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Can anyone identify this large fly?

I encountered it in Okefenokee Swamp (Georgia, U.S.A.) on May 1, 2015. Weather: 53-78°F (avg. 66°F), avg humidity = 60, dew point = 47°F.

The fly was about the size of an adult thumb (more accurately approx. 5cm long and 2cm wide) -- this fly was HUGE! It was fairly docile and allowed me to take a good close-up or 2.

Large Swamp Fly

enter image description here

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Tabanus nigrovittatus as my initial guess. "Greenhead horse fly."

enter image description here

Source

This is probably the biggest give-away for the genus Tabanus (namely the features of the veins at the wing tip, denoted R4 and R5, versus other Diptera orders), still searching out a good identification key to solidify the ID. Furthermore, also known as the salt marsh greenhead, a swamp is a good candidate to find these (1).

So what's particular is entomologists have gone through and collected many specimens, and data about their wing venation, and based on the Comstock-Needham system, you can pin presence/absence of major veins, and branching of said veins to specific types on insects. In this case, R refers to the Radius or 3rd longitudinal vein, and the number refers part of the branching that reaches the margin of the wing. Pg 156 here helps us ID Tabanus vs other orders.

Problematic with identifying these spp. is you need to get a good look at the sternites on the under side as well, if you look at how they ID Tabanus sudeticus here, but the photos tend to match nicely with those of T. nigrovittatus you can widely find.

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    $\begingroup$ You should add a bit on why you think this a good fit (traits etc), but my first impression was also Tabanus sp. As the moment, this is more of a comment than a full answer. FYI it was caught by the low-quality review queue "because of its length and content". $\endgroup$ – fileunderwater Jan 22 '16 at 19:39
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    $\begingroup$ Believe it or not it's really hard to find a good identification key for Tabanus species! $\endgroup$ – CKM Jan 22 '16 at 19:51
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    $\begingroup$ @theforestecologist I've done so in an edit to my post, let me know how I can better clarify this! $\endgroup$ – CKM Jan 23 '16 at 20:55
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    $\begingroup$ @theforestecologist Most of the nomenclature discrepancy I suspect is from the fact the article by Hamilton is trying to do something different. I can't quote you on when the distinction was made, but R2+3 means that the vein R2 and the vein R3 coalesce through their extent in a single vein. Here's your image source biodiversitylibrary.org/page/6270326#page/328/mode/1up .. for more on the nomenclature, here $\endgroup$ – CKM Jan 23 '16 at 22:20
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    $\begingroup$ It's my thought while researching this that Tabanus is rather poorly defined! $\endgroup$ – CKM Jan 23 '16 at 22:24

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