Congratulations to Stein Feick, the first person to correctly identify the Mystery Photo as a Whirligig Beetle! You usually see this aquatic beetle swimming around and around in circles on the surface of a pond searching for prey. A unique feature of most beetles in this genus is their divided eyes. Each eye is completely separated into two portions (see photo). One portion (dorsal) is above the water line and the other (ventral) is beneath the water on each side of their head, allowing them to see both in the air/on the surface of the water as well as under the water. The dorsal eyes have a limited field of view, so these beetles rest one of their antennae on the surface of the water to help them detect any motion caused by prey.
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There is a group of butterflies known as greater fritillaries, or silverspots (their underwings often have multiple silver spots). Three species of greater fritillaries can be found in the Northeast: Great Spangled, Atlantis and Aphrodite. All three are similar in appearance, with differences so subtle that the butterfly in a Naturally Curious post last month was mis-identified as a Great Spangled Fritillary, when it was actually an Atlantis Fritillary. Thanks to a sharp-eyed reader, Sue Elliott, this was brought to my attention. If you can approach a fritillary close enough to see the color of its eye, identification is a snap! Great Spangled Fritillaries have amber-colored eyes, Atlantis Fritillaries have blue-gray eyes, and Aprhrodite Fritillaries have yellow-green eyes. Can you identify the two species of fritillaries that are pictured? (Upper right, on thistle – Great Spangled Fritillary; main photo, on Joe-Pye Weed – Atlantis Fritillary)
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Dragonflies (and bees) have the largest compound eyes of any insect, each containing up to 30,000 facets, or ommatidia (house flies have 6,000). Each facet points in a slightly different direction and creates its own image, and the dragonfly’s brain compiles these thousands of images into one picture. This eye structure enables dragonflies to be extremely sensitive to motion. Because a dragonfly’s eyes wrap around its head, it can see in all directions at the same time (though its forward-looking vision is the sharpest). When capturing prey, a dragonfly doesn’t chase it – it intercepts it in mid-air, and it’s successful nearly 95% of the time. This hunting technique entails calculating the distance of its prey, the direction it’s moving and the speed that it’s flying – an impressive feat any tennis, baseball or football player would especially appreciate!
(Photo is looking down on the eyes of a Common Green Darner. Three simple eyes, or ocelli, are located in the black section below (above in the photo) its two tan compound eyes. The short, thin black lines are its antennae, which can detect wind direction and speed. The yellow section is part of the upper half of its face, or frons.)