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My favorite bee, Augochlora,
on cone flower.
© Beatriz Moisset
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I often marvel at the exquisitely colored bees that visit my
flowers. My favorite ones have a metallic green sheen that makes me think of
them as miniature robots. How do they get such an interesting hue? Many other
insects, such as wasps, flies, butterflies and beetles, also dress up on shiny
armors. Metallic colors range from blue to copper and even red, with green
being perhaps the most common. I decided to learn more about these shimmering hues,
so different from ordinary colorations.
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Another view of my favorite bee, the pure golden-green bee (Augochlora pura)
© Beatriz Moisset
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It turns out that biologists have a lot to learn about this
subject. The physics is quite complicated and I will not attempt to give more
than the simplest explanation here. Most colors we see in nature are produced
by pigments. The shiny and iridescent effects we see on certain animals are
caused, not by pigments, but by tiny structures in the cuticle (the skin) of
insects and thus they are called structural colors. Those microscopic ridges or
plates or cross ribs make the light rays bounce off, scattering them in ways
that produce special effects. The term iridescence lumps together three
different types of chromatic effects: metallic looks, spectral iridescence
(rainbow effects) and opal-like effects. Peacock feathers and Morpho butterfly
wings are fine examples of iridescence. Some metallic colors observed in
insects show a certain degree of iridescence. They change from green to copper
or red depending on the angle of the light rays
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Parasitic wasp, Perilampidae.
© Beatriz Moisset
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Pigments tend to decay after death, so ordinary tints usually
fade away. Museum collections of dead specimens may look rather boring. In
contrast, structural colors remain vibrant for a long time because the
miniature structures don't change as long as the cuticle is intact. They may
even be present in fossils.
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Green-bottle blowfly, Lucilia.
© Beatriz Moisset
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Let us get back to metallic colors, what function do they
serve? Why do so many different insects wear these shimmering hard-looking
coats? Some biologists think that structural colors serve functions of
camouflage, signaling or disguise. What do these words mean? Can we find
examples of these functions?
"Camouflage" enables the insect to blend with its
surroundings and escape notice by predators. Let us look at my shiny bee. It
blends moderately well with the foliage on which it often rests despite the fact
that it is not a perfect imitation. It turns out that, while plants make green
pigment in abundance, insects have trouble producing such pigment. The best approximation
to the appearance of leaves is a structural color.
"Signaling" refers to sending a sign or a coded
message to others. It may be a warning to predators, such as: I am poisonous;
you don't want to eat me. It may be a message to other males of the same
species: I am stronger, I am brighter; you'd better give up. Or, it may be
telling females: I am the best male around.
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Dogbane beetle.
© Beatriz Moisset
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Finally "disguise" is a little different from
camouflage. It is an imitation or mimicry of something else. The purpose is to
deceive the observer
I hope you enjoy this gallery of metallic insects. It
includes bees, wasps, flies, beetles and a butterfly. You may recognize some of
these beauties in your garden or notice other ones.
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| Augochloropsis, a relative of Augochlora. © Beatriz Moisset |
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| Parasitic wasp, Torymus. © Beatriz Moisset |
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| Cuckoo wasp. Chrisididae. © Beatriz Moisset |
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| Longlegged fly, Dolichopodidae. © Beatriz Moisset |
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| Syrphid fly, Copestylum. © Beatriz Moisset |
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| Tiger beetle. Cicindelidae. © Beatriz Moisset |
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| Buprestid beetle. © Beatriz Moisset |
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| Mating dogbane beetles. © Beatriz Moisset |
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| Red spotted purple butterfly. © Beatriz Moisset |
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| © Beatriz Moisset |
List of articles
© Beatriz Moisset. 2014
