I. Biology

Morphology

Like all other insects, adult butterflies have three body parts - the head, thorax and abdomen, as well as three pairs of legs attached to the thorax. Butterflies are known for their two pairs of charismatic wings, often dotted and striped in different colours. 

Images by Didier Descouens, licensed under CC BY-SA 4.0

With wing spans ranging from 5.1 to 7.3 cm, the painted lady butterfly exhibits different colour patterns on the dorsal (upperside) and ventral (underside) wing surfaces, contributing to its conspicuity. 

The dorsal surface consists of white and black pattern elements like spots and stripes, against an orange background. The ventral surface, on the other hand, had a greater variety of colours including blue, yellow, brown and grey on top of the three colours observed on the dorsal surface. Of particular interests are the eyespots, which are made up of concentric rings of different colours, and found on the ventral hindwing of these butterflies. 

Wings of these lepidopteran insects are widely diverse in colours and patterns. So what are these wings made of? Studies have found that butterfly wings are made up of scale cells, and each wing scale (approximately 0.8 to 40 mm²) only one colour. This means that a single eyespot is dotted by a variety of scale cells of different individual colours. It is a wonder how dead scale cells give rise to the vibrant array of colours on the wings!

Life cycle

Found in temperate and subtropical regions, the painted lady butterfly exhibits seasonal migration and breed throughout the year. The female lays eggs during its migration, contributing to its almost global distribution. It does not have subspecies, possibly a result of migratory behaviour that prevents the diversification of local populations¹ .

Like other holometabolous organisms, the painted lady butterfly undergoes metamorphosis as part of its life cycle that ranges from 1 month in subtropical areas to 2 months in the temperature region² . They can be found in open or disturbed urban areas. Eggs are laid singly on plant leaves and develop over the course of 5 days, before the caterpillar hatches. Five larval stages, also known as instars, span over 25 days where the caterpillar feeds continuously and grows extensively, followed by a pupation stage when metamorphosis takes place. The adult butterfly emerges from the pupa after a week and finds a mate before settling on a leaf to lay eggs and start the cycle again. 

A single V. cardui egg. Image by Harald Süpfle, licensed under CC BY-SA 3.0

Eggs are laid singly on upperside leaves of foodplants. The adult female butterfly first finds a host plant where its caterpillars would have abundant food, then oviposits about 500 eggs.

After hatching, the caterpillar crawls to the underside of the leaf and start its voracious feeding life. Born with six eyes, it then undergoes a total of four moults, where it sheds its skin to grow larger.

Vanessa cardui larva hiding under a silk tent made on a thistle leaf. Image by Michael J. Plagens (permission pending).

The caterpillar builds a silk tent across one or more leaves that it feeds on and excrement accumulates, making it easy to spot. It leaves the tent when it is ready to pupate. 

Vanessa cardui undergoing pupation (left) then metamorphosis (right). Video adapted from Birdy Official (permission pending).

After emerging from its chrysalis, the adult butterfly rests and unfolds its wings slowly to dry. A few hours later, it will be able to find food and mates to produce the next generation of painted ladies. 

Diet

More than 100 host plant species of the painted lady butterfly has been recorded around the world. Among the host plants, V. cardui prefers thistles (Asteraceae), hollyhock and mallow (Malvaceae), and various legumes (Fabaceae)³ . 

Tricoloured buddleia. Image by © YouGarden 2018

While V. cardui is a polyphagous species, some of its favourite food as a larva is thistle like Buddleia, which can be an invasive weed, and common mallow Malva sylvestris (Malvaceae). Unlike the adults with a proboscis mouthpart, larvae have chewing mouthparts. These mouthparts allow them to munch on an immense volume of leaves that support their size increase⁴ .

The painted lady butterfly prefers open fields and urban areas, like flower meadows and rural hills. In Singapore, they have been observed a few times, at one of our urban hill parks, at an open wasteland in the north and one at Pasir Panjang Nursery⁵ .

Multi-generational migration

Migration

The most amount of attention diverted to V. cardui is its world's longest return migratory journey. In particular, the most well documented trip was by Stefanescu et al. (2013), who reported an exceptional large number of V. cardui migrating across Europe in 2009. Vanessa cardui returned to the Europe from its overwintering range in Africa and the trip was observed to climb to high altitudes about 1000 metres above sea level. 

Distribution

Described as cosmopolitan, the extant Vanessa cardui has one of the world's largest distribution, found in almost all continents including Europe, Africa, Asia, North and South America, except Antarctica⁶ .

Proposed global distribution of Vanessa cardui. Map by Discover Life. 

Exhibiting multi-generational migratory behaviour, the painted lady butterfly stops in Singapore occasionally during winter migrations from East Asia, with sightings since 2007⁷ . Migratory behaviour that moves them from temperate regions, where they spend spring and summer, to subtropical regions in autumn and winter ensures constant access to resources and avoid parasitism⁸ . Travelling over thousands of kilometres, these butterflies have been found in Svalbard, Norway, vastly distant from breeding grounds. The abundance and wide range habitats of V. cardui allows for the non-listing as an endangered species. 

