Do butterflies eat dirt?

Canadian tiger swallowtail (and one pale swallowtail) butterflies near Dutch Lake, Clearwater, BC.

I have seen this before, in fact, many times.  I wrote a similar blog on this subject here.  Butterflies on dirt, proboscis extended, probing the substrate for something.  But what?

It turns out that many butterflies do this.  The soil they are on is usually damp.  The tiny tube, called a proboscis, has an internal diameter of less than 0.04 mm (about 40 microns).  To give you an idea how narrow this is, you couldn't insert a human hair into it; it's too narrow.  

One advantage of the extremely narrow gauge of the proboscis is that fluids will naturally climb it through a process known as capillary action.  There is also a small muscular organ called the cibarial pump.  This is found in many sucking insects; it creates a low-pressure area at the end of the tube that facilitates fluid motion.  The combination of the two pulls liquids towards the digestive tract.  I found a good diagram of the organ here.

The fluid they are extracting is water, but there are also salts.  Some butterflies, especially the ones that are active early in the spring, search out decaying matter to extract water, salt, and any viable nutrients. The morning cloak butterfly is a good example of this (see my blog here). 

I often find several butterflies taking briney fluid from damp, sandy deposits at the same time, but I have never seen so many at once.  It was a wonderful moment.

Thanks for reading.

Eric Svendsen     www.ericspix.com