City Creek Nature Notes – Salt Lake City

November 11, 2016

November 11th

Filed under: Insects, People, Plants, Robber fly, Teasel — canopus56 @ 5:59 pm

Bee Killer

9:30 a.m. The bloom of a dried teasel (Dipsacus sylvestris) just below the red bridge at mile 0.8, is surrounded by four long spines. A single Robber fly (Promachus fitchii), with its distinctive segmented long abdomen and hairy body and legs, is wrapped around and hanging upside down from a spine. This example probably is Promachus fitchii, but Nelson at Brigham Young University notes that there are 50 types of robber flies in Utah. The air is freezing cold and it seems late in the year to see one. This Robber fly is torpid and unmoving; it is waiting for the morning sun to arrive. We are instinctively repelled by flies because we mostly have experience with the unclean, common house fly. Common house flies live in the canyon on trash can rubbish and on fecal matter at picnic site septic tank toilets.

Because this freezing Robber fly cannot move, I am able to examine it very closely. The Robber fly is fascinating in its many details, and it has characteristics in common with bees such as hairs on its thorax, head, and legs, and its tubular segmented abdomen, but the Robber fly’s mouth is looks like a short, fat proboscis. The physical features of this Robber fly makes me question whether bees evolved from flies, and not wasps, but British bee expert Goulson concludes that wasps are the grandparents of bees. Robber flies are predators: they hunt bees and other small insects, and they have the nick-name of “bee-killer”. The fat proboscis of the Robber fly is not for nectar drinking. The fly’s proboscis is a stabbing instrument, and it plunges the spike into its prey, and then injects venom. While there are several types of smaller bees in the canyon, this Robber fly is perhaps three-sixteenths of an inch in size, and I cannot image it taking on a larger red-rumped central bumble bee. Small bees are sparsely present in the canyon this late in the year, and about one bee can still be seen every other day along the first mile of the road, including today.

1 p.m. It is holiday, and when I return for a second jog, the canyon parking lot is full. There are about a seventy people on the road to mile post 1.5. Two have fishing poles, and trout seen in pools in the morning are now all hiding.


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