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Forelegs and four legs in butterflies!

The subject of reduced or modified forelegs in butterflies might strike some people as being rather obscure, but it is – as I hope to demonstrate – a fascinating tale of evolution and adaptation. In addition, there is an awful lot of basic biology and behaviour which is not fully understood in many of our common butterflies in the United Kingdom and Europe. The extent to which the forelegs (prothoracic, or T1 legs) of butterflies are reduced in size in different species – and also how they differ between the sexes – has traditionally been something which taxonomists got interested in. For example, they have used it to classify butterflies into different families. Butterflies probably started out, in the dim and distant past, with six equally sized legs, but over time the front pair of legs have been reduced or miniaturized in some families, and in many cases their function has changed from that of walking, to that of chemoreception and mechanoreception. In other words, the front legs evolved, and were co-opted into a different use: that of sensing, or detecting different stimuli: touch and taste for example. And it is likely that some of these reductions occurred independently (convergent evolution) over time (Wolfe et al., 2011).

As is commonly known, brush-footed butterflies (Nymphalidae) only have four (two pairs) of fully functional legs; the forelegs are markedly reduced – or even vestigial – and are not used for walking or perching. The forelegs are highly reduced and vestigial in many common temperate species, like the Painted lady (Vanessa cardui) and the Red admiral (Vanessa atalanta) for example. The tiny little appendages are covered in hairs and are pressed tightly against against the thorax (below). As far as I am aware they are not mobile?

Red admiral (Vanessa atalanta) on bell heather with right vestigial foreleg highlighted. Galicia, Spain. Photograph by Raymond JC Cannon
The painted lady (Vanessa cardui) on bell heather, Galicia, Spain. Vestigial foreleg highlighted. Photograph by Raymond JC Cannon

In other nymphalids, like graylings (of many species), there are tiny little legs which hang down in front of the thorax. There is a good photograph of a Woodland Grayling (Hipparchia fagi) on the UK Butterflies website (Female Underside Photo © Roger Gibbons) which shows this nicely. I have also managed to capture photographs of Rock Graylings (below) where the forelegs are visible (below). The fact that they are not fixed against the body, suggests that they may be used by the butterflies, for example to ‘taste’ leaves by the female to see if they are suitable for laying eggs on. I am only guessing, based on what is known about Heliconius species (see below).

Rock Grayling (Hipparchia hermione) with reduced forelegs highlighted. Galicia, Spain. Photograph by Raymond JC Cannon

Females of Heliconius butterflies – such as Heliconius erato and H. melpomene (below) (Lepidoptera, Nymphalidae) – are well known for ‘tasting’ plant surfaces with their atrophied foretarsi, by ‘drumming’ on it (Bell, & Cardé, 2013). Females of other very different species, such as Pieris brassicae and P. rapae (Pieridae), for example, also carry out this drumming behaviour on leaf surfaces (Fox, 1966). This behaviour probably liberates plant saps, which the butterfly is able to taste using gustatory receptors (Briscoe et al., 2013), and thus, determine the chemical properties of the plant. In some Heliconius species, the male forelegs lack the chemosensory hairs needed to taste the plant, whilst female forelegs (the foretarsi again) have groups of special sensory hairs which they use to identify sugars and amino acids, as well as secondary plant compounds (Briscoe et al., 2013).

Heliconius melpomene with atrophied forelegs highlighted. By Richard Bartz, Munich aka Makro Freak – Own work, CC BY-SA 2.5

Some lycaenids (Lycaenidae), such as Miletus species in the sub-family Miletinae, also hold their forelegs closely – i.e. adpressed – against the body (see below).

Narrow-banded Brownie (Miletus mallus) with adpressed forelegs- Da Lat, Vietnam. Photo by Charlie Jackson (Flickr CC) with adpressed forelegs

The forelegs of the males can also be reduced in some lycaenid species, with the terminal claws absent (Robbins, 1988; Braby, 2016).  For example in Jalmenus evagoras, the imperial hairstreak (see below).

