A swarm of puddling swallowtails

By Jeff Mitton

I had been at Heil Ranch the previous day and had spotted colorful melissa blue butterflies, but gusty winds kept me from getting a decent photo. Now I was returning to get a good photo of the tiny butterfly.

The arid heat of late May had evaporated the water from a shallow ephemeral pond near Heil Ranch, exposing a shallow depression of slippery mud. While mud is not high on the aesthetically pleasing scale, it certainly attracts butterflies. Knowing that, I glanced at the muddy spot as I drove by and was astounded and delighted by the sight. I hit the brakes and swung around.

Swarms of butterflies congregate at moist mud to collect salt and minerals, an activity called "puddling." On this day, nine species formed a brilliant, flickering swarm on the mud.

Pale swallowtails, Papilio eurymedon, were the most abundant. Dozens of these large butterflies packed together, bumping and jostling for positions on patches of mud that were evidently more alluring than other patches of mud. Pale swallowtails are predominantly black and white accented with brilliant patches of blue, orange and yellow on the upper and lower surfaces of the tails.

Western tiger swallowtails, P. rutulus, were less common, but they mingled with the pales. Western tigers have similar color patterns except that they are yellow where pale swallowtails are white.

Three species of black swallowtails were also present: black swallowtails, P. poyxenes, anise swallowtails, P. zelicaon, and short-tailed swallowtails, P. indra. Black swallowtails are velvety black with yellow spots on the upper wings, orange spots on the lower wings and yellow dots on a black body. Anise swallowtails are similar to westerns except that the yellow on the wings has a different pattern. Short-tailed swallowtails have, indeed, much shorter tails.

In addition to the five swallowtail species, orange and black northwestern fritillaries, Speyeria hesperis, floated gracefully about, but did not mingle with the swallowtails. Showy green and orange green skippers, Hesperia viridis, were also puddling, but they seemed aloof and solitary. Silver spotted skippers, Epargyreus clarus, were the most wary in the swarm and would not allow me to approach.

A few of the tiny, brilliant melissa blues, Plebejus melissa, were present, and once they began sipping from the mud I was able to get very close. A pale blue glow seemed to emanate from their upper wing surfaces. Their underwings had a pale blue and silver background with small black dots and arcs fringed with white and rows of brilliant orange edged with black. Their antennae have alternating white and black bands and their body is covered in fine white hairs -- they are a fluffy as kittens.

Some avid lepidopterists noticed the spectacle and joined me. At one point something spooked the swallowtails and as they took flight, we all froze. The swarm swirled around and among us several times and then settled down to resume puddling. It was enchanting.

Puddling butterflies can be a mixture of several species, but they are usually young males. And while puddling butterflies might be extracting proteins and amino acids from the mud, multiple studies indicate that the most butterflies are seeking salts, usually sodium. Males fly more than females, and vigorous courtship displays generate a greater need for sodium.

Males also increase their reproductive success by passing sodium to their mates during copulation. Females deposit much of the sodium in their eggs, and this investment of the nuptial gift enhances development and survival of offspring.

I left the butterflies with a new insight: Mud is a good thing, not only for its concentrated salts, but also because it attracts swirling swarms of brilliant butterflies.

Jeff Mitton (mitton@colorado.edu) is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

July 24, 2012