Katie’s Sightings

By Katie O’Hara Kelly

A Bejeweled Blog

This week I spotted two tiny (1 cm) beetles on a Silk Tassel Bush! They didn’t look like anything in my field guide, so I posted them on bugguide.net. Right away they were identified as Leaf Beetles, in the Chrysomelidae Family. Their brilliant, metallic, blue-green color was amazing!!! It made me wonder what causes this iridescence, or “luminous colors that seem to change when seen from different angles.” It occurs naturally in some beetles, some butterflies, and some birds. The color is not from pigment, but rather from structure. Interestingly, the color works as camouflage, not as warning to a predator or as an advertisement for a mate!

The following information from http://www.webexhibits.org/ explains how this coloration works.

“The epicuticle, or outermost surface, of iridescent beetles is made of many stacks of slanting, plate-like layers, which are oriented in different directions. These layers bend, and then reflect the incoming light in the same way as the ridges of iridescent butterfly and moth scales. A layer of pigment below the refractive plates of beetles and the ridges of iridescent butterfly scales enhances the effect of the iridescence. In some species, the epicuticle acts as a reflection diffraction grating to cause iridescence.

Most insect structural colors are in the green-blue-violet range, but red, gold, and copper colors may also be produced in this way. The shade of color and its intensity are determined by several factors, including the thickness and spacing of the layers of the scales, or epicuticle, the number of these layers, and the angle of the incoming light.”

The following information from https://indianapublicmedia.org/amomentofscience explains how this coloration works as camouflage!

“Some beetles have beautiful, shiny carapaces [hard outer shell] that look like metal, or a jewel. That shininess is called iridescence. It’s caused when tiny structures in the carapace interfere with certain wavelengths of light, so that different colors are seen from different angles.

It’s actually fairly common in insects, and the feathers of some birds, too. Beetles don’t use the bright, shiny colors to attract a mate, because in most species iridescence is found in both sexes and during non-reproductive stages of the insect’s life. Neither is irridescence used as a kind of warning to predators, because most iridescent beetles aren’t poisonous.

The best experiment would be to test whether predators are better at finding iridescent of non-iridescent targets, which is what they did next. They took the wing cases of iridescent and non-iridescent beetles and baited them with a tasty mealworm. They put out more than 800 targets like this in a wooded area.

Birds found and ate 85 percent of the non-iridescent targets, but only 60 percent of the iridescent ones. Humans also found the iridescent targets harder to find. The iridescent targets were especially hard to find on glossy leaves. So, as unlikely as it may seem, iridescence is an effective form of camouflage.”

This week some Anna’s Hummingbirds showed up, with the males displaying their amazing, metallic, iridescent colors! Iridescence in hummingbirds is structurally different than iridescence in beetles. Their iridescence is formed in the bubbles found in their pigment producing melanosomes! What??? An article posted below* explains this process.

Before you read it, I wanted to share part of an essay that one of my favorite authors, Brian Doyle, wrote about hummingbirds. Just Google “Joyas Voladoras” by Brian Doyle to read the whole essay, it’s beautiful!

“Consider the hummingbird for a long moment. A hummingbird’s heart beats ten times a second. A hummingbird’s heart is the size of a pencil eraser. A hummingbird’s heart is a lot of the hummingbird. Joyas voladoras, flying jewels, the first white explorers in the Americas called them, and the white men had never seen such creatures, for hummingbirds came into the world only in the Americas, nowhere else in the universe, more than three hundred species of them whirring and zooming and nectaring in hummer time zones nine times removed from ours, their hearts hammering faster than we could clearly hear if we pressed our elephantine ears to their infinitesimal chests.” by Brian Doyle

* Visit https://www.sciencedaily.com for an explanation of how iridescent colors occur in hummingbirds.

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