Friday, August 10, 2012

Sharing the Risks and Dividing the Dangers

A quote from the preamble to the U.S. Constitution sums up the idea of Müllerian mimicry very well – "We the people ... in order to ... provide for the common defense, promote the general welfare ..."

Müllerian mimicry is named in honour of the German naturalist Fritz Müller, who first observed and offered an explanation for the phenomenon. Simply put, several species, not necessarily related, that have predators in common will also tend to mimic each other and share aposematic colors or warning signals. Of course, unlike the U.S. model there's no conscious or intelligent decision to co-operate, and the mutations that eventually give rise to a given defensive game plan are random – but the process of natural selection is not. A given shared color scheme benefits and promotes the survival of all the individuals within the species that share it, and it's survivors that (naturally!) hand their genes down for another roll of the dice to the next generation.

Members of the order Hymenoptera that can sting tend to exhibit bold black-and-yellow stripes. A child, a bird, or a frog that has been stung by a Honeybee will likely remember the painful experience and in future also avoid a Bumblebee, a Paper Wasp and a Mason Wasp – the individual Honeybee (a non-reproductive worker in any event) loses, but its relatives and the other Hymenopteran species gain a measure of immunity from predation.

Beetles belonging to the family Coccinellidae, better known as Ladybird Beetles or Ladybugs, secrete toxic alkaloids to discourage predators, exuding fluid from their leg joints if threatened. The colors and patterns of many species are variations on a similar black-and-red theme, and not surprisingly, their family name is derived from the Latin word for scarlet.

Three-Banded Lady Beetle (Coccinella trifasciata perplexa)

A mating pair of Polished Lady Beetles (Cycloneda munda)

Convergent Ladybird (Hippodamia convergens)

Glacial Ladybird (Hippodamia glacialis glacialis)

Milkweeds produce cardiac glycosides to ward off insects, but over time a diverse assortment of insect species have adapted and appropriated the milkweed's chemical defense for their own protection. Such insects sequester the toxins in their own bodies and boldly advertise their noxious natures with a combination of showy black and red/orange colors.

The well-known Monarch (Danaus plexippus)

Small Milkweed Bug (Lygaeus kalmii)

Large Milkweed Bug (Oncopeltus fasciatus)

Larvae of the Milkweed Tussock Moth (Euchaetes egle) ... a group of these caterpillars can annihilate a leaf, within a short time there's little left but the veins and frass.

Red Milkweed Beetles (Tetraopes tetrophthalmus) – their scientific name, derived from Greek, means four eyes. Apropos, as their antennae arise in a manner that divides their compound eyes into a pair of eyes above and below the antennae. And in the background it looks like there's an exception to the black-and-red milkweed color rule, a tiny Monarch caterpillar.

Another image of the Monarch larva, and that's nature for you ... as soon as you think you've found a rule or underlying logical principle, there's a departure or deviation that challenges your assumptions as to how things work.

On the other hand – Monarch caterpillars aren't toxic from the get-go when they hatch, they acquire their bad taste over time as they consume milkweed leaves. It's been my observation that the yellow, white and black stripes on the tiny first instar larvae are indistinct and blend together to form green, thus camouflaging the caterpillars while they are still vulnerable to predators. And the strongly marked later instars do in fact possess the black-and-yellow aposematic warning scheme common to many Hymenoptera. So it seems that the Monarch participates in two different Müllerian defensive strategies throughout its life cycle.