Thursday, September 21, 2017

A Butterfly's Flashy Colors

Male monarch butterfly
(Danaus plexippus)
© Beatriz Moisset
People fall in love with monarch butterflies because of their bright black and orange pattern. Many monarch enthusiasts are busily raising these butterflies in their gardens and homes. Some have become real experts on these insects and their life cycle. They are profoundly distressed when something goes wrong and one of them fails to make it to adulthood and freedom.

Some particularly despise butterfly enemies. If they catch a predatory bug sucking the juices out of an unfortunate caterpillar, they rage against the predator. Parasitic flies also generate a violent reaction. It is hard to believe, but monarchs owe their beautiful colors to their enemies. Here is the whole story.

Monarchs feed on milkweeds. They are dependent on these plants and cannot digest others. Milkweeds, like many plants produce powerful toxins as a defense against herbivores. These toxins go by the name of cardiac glycosides because they cause heart paralysis. As an additional defense they produce a sticky milky-looking substance that gives them their name. The milk is present in most tissues of the plant and bleed easily, gumming the yaws of a hungry attacker that tries to eat the milkweed plant.

This is enough to deter most plant eaters, but monarch butterfly caterpillars and more than a dozen other creatures have learned to overcome such defenses. Earlier milkweeds, millions of years ago, had milder forms of the toxins. That was all they needed, but some early insects learned to tolerate them and proceeded undeterred to feed from these plants. Thus, milkweeds were forced to create stronger and stronger glycosides and, in turn their feeders found ways to deal with the more powerful toxins. Arms races of this type abound in the natural world.

The monarch butterfly adapted itself to these plants by several means: it avoided the most toxic plants or their most toxic parts; it developed enzymes that could deal with the toxins, or it stored them in parts of its body where they could do no harm. In doing all this, it became dependent on milkweeds. This dependence added a bonus to the monarch's survival, its body is loaded with bad tasting, toxic glycosides, which constitute a powerful defense against its enemies. Most predators avoid the toxic butterfly. However a handful of these predators developed ways to handle the monarch's toxins by eating only the parts with less glycosides, or by evolving enzymes that neutralize these toxic substances. This is another case of the arms race at work.

This is not all. The monarch advertises its toxicity and horrid taste to possible predators. Birds who never saw a monarch butterfly before eagerly take a bite of one. The immediate reaction is that of disgust, spitting up the morsel and shaking their heads or rubbing their beaks in an effort to remove the unpleasantness. They have no trouble remembering the strikingly colored creature and its bad taste. They are not likely to repeat such experience.

Large milkweed bugs (Oncopeltus fasciatus)
on milkweed seed pod. Adults and nymphs
© Beatriz Moisset
Milkweed beetle (Tetraopes tetrophthalmus)
© Beatriz Moisset
The monarch butterfly is not the only animal that advertises its bad taste by sporting bright colors. Other insects that feed on milkweeds, like the milkweed beetles and milkweed bugs, are also colorful, in red and black; we can be sure that they are just as bad tasting. Similar cases abound in nature, not only insects but also vertebrates. Most frogs are green or have brown spots, colors that blend well with vegetation and help them remain unnoticed. The so-called poison dart frogs are the exception. Their backs are glossy red. The name tells you that these frogs produce powerful toxins, so much so, that native peoples use them to smear the tip of their darts in order to make them more lethal.

Dendrobatid frog, Peru
© Tim Ross. Wikicommons

Polished Lady Beetle (Cycloneda munda)
Ladybeetles are another example of
brightly colored bad tasting insects
© Beatriz Moisset
So, as I said at the start, a monarch's lovely colors are due to a constant battle with their enemies. In a perfect world (perfect for monarchs, that is) these butterflies wouldn't need to be loaded with toxins, nor would they need to tell their enemies to keep away. In such a perfect world, monarchs would have plain colors. Is this what we want?

The monarch caterpillar is also toxic
and also has bright warning colors
© Beatriz Moisset

Further readings:

Sunday, September 3, 2017

Imitation is the Best Form of Protection in Some Cases

Not a bee, but a mimic, the fly Eristalis© Beatriz Moisset
A stroll among the flowers allows me to see numerous bees of different sorts, zipping along from blossom to blossom. Just as numerous are other winged insects that look very much like bees. The experienced eye recognizes them for what they are, flower flies impersonating the stinging insects. But those unfamiliar with bees or with insects, in general, may assume that these flower flies are bees. They may even recoil in fear of a sting although the flies are innocuous, lacking such powerful weapon.

A wasp mimic, Spilomyia sayi © Beatriz Moisset
Those flies who imitate bees or wasps have a reason for playing this masquerade. They are not interested in fooling us, humans. Their deception is aimed at their predators, mostly birds. However, the ruse is so effective that it even we fall for it.

Syrphid flies, also known as flower flies or hover flies, are almost as assiduous in visiting flowers as bees are. They drink nectar and sometimes also feed on pollen. Although not as efficient as bees as pollinators, they deserve some credit and occasionally their contribution to pollination is significant. Another group of flies that visit flowers with great frequency includes the so called bee flies.

Both, flower flies and bee flies mimic bees or, in some instances, wasps. The imitation is very specific in some cases. The so called drone fly, Eristalis tenax, looks like a honey bee. The name is appropriate, considering that male honey bees, drones, have large eyes, and these flies have even bigger eyes. Both, the fly and the honey bee are European. The imitator evolved along with its model in that continent. Another European flower fly that mimics honey bees effectively is the narcissus fly, Merodon equestris.

Other flower flies and also a robber fly imitate bumble bees. They have a furry coat and even buzz like bumble bees.

Bumble bee mimic, Mallota bautias © Beatriz Moisset
The smaller flies, members of the Syrphinae subfamily apparently are mimics of some or another of the many solitary bees. In most cases one cannot be sure of the model chosen for this mimicry.

A honey bee, the model for many mimics © Beatriz Moisset
It is a peculiar thing that some imitations are extremely good, while others are rather general in character. Biologists speculate that even a not-so-good imitation may serve the purpose of deceiving the enemies and this is why such types of mimicry persist in nature. It is also possible that the predators don’t see exactly what we see and the mimicry is convincing enough for them.

This is just one type of mimicry. Other insects take the appearance of their surroundings, which makes them nearly invisible. Still others look like bird poop, not an appetizing sight for a snack seeker. Stay tuned for descriptions and illustrations of more of these ingenious survival mechanisms.