City Creek Nature Notes – Salt Lake City

September 20, 2016

August 11th

Filed under: Birds, Dragonfly, Hawk, Insects — canopus56 @ 10:50 pm

Dragonfly Feast

5 p.m. The Sun is lower in the sky at 5 p.m. and is less fierce as compared to the summer solstice over 45 days ago. At mile 0.3 at a bend in the road, the group of about 15 dragonflies are doing dramatic acrobatic maneuvers. They’re doing rapid right angle turns, barrel rolls, and inverted loops. Because they are back lit against the low Sun on the western ridge, the cause of their gyrations is also visible. The air is filled with small gnats. Each gnat is barely one fiftieth the size of a dragonfly and if not back lit, they would be invisible. The wild flight of the dragonflies are attempts to catch the gnats in flight. The dragonflies wait for the gnats to fly up towards the Sun and then streak in from behind or below. At the last second the dragonflies flip over and try to catch their prey. Presumably, the dragonflies are waiting for the gnats to be blinded by the Sun as they fly upwards. But the physics of the matter is against the dragonflies. Being so much smaller and lighter, the gnats can make instantaneous right angle turns that the dragonflies cannot match. Being much smaller, the distance between the brain and muscles of the gnats is so much less that their reaction time is much faster than the dragonflies’. As a result, the dragonflies are rarely successful. I follow one dragonfly who manages to catch a meal on its sixth attempt. When I run past this spot again on my way out of the canyon, it is in complete shade, but now the swarm of dragonflies has grown to about 50 individuals. High above, a hawk circles.

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