The butterfly proboscis (plural: proboscides) is an exquisitely evolved instrument for exploiting sources of nectar at the base of flowers. In fact it has evolved in concert – co-evolution – with the flowers it relies on for sustenance, so not surprisingly the length of the proboscis is usually correlated with the depth of the corolla tube of the flower species on which it feeds. The flowers visited by butterflies tend to produce nectar that contains sucrose and is rich in amino acids. Foraging for nectar is one of the most important activities in a butterfly’s life and all species have distinct preferences for certain types of flowers. Nectar provides them with a source of fuel and the proboscis enables them to suck it up. They can also discharge saliva via the proboscis, which is used to dissolve dried up nectar, but – unlike some insects – they cannot suck and spit at the same time! Male butterflies in particular, use their proboscides to obtain additional supplies of sodium and nitrogen – via puddling, sometimes called mud puddling – in order to manufacture spermatophores. They stick their proboscides into all manner of disgusting substrates in order to gain the nutrients they need (see below).

 It is interesting to speculate on how butterflies and flowers might have conspired together to evolve such a close relationship. Butterflies are important pollinators and a flower which attracts certain butterflies on a regular basis has an advantage in terms of less wasted pollen, compared to flowers which attract any old pollinator. So if the flower evolves together with the butterfly towards a structure which only that species can use, i.e. a deep flower for a long tongue, then the flower avoids having its nectar taken by less efficient pollinators. This tight arrangement also prevents other species from taking the nectar. The flower provides it’s dedicated worker with a private source of food. There are however, limits – or constraints – on how long a proboscis can be in relation to the size of the butterfly.

The proboscis is a remarkable combination of rigidity and elasticity and the butterfly has a superb control over its movement. The speed with which they can uncurl the proboscis and guide the tip down into a flower tube – which may be very small and narrow (see below) – never fails to amaze me.

 I would refer anyone interested in the details of the form and function of the proboscis to the papers by Professor Harald W. Krenn who describes the proboscis as follows:

The proboscis “consists of two elongated mouthpart structures that are tightly linked together, at the tip are slit openings for the uptake of fluids. Precise and rapid action of the proboscis is crucial for efficient foraging in flower-visiting butterflies. Apart from the dimensions of the feeding apparatus, the importance of the sensory organs in localizing concealed floral rewards is paramount.” (Link)

The proboscis is not just a drinking straw though! It also functions as a sort of sponge (a ‘nanosponge’ according to Monaenkova et al., 2011) which allows it soak up fluids via capillary action, if I have understood it correctly, as well as by the means of a suction pump in the head of the butterfly. There are differences in the proboscides of nectar and fruit-feeding butterflies. The latter have enlarged chemosensory structures which form a brush near the tip of the proboscis; this helps them when feeding on liquid films of the sort found in rotting fruit (below).

 Many butterflies are sweat-feeders, a form of puddling where humans – and other animals – are the substrate!

The proboscis of most European species usually measures about two-thirds of the body length but reportedly increases to an average of about 80% in Neotropical butterflies. Whether this holds true for all tropical butterflies, I don’t know. It is said that there are few true butterflies (Lepidoptera: Papilionoidea) which have a proboscis that greatly exceeds the length of the body, although I have some doubts about this. One of the longest proboscides – relative to body size – is found in the Blue-winged euyrbia,  Eurybia lycisca (Riodinidae), found from Mexico to Colombia. The proboscis is about twice as long as the body, measuring up to 45.6 mm in actual length (Bauder et al., 2011). Not surprisingly for a butterfly with such a long, thin instrument, it took quite a long time at each flower to uncoil the proboscis and suck up the nectar; the so-called ‘handling time’ per flower. So a really long proboscis comes with some draw-backs, even though it lets the butterfly feed where few others can reach! The the Blue-winged euyrbia is said to fly from flower to flower without recoiling its proboscis, but it is not alone in this respect, many butterflies can be seen flying away from a flower whilst in the process of recoiling their proboscides (see below). 

 Butterflies frequently accumulate pollen on their proboscides (see below) and some species (Neotropical butterflies in the genera  Heliconius and Laparus) have taken this further, such that they can actually feed on pollen, agitating it in a fluid for hours by coiling and uncoiling the proboscis. I wonder whether all butterflies can do this to some certain extent?

Finally, there is the question of colour. Why are the proboscides of some butterflies so brightly coulored? I don’t know. I recall reading somewhere that sap- and fruit-feeders have brighter proboscides than nectar feeders, but I’m not sure if this is correct? Is there any relationship between colour and function. I don’t know. Or is it simply an artifact caused by pigments that have some other function? I would be grateful for any suggestions. So, what better way to finish this blog than with a few photographs of butterflies with brightly coloured proboscides (I prefer probocises!). Like thin neckties, they set off their gorgeous wing colours to great effect. Butterflies just know how to dress to impress!

References

1. Agosta, S. J., & Janzen, D. H. (2005). Body size distributions of large Costa Rican dry forest moths and the underlying relationship between plant and pollinator morphology. Oikos, 108(1), 183-193.
2. Bauder, J. A., Lieskonig, N. R., & Krenn, H. W. (2011). The extremely long-tongued Neotropical butterfly Eurybia lycisca (Riodinidae): proboscis morphology and flower handling. Arthropod structure & development, 40(2), 122-127.
3. Kunte, K. (2007). Allometry and functional constraints on proboscis lengths in butterflies. Functional Ecology, 21(5), 982-987.
4. Krenn, H. W. (2010). Feeding mechanisms of adult Lepidoptera: structure, function, and evolution of the mouthparts. Annual review of entomology, 55, 307-327.
5. Lehnert, M. S., Monaenkova, D., Andrukh, T., Beard, C. E., Adler, P. H., & Kornev, K. G. (2013). Hydrophobic–hydrophilic dichotomy of the butterfly proboscis. Journal of the Royal Society Interface, 10(85), 20130336.
6. Monaenkova, D., Lehnert, M. S., Andrukh, T., Beard, C. E., Rubin, B., Tokarev, A., … & Kornev, K. G. (2011). Butterfly proboscis: combining a drinking straw with a nanosponge facilitated diversification of feeding habits. Journal of the Royal Society Interface, rsif20110392.
7. Romeis, J. O. R. G., Städler, E., & Wäckers, F. L. (2005). Nectar-and pollen-feeding by adult herbivorous insects. Plant-Provided Food for Carnivorous Insects: A Protective Mutualism and its Applications. Cambridge University Press, Cambridge, UK, 178-219.