30 March 2019

The Butterfly Legs

The Butterfly Legs
An Anatomical Discussion of the Legs of a Butterfly


A Common Mormon tiptoes on its slender long legs as it puddles on damp sand

Insects form the largest group within the phylum arthropoda. In general, insects have a chitinous exoskeleton, a body that has 3 distinct segments (head, thorax and abdomen), three pairs of jointed legs, compound eyes and one pair of antennae. Butterflies belong to the Order Lepidoptera (Scaled wings) and are classified as insects.


A Painted Jezebel uses all its six legs to grasp the flower as it feeds on nectar

Fundamentally, this means that all butterflies have six legs. However, even though all six legs are present, the forelegs of some butterfly species have evolved and adapted to perform a totally different function from their original purpose of locomotion (walking). In particular, the species in the family Nymphalidae or often referred to as "Brushfoot" butterflies, have their forelegs reduced to such an extent that they become imperfect and brush-like (hence the name brushfoot) and useless for walking.



A typical butterfly leg has the following five parts, in sequence, from where the leg is attached to the thorax to the extremities (usually the tarsal claws) - coxa, trochanter, femur, tibia and tarsal segments. Butterflies have three pairs of legs - the forelegs (nearest to the head), the middle legs and the hind legs, each attached to a segment of the thorax.



Although each typical leg consists of the standard segmented parts, there is a wide variation of the legs across the butterfly families. This blog article takes a look at some of the differences of the butterfly species' legs from different families. The majority of the photos depicted here are cropped from a full frame DSLR sensor output and may not be as sharp or of high quality as expected.

Papilionidae


Papilionidae species have long slender legs that allows the butterfly to "stand tall"

All six legs are present in all the species of the Papilionidae family. A typical Papilionid has the longest legs (measured from coxa to tarsus) compared to the species in other families. The femur of the Papilionid's leg appears more 'muscular' compared to species of other families. The tarsal segments are typically long and when a Papilionid butterfly puddles or uses its legs to hold onto a flower as it feeds, the body of the butterfly is held high and some distance away from the food source.


A Lime Butterfly uses its sharp tarsal claws to grasp its perch as it stops to rest

A Five Bar Swordtail's legs in close up.  Note the greenish hue and sharp spiny hairs on its legs

Most Papiliond species have tibia spurs and the femur, tibia and tarsal segments may be covered with small spiny hairs which aid in holding on to perches and help in gripping onto flowers as the butterfly feeds. The tarsal claws are prominent on all the legs and also help to hold on to branches, leaves and any other perch that the butterfly is resting on.

Pieridae



All Pieridae species have six legs fully developed and functional. Many of the species usually have their legs concolourous with the butterflies' abdomen or wings. There is usually a darker longitudinal stripe throughout the length of the leg. Compared to the Papilionidae, the Pierids have shorter legs. The femur is usually covered with short and soft hairs.


Pieridae have all six legs developed

As with the other species, the Pierids have tarsal claws and fine hairs on the tarsal area of the leg to help it hold on to its perch whilst it stops to rest or when feeding. They usually utilise all six legs to grasp the perch or balance on a flower.

Nymphalidae


An Autumn Leaf perches with its four legs and balances well with its legs spread out

The Nymphalidae family (or Brushfoots) are unique in that the forelegs of the butterfly species in this family are under-developed and reduced to mere tufts of hair. These 'legs' are therefore are useless and unfunctional. Most of the time, these forelegs cannot be seen, whilst the middle and hindlegs are well developed and robust. The butterfly species in this family appear to have only 4 legs.


A Common Tiger uses its sharp tarsal claws to clutch onto a flower as it feeds on the nectar


A Blue Pansy perched on a flower on its four legs.  Note the sharp spiny hairs on its legs

The variety of species in the sub-families Danainae, Satyrinae, Nymphalinae and so on clearly show that they are not in any way disadvantaged with only 4 legs. They go about their usual daily routine using just the remaining 4 legs (middle and hind legs) without any problems at all.

Riodinidae


A Female Harlequin has all six legs developed.

The Riodinids are unique in that only the females have six fully developed legs. In the males, the forelegs are reduced to tufts of hair like in the Nymphalidae and these legs are not functional. However, the females tend to fold their forelegs tightly to the front of the thorax and appear to also have only 4 legs to stand on.


A male Harlequin has four developed legs whilst its forelegs are reduced to just tufts of hair

The legs of the Riodinids are relatively short and thick with the tarsal segments, particularly at the extremities close together giving them a banded appearance. The legs of the Riodinids also appear to lack the prominent tibia spur that can be found in other species of butterflies.

Lycaenidae


A Blue Brownwing's unique shaped legs. Note the compressed tarsal segments of its foreleg


A Bigg's Brownwing with its rather specialised foreleg that appears spatulate and flat

The variety of leg designs across the many genera of Lycaenidae is quite amazing. Of special note are the legs of some species of sub-family Miletinae. Of interest are the legs of the Miletus spp. The legs appear flattened and taper towards the joints. The tarsal are broader towards the extremities and as least in one of the species, the forelegs appear to lack the tarsal claws.





A variety of six-legged Theclinae with banded and pure white legs

In other species of the subfamily Theclinae, the legs are short and thick, and may sometimes be black and white banded. In other species, all six legs appear as if they are clad in white stockings. The tibia spur occur in many species across the family and the tarsus is also covered with small spiny hairs which allow the butterfly to hold on to its perch firmly.

Hesperiidae




Note the exceptionally hairy femur and tibia of some Hesperiidae species

The Skippers have all six legs fully developed. In many of the species, the femur is exceptionally hairy and covered with thick long hairs, giving this segment of the leg a rather thick and muscular appearance. In some species, the hairs extend to the next segment, the tibia. The tibia spurs are thick and prominent. On the hind legs, there is usually an additional pair of tibia spurs in a number of species.


The White Banded Flat has evolved a rather strange tuft of hair on the tibia of its hind legs

In some unique species, like the White Banded Flat, the hind legs are further evolved to have a prominent hair tuft covering the tibia and extending beyond the leg.

So the next time you have an opportunity to observe a butterfly closely, take a look at its legs and see what interesting observations you can make about the legs of different species of butterflies.

Text by Khew SK : Photos by Huang CJ and Khew SK

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