Bot Fly

Giant Fly
Location: Yakima, WA
November 1, 2010 4:18 pm
Biggest dipteran I’ve ever seen.
Signature: Paul Huffman, President-for-Life, Moclips Surf Club

Bot Fly

Hi Paul,
We strongly believed that you had submitted an image of a Bot Fly in the family Oestridae, but most individuals we have identified in the past are marked with black and white patches similar to the patterns on a Holstein milk cow.  We quickly found a matching photo on BugGuide that is identified as the Bot Fly
Cuterebra tenebrosa, and Natalie McNear from Marin County California who submitted the photo wrote:  “Looking on here it most closely resembles the New World skin bot flies of the subfamily Cuterebrinae, but I don’t see any on here that are all dark with a metallic blue abdomen.”  There is a comment by Jeff Boettner on the posting that indicates:  “I am pretty confident this one is likely Cuterebra tenebrosa. There are a few other species that have all black females, but you have shots from all angles, so likely this is correct. The bot uses Neotoma (wood rats) as a host. … There is a very robust comment dialog on that posting that is well worth the time to read.  The genus information page on BugGuide provides this information on the life cycle of the Bot Flies:  “Females typically deposit eggs in the burrows and ‘runs’ of rodent or rabbit hosts. A warm body passing by the eggs causes them to hatch almost instantly and the larvae glom onto the host. The larvae are subcutaneous (under the skin) parasites of the host. Their presence is easily detected as a tumor-like bulge, often in the throat or neck or flanks of the host. The larvae breathe by everting the anal spiracles out a hole (so they are oriented head-down inside the host). They feed on the flesh of the host, but only rarely does the host die as a result. Bot Flies are also known as Warble Flies. These Bot Flies really are quite large and they resemble bumble bees in both appearance and sound.

Bot Fly
Photo of author


BugMan aka Daniel Marlos has been identifying bugs since 1999. is his passion project and it has helped millions of readers identify the bug that has been bugging them for over two decades. You can reach out to him through our Contact Page.

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