Jalmenus evagoras male with slightly reduced forelegs highlighted. Photo by Ross Field Wiki CC BY 3.0 AU

Riodinids (Riodinidae) are also unusual in that only females have a full complement (six) of fully developed legs! In males, the forelegs are largely absent, or reduced to non-functional, hairlike tufts (similar to nymphalids) according to an excellent blog on the Butterflies of Singapore website (The Butterfly Legs). Females tend to fold their forelegs up against the front of the thorax, and stand on the four ‘back’ legs (see below). But the forelegs look functional to me, and I would not be surprised to learn that they are covered in ‘taste hairs’ just like Heliconious butterflies? There is so much that could be found out about the function of these appendages using modern techniques, e.g. to identify chemoreceptors.

Punchinello (Zemeros flegyas) female on a leaf with forelegs held high. Thailand
The Tailed Judy (Abisara neophron chelina) female with forelegs held above leaf. Chiang Mai, Thailand. Photograph by Raymond JC Cannon

In skippers (Hesperiidae), the forelegs are only slightly reduced according to Fox (1966), and is not apparent from casual observation. As far as I am aware, all skippers are perfectly capable of using all six legs! Nevertheless, many skipper species perch on leaves with their forelegs held tucked against the body (below).

Large skipper (Ochlodes sylvanus) male perching with forelegs held up (right foreleg marked by arrow). Spain. Photograph by Raymond JC Cannon

Quite why skippers sometimes perch with their forelegs held against the thorax (like a rabbit!) I have no idea? Perhaps it is an indication that the forelegs are used for different – sensory – purposes? Skippers are the oldest family of butterflies, evolutionary speaking, so presumably there has been little morphological specialization. But might they have evolved some of the sensory structures seen on other butterflies?

Clavate Banded Demon (Notocrypta clavata) with forelegs held above leaf. Chiang Mai, Thailand. Photograph by Raymond JC Cannon
Chestnut bob (Iambrix salsala) with forelegs held above leaf. Chiang Mai, Thailand. Photograph by Raymond JC Cannon

Finally, I am sure that there is much more to this subject than I am aware of. Some of the academic literature is very old and difficult to obtain. If any readers have any observations, particularly on the behaviour and use of reduced forelegs (the front legs!) in butterflies, I would be delighted to hear from you.

References

Bell, W. J., & Cardé, R. T. (2013). Chemical ecology of insects. Springer.

Braby, M. (2016). The complete field guide to butterflies of Australia. Csiro Publishing.

Briscoe, A.D., Macias-Munoz, A., Kozak, K.M., Walters, J.R., Yuan, F., Jamie, G.A., Martin, S.H., Dasmahapatra, K.K., Ferguson, L.C., Mallet, J. and Jacquin-Joly, E., 2013. Female behaviour drives expression and evolution of gustatory receptors in butterflies. PLoS genetics9(7).

Fox, R. M. (1966). Forlegs of butterflies I. Introduction: Chemoreception. J. Res. Lepidoptera5, 1-12.

Robbins, R. K. (1988). Male foretarsal variation in Lycaenidae and Riodinidae, and the systematic placement of Styx infernalis (Lepidoptera). Proceedings of the entomological Society of Washington.

Wolfe, J. M., Oliver, J. C., & Monteiro, A. (2011). Evolutionary reduction of the first thoracic limb in butterflies. Journal of Insect Science11(1), 66.

 

rcannon992 View All

I am an entomologist with a background in quarantine pests and invasive invertebrates. I studied zoology at Imperial College (University of London) and did a PhD on the population dynamics of a cereal aphid (Metopolophium dirhodum) in the UK. I spent 5 years with the British Antarctic Survey studing cold hardiness of Antarctic invertebates and 17 years with the Food and Environment Research Agency. My main interests now are natural history, photography, painting and bird watching.

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