and butterfly recorders.

Wingspan: 32–42 mm; UK guides; females are larger. Other sources give 24–34 mm.⁠[7, 8, 1]
Egg width: 0.5 mm; a flattened pale disc laid on a bud⁠[7, 1]
Egg to butterfly: 15–35 days; in studies in India and Bangladesh; slower in England⁠[12, 11, 13, 7]
Caterpillar length: 16–19 mm; fully grown, measured while crawling⁠[7]
Caterpillar stages: 4; instars, with a moult between each⁠[11, 13, 7]
Adult life: 8–9.5 days; males and females in a lab study on field pea⁠[11]
Host plant families: 6+; the pea family is the main one⁠[9]
Continents: 4; Europe, Africa, Asia and Australia, plus islands such as Hawaii⁠[9]
DNA difference: 0–0.5%; between most pea blues sampled more than 9,000 km apart⁠[9]
First British record: 1859; Brighton and Christchurch, August 1859⁠[7]

Pea blue myths and facts

Some things people say about pea blues are true. Some are not. Tap to check.

0 of 8 answered

“Pea blues are blue all over.”
“The tails on its wings are its real antennae.”
“The false head saves pea blues from predators.”
“Pea blue caterpillars are looked after by ants.”
“The pea blue is one of Hawaii’s native butterflies.”
“Long-tailed blues live in Britain all year round.”
“Its caterpillars can damage pea and bean crops.”
“The male’s blue comes from a blue pigment.”

Pea blue records

Superlatives backed by measurements and records.

One of the most widespread butterflies
4 continents
Pea blue
from Britain to Japan, across Africa, southern Asia and Australia, and out to Hawaii⁠[9]
Same butterfly, far apart
0–0.5%
Pea blue
DNA difference between most pea blues sampled more than 9,000 km apart⁠[9]
Alone in its genus
1 species
Lampides boeticus
the genus Lampides has no other species and no commonly accepted subspecies⁠[9]
Quickest life cycle we found
15 days
Pea blues in southern India
the shortest egg-to-adult time in a study in Kadapa, India (15–22 days)⁠[12]
First seen in Britain
1859
Brighton and Christchurch
the first British records, both in August 1859⁠[7]
Biggest early British arrival
38 sightings
Summer 1945
the only major arrival recorded before recent decades⁠[7]

A pea blue’s life

From egg to butterfly in a few weeks. Times are from studies in warm countries and from England.

Day 0
An egg on a bud
A female lays a single pale egg, about 0.5 mm wide, on a flower bud of a pea, bean or relative.⁠[1, 7]
Days 2–7
Hatching
The caterpillar, under 1 mm long, eats a round hole in its eggshell and bores into a bud.⁠[12, 7]
Next 1–4 weeks
Flowers, then pods
Over four stages it grows to about 16–19 mm, moving from flowers into a pod to eat the seeds.⁠[11, 7]
Older caterpillar
Ant visitors
Ants often visit the older caterpillars for sugary droplets.⁠[10, 9]
Fully grown
Lilac-pink
The caterpillar turns a dull lilac-pink and gets ready to pupate.⁠[7]
3–10 days
Chrysalis
The chrysalis, about 1 cm long, is held by a silk girdle. It lasts 3–8 days in India and about 10 days in England.⁠[12, 11, 7]
2–5 weeks after the egg
Butterfly
The adult emerges and drinks nectar. In a lab study, adults lived about 8–9.5 days.⁠[12, 11]
Late summer in Britain
Migrants arrive
Migrants from the south are seen mostly from late July to September. Their young cannot survive the winter.⁠[7]

Pea blue vs Lang’s short-tailed blue vs common blue

Three small blue butterflies that can turn up in the same gardens, side by side.

Pea blue (long-tailed blue) vs Lang's short-tailed blue vs common blue
	Pea blue	Lang’s short-tailed blue	Common blue
Scientific name⁠[20]	Lampides boeticus	Leptotes pirithous	Polyommatus icarus
Wingspan (UK guides)⁠[7, 18, 22]	32–42 mm	26–34 mm	29–36 mm
Tails⁠[1, 19]	One long, thin tail per hindwing	Two small tails	—
Underside⁠[1, 19]	Wavy brown lines edged with white	Striped (also called common zebra blue)	—
Caterpillar food⁠[8, 9, 19, 23]	Peas, beans, broom and other legumes	Legumes, plumbago and other plants	Mainly bird’s-foot trefoil
In Britain⁠[7, 18, 23]	Rare migrant	Rare migrant	Resident; the most widespread blue
Ants⁠[9, 22]	Tended by many ant species	—	Last-stage caterpillars and pupae attract ants
IUCN status⁠[20]	Least Concern	Least Concern	Not evaluated globally

“—” means we found no reliable source to compare. Wingspans are from UK Butterflies; other guides give smaller ranges.

