Residents of Tucson awoke on the morning of August 9, 1966, to a sight that must have seemed almost apocalyptic.  The normally fierce summer sun was invisible, obscured by the fluttering wings of millions of butterflies.  The streetlights had to be turned on to keep the city from grinding to a halt.

The culprit was the ignobly named American Snout (Libytheana carinata), one of our most eccentric butterflies.  Often placed in their own tiny family, Snouts are most easily distinguished from their various nymphalid (brush-footed) cousins by their labial palps, the hyperextended mouthparts for which they’re named.  Such an extravagant appendage is bound to have a vital function, and in the Snout’s case, it seems to function in camouflage:  perched nonchalantly on a twig, with only the mottled underwing visible, the creature looks remarkably like a dead leaf of which the snout is the stalk.

In most of the East, the Snout is present in modest numbers anywhere a concentration of hackberries–the caterpillar’s food plant–is found.  It’s in the arid, unpredictable Southwest that Snouts really hit their stride, periodically undergoing staggering, short-lived, apparently one-way migrations.  A legendary Snout flight in south Texas in September 1921 is thought to have included over 6 billion butterflies, with as many as 25 million per minute passing along a 250-mile front.  Oddly, the great majority of these vagabonds seem to be males.  What’s the story here?

As lepidopterists understand it, the first overture to a “Snoutbreak” (forgive me) is usually an extended drought period.  Snouts are well adapted to drought, putting their reproductive processes on hold and patiently waiting things out, while many of their enemies–especially the tiny wasp Brachymeria, which lays eggs in Snout pupae–suffer population hits.  When a big summer rainstorm arrives after such a period, the similarly adapted Spiny Hackberry or Granjeno tree puts out a massive flush of new foliage, attracting swarms of egg-laying female Snouts.  The caterpillars emerge very quickly, and without parasitoids around to cramp their style, they are capable of totally defoliating vast areas of Granjeno.

What happens next is disputed, but one theory is that once this boomer generation has pupated and emerged as adults, the different sexes meet with different success.  In particular, newly emerged females may prefer the more worldly males of the previous generation–leaving thousands or millions of young bachelors to take flight and aim their unrequited lust elsewhere.  Understandably, they can be quite single-minded in their mission; south Texans during a Snoutbreak have reported needing to brush all the Snouts off their clothing before going indoors.

The mass migration of the American Snout, one of the great insect spectacles, meets its match in the migration of its congener the African Snout, as this picture shows.  I don’t know much about the African Snout except that its host plant is the African Hackberry or “Stinkwood,” a plant with various reputed magical powers.  One source states matter-of-factly:  “Used with crocodile fat against lightning.”  A handy tree to have around.

Photo credits:  Clinton & Charles Robertson (upper), Bruce Marlin (lower)