Formation of bubble droplets when a Butterfly "pees"
When I was thinking of a title for this blog article, I had originally thought of a more catchy "Bubble Butts" or something to that effect. However, a quick Google uncovered a series of rather uncomplimentary definitions that this particular pair of words alluded to, so I had to decide on a more decent title, lest this blog be subject to a more stringent censorship scrutiny!
The power of high speed photography - A stream of droplets ejected from a puddling FiveBar Swordtail
Over the years of photographing butterflies, particularly those that were puddling, ButterflyCircle members have come across the puddlers forcefully excreting liquid from the business-ends of their abdomens. When puddling, it is quite obvious that butterflies take in copious amounts of mineral-enriched fluids that they require for various biological functions.
Jet-propelled FiveBar Swordtail! The butterfly ejects a stream of fluids whilst puddling
The biological processing of these fluids within the bodies of the butterflies to extract the requisite minerals must be extremely efficient. Once the minerals are absorbed, the remaining fluids are ejected from the butterfly's body, so that there is no superfluous build-up of the excess fluids that the butterfly does not need.
However, it has been observed that in many cases, the excretion of the waste fluids is much slower, as the butterfly passes the fluids slowly, creating a droplet of fluid that remains attached to the tip of the abdomen of the butterfly. Given the viscosity of the liquid that the butterfly excretes, and the low speed at which the liquid forms, a droplet is formed.
Before we delve into the physics of the formation of drops of liquid, let us first recall the reasons why butterflies puddle. In an earlier blog article
, puddling was discussed. Adults of many butterflies, principally males, frequent mud puddles, stream banks that have been contaminated with animal excretions, carrions and bird droppings where their imbibe sodium, chlorides and nitrates that are essential to their biological functions. A study showed that virgin females have reduced longevity. This is attributed to virgins not obtaining important nutrients which the males transfer to females during mating.
When settled down to puddle, some butterflies can remain still for long periods of time, often to their own detriment, as they become vulnerable to nearby predators. At times, their preoccupation with puddling can prove fatal. We have observed, for example along forest road in Malaysia, where puddling butterflies are so engrossed in puddling, that they even forget to fly out of the way of an oncoming vehicle's tyres!
A Burmese Batwing forms a little bubble of fluid (taken in Thailand)
Butterflies from the families Papilionidae, Pieridae, Lycaenidae, Hesperiidae, and to a lesser extent, Nymphalidae, engage in puddling activities. In many instances, the process of excreting the excess fluids from the abdomen has been photographed. Besides the jets of fluid excreted, we have often observed the formation of small bubbles of fluid oozing out from the tip of the abdomens of butterflies.
In physics, a drop is a small column of liquid, bounded completely or almost completely by free surfaces. A drop may form when liquid accumulates at the lower end of a tube or other surface boundary, producing a hanging drop called a pendant drop. When a butterfly excretes the liquid that flows slowly from its anal tube, the liquid forms a drop due to surface tension. A simple way to form a drop is to allow liquid to flow slowly from the lower end of a vertical tube of small diameter.
Source : Wikipedia
The surface tension of the liquid causes the liquid to hang from the tube, forming a pendant. When the drop exceeds a certain size it is no longer stable and detaches itself. The falling liquid is also a drop held together by surface tension. As the drop detaches itself from where it previously hung, surface tension will pinch it and the drop falls as a sphere.
Top : A drop of water falls off after exceeding a certain size when gravity exceeds surface tension forces. (© Wikipedia) Bottom : An Eurema simulatrix excretes a drop of fluid from its abdomen
In butterflies, it is interesting to observe the variations of the size of the drops of excreted fluids before they detach from the anal tube. There doesn't appear to be any direct relation of the size of the droplet to the size of the butterfly. In many cases, the smaller butterflies hold much larger droplets in relation to the size of the butterfly.
The drop of fluid can vary in clarity from crystal clear and totally transparent, to a muddy and cloudy constituency. This is probably dependent on the fluids that the butterfly is feeding on, or the process of digestion that the fluids go through in the alimentary canal of the butterfly.
Two examples of slightly cloudy/murky drops of excretion, compared to the usually crystal clear drop
When feeding on bird droppings, the Hesperiidae exhibit an interesting phenomenon of "recycling" their excretion. With its proboscis extended whilst feeding on a bird dropping, a skipper has often been observed to arch its abdomen and deposit a drop of its own excreted fluid onto the bird dropping and then feeding off the mixture. It repeats the process as it adds its own excretions to the bird dropping to wet it, and then feeds for extended periods of time in this manner, if not disturbed.
Skippers recycling their pee onto the bird dropping that they are feeding on
Amongst the puddlers, butterfly species from the families Papilionidae, Pieridae, Lycaenidae and Hesperiidae tend to display the "bubble drop" phenomenon as they puddle. Of those Nymphalidae that do puddle, we have rarely come across any of the species retaining a bubble at its abdomen. Could it be perhaps that they imbibe smaller amounts of fluid as they puddle? Or maybe they have a slower metabolic process that retains in their bodies, most of the fluids that they imbibe?
In the meantime, keep observing and recording any interesting notes that could yield a better understanding of this simple phenomenon and how each different species of butterfly displays different behaviour. Click on the individual photos for a larger view of these bubbles on the butterflies.
Text by Khew SK : Photos by Chng CK, Antonio Giudici, Khew SK, Koh CH, Loke PF, Lemon Tea, Anthony Wong & Mark Wong