Is the pea blue endangered?

No. The pea blue is common across a huge range and is even a crop pest in places.

IUCN Red List

Least Concern (LC)

Population stable

Listed as Least Concern because it is common across a huge range. Butterfly Conservation lists its European status as stable.⁠[24, 20, 25, 8]

IUCN Red List, 2020 assessment

Threats

Cold winters
The temperature of the coldest month limits where it can live, and it cannot survive British winters.⁠[17, 7]
Treated as a pest
Because its caterpillars damage pulse crops, farmers in many warm countries try to control it.⁠[10, 17]

What’s helping

Garden legumes
Everlasting-peas, broom, bladder-senna and mange-tout peas in gardens and allotments give migrants places to breed.⁠[8]
A warmer climate
Experts think climate change is behind more frequent arrivals in Britain, and models predict its range will spread north.⁠[8, 17]
Recording sightings
Records of sightings since 1859 show how arrivals in Britain have changed over time.⁠[7]

Look-alike

Lang’s short-tailed blueLeast Concern
Also Least Concern; a regular migrant in southern Europe, Africa and Asia.⁠[20, 19]

Pea blue quiz

12 questions. Every answer is somewhere on this page.

Question 1 of 12Score 0

Where is the pea blue’s “false head”?

Pea Blue field notes

Diet, food web, safety and badges from the Snappit field guide.

Diet & Feeding

Caterpillars feed mostly on the flowers and seed pods of leguminous plants, while adults forage for floral nectar.

Primary Foods

Legume flowers
Pea pods
Bean pods
Broom flowers
Alfalfa
Foraging Method: Foraging

Age differences: Larvae consume solid plant material, specifically targeting nitrogen-rich reproductive parts of legumes, whereas adults exclusively consume liquid nectar.

Ecological connections

Garden PeaHost plant
Host plant for laying eggs and caterpillar feeding.
Carpenter AntsMutualism
Ants visit the caterpillars for sugary droplets and may protect them in return.

Safety

Danger 1/5 · Very low

No special safety notes yet.

Classification

Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Lepidoptera
Family: Lycaenidae
Genus: Lampides

Sources for these pea blue facts

25 studies and references · checked Oct 16, 2026

Frequently Asked Questions

How to identify Pea Blue?

Compare the wing outline, spots, bands, eyespots and wing edges. The underside can look completely different from the top. Check for tails, the antennae and where the markings sit instead of relying on color alone. Butterfly Identifier: upload a photo and compare the visible details.

How long is Pea Blue?

1-2 cm

What is Pea Blue's wingspan?

2.4-4.2 cm

How much does Pea Blue weigh?

0.0001-0.0003 kg

How long does Pea Blue live?

up to 1 years

What does Pea Blue eat?

Caterpillars feed mostly on the flowers and seed pods of leguminous plants, while adults forage for floral nectar.

Where is Pea Blue usually found?

Found in open woodlands, agricultural fields, meadows, and urban gardens, typically wherever leguminous host plants are abundant.

Is the pea blue butterfly a pest?

Its caterpillars eat flowers and the seeds inside the pods of peas, beans and other legumes, so it is a pest of pulse crops such as cowpea and pigeon pea in many warm countries. In southern Europe it can damage pea crops.

Why is it called the long-tailed blue?

Because of the long, thin tail on each hindwing, which flutters in the breeze. The name pea blue comes from its favourite food plants and the male’s blue colour.

Does the long-tailed blue live in the UK?

Not all year. It is a rare migrant from the Mediterranean, seen mostly in late summer in southern England. Migrants can breed here, but the species cannot survive British winters.

Why do ants look after pea blue caterpillars?

The caterpillars give ants sugary droplets from special glands. In return, ants may protect them from enemies. The pea blue can also survive without ants.

What is a false head on a butterfly?

A pattern at the back of the hindwings that looks like a head: tails as antennae, eyespots as eyes and lines that lead to them. Scientists think it makes predators attack the wrong end, but the evidence is still limited.

How long does a pea blue butterfly live?

In warm countries it takes about 15–35 days to go from egg to butterfly. In a lab study, adult males lived about 8 days and females about 9.5 days.

How can you tell a male from a female pea blue?

Look at the upperside: males are violet-blue with brown edges, females are mostly brown with a little blue near the body. Underneath, both look the same.

What is the difference between the long-tailed blue and Lang’s short-tailed blue?

The long-tailed blue is bigger (32–42 mm in UK guides) with one long tail per hindwing and wavy underside lines. Lang’s short-tailed blue is smaller (26–34 mm), with two small tails and a striped underside.