Pollinators: My Metallic Green Bees

My favorite bee, Augochlora,
on cone flower.

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.

Another view of my favorite bee, the pure golden-green bee (Augochlora pura)

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.

Parasitic wasp, Perilampidae.

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.

Green-bottle blowfly, Lucilia.

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.

Dogbane beetle.

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.