The interesting thing about migration in V. cardui is the span of the migration cycle over multiple generations. Unlike vertebrates including large mammals, which have generation times longer than the migration cycle, migratory insects with short lifespans either travel a shorter distance, or make stops to reproduce during migration⁹ . Well studied migration routes include between North Africa and Europe, and between North and South America. In both cases, these butterflies make a round trip and these annual circuits span a minimum of 10 generations¹⁰ . Other than the Americas, Europe, Africa and Asia, the painted lady is not found in areas east of the Wallace line including south Australia, as well as Antarctica.

Locally, there has only been sporadic sightings of V. cardui in Singapore between 1990 and 2017¹¹ . Hopefully, we get to see more of this elegant Painted Lady in years to come. 

Heat regulation

As with other invertebrates, V. cardui is ectothermic. This means that it is unable to regulate its body temperature and is heavily dependent on the environment. Correspondingly, it usually avoids the cold due to its inability to overwinter as well, by migrating to other regions where the temperature is warmer. The painted lady butterfly takes advantage of the sun by basking in the heat. 

V. cardui basking on a hot sunny afternoon. Image by © 2018 North American Butterfly Association. 

Basking is the act of using specific wing postures and orientation according to solar and wind direction. When it gets too hot, the butterflies simply fold their wings back to prevent overheating. 

Apart from warming up, V. cardui also have to face ephemeral periods of cold in their breeding regions in the Northern Hemisphere, before they make their long migration. Shivering behaviour in butterflies involves the contraction of flight muscles against each other to produce heat¹² . This behaviour is observed in insects generally, but is not well studied. A further investigation of this shivering behaviour could elucidate the ability of such tiny insects to traverse across long distances and high elevations. 

II. Species diagnosis

As of today, there are a total of 22 species in the genus Vanessa. 

Vanessa species with dorsal (left) and ventral (right) wings shown¹⁵

.

The most closely related species to V. cardui is Australian painted lady, V, kershawi.

Enlarged images of dorsal and ventral wing surfaces of V. cardui and V. kershawi. Adapted from Abassi & Marcus (2015).

In Singapore, V. cardui is distinct from other butterflies in Singapore so any luck with finding a butterfly with a white stripe with splodges of orange and black should make it easy to identify as V. cardui.

Vanessa kershawi, the Australian painted lady. Image by Fir0002/Flagstaffotos, under CC BY-NC.

Note the huge similarity in wing colour patterns between the two Vanessa species. Despite the glaring similarities of the two species in behaviour and morphology, the most prominent difference is phenotypic plasticity in V. kershawi. Phenotypic plasticity is exhibited by different appearance or behaviour of organisms when they are exposed to different environments like varying temperatures. In V. cardui, stress or low temperatures can induce darker colour wing patterns like those in V. kershawi¹⁶ . Talk about becoming darker under stress!

III. Uses in scientific studies

Vanessa cardui is widely used for scientific studies due to its abundance and ease of rearing. The studies range from examining nanostructures of wings to biological studies of environmental effects on organisms. 

Material Science

Following the studies of shells, bones and honeycombs, butterfly wings have become the next natural architecture to be studied¹⁷ . As mentioned previously, butterfly wings are made up of many scales, each contributing a single colour. These scales have been found to be made of chitin, which is the second most abundant organic compound after cellulose, and chitin is also a component of crab shells and squid pens (a hardened internal body part)¹⁸ . The dazzling structures of butterfly wings make them candidates for replicating these biomimetic structures for solar cells and advanced sensors. However, these nature-engineered structures are difficult for mass production at the moment, and the ongoing evolutionary process means that these structures are continuously altered but it is difficult to replicate natural modifications¹⁹ . These studies are pioneering the field and future research could resolve these issues for the replication of naturally occurring structures. 

Phenotypic plasticity

Vanessa cardui was first found to be temperature-sensitive in a study by Nijhout (1985), where cold-shocks administered to led to individuals developing aberrant colour patterns²⁰ .

This observation was then further explored by Yamanaka et al. (2009), where pupae of V. cardui were found to exhibit phenotypic plasticity. Different shades of wings appeared under varying temperature conditions, as observed from the image below. 

An increase of 14°C led to a shrink in the black central spot (indicated by the white triangles) in the eyespots on the ventral hindwings of V, cardui. Image © 2016 Connahs et al.²¹

This led on to further investigation of plasticity in nymphalid butterflies, where Junonia coenia and Bicyclus anynana have also been observed to exhibit varying eyespot sizes with different temperatures. Preliminary comparative work has identified plasticity in nymphalid butterflies to have multiple, independent origins²² .