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Giant black fly - Cuterebrinae? - Cuterebra tenebrosa - female

Giant black fly - Cuterebrinae? - Cuterebra tenebrosa - Female
Marin County, California, USA
October 3, 2009
Size: 22 mm body length
This extremely large fly suddenly flew into my house a little while ago. Its size and loud buzzing had me thinking it was a large bumble bee at first, but then I soon realized it was actually a fly. I then assumed it was just a horse fly, but when it landed on the window I saw that the eyes were much too small and it held its wings differently. Eventually I convinced myself that it couldn't bite and worked up the nerve to grab it for some photos. 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.

Images of this individual: tag all
Giant black fly - Cuterebrinae? - Cuterebra tenebrosa - female Giant black fly - Cuterebrinae? - Cuterebra tenebrosa - female Giant black fly - Cuterebrinae? - Cuterebra tenebrosa - female Giant black fly - Cuterebrinae? - Cuterebra tenebrosa - female Giant black fly - Cuterebrinae? - Cuterebra tenebrosa - female Giant black fly - Cuterebrinae? - Cuterebra tenebrosa - female

nice bot! thanks to Natalie for sharing and to Jeff for the ID
Moved from Cuterebra.

I just caught one of these in my house. I put it in a mason jar because i didn't know what it was. I'm glad there was a picture of one on here. Antway, it started laying eggs inside the jar within a couple hours, probly in desperation. i still have it if any of you bug enthusiasts would like it to study.

Cuterebra tenebrosa female
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. They can get in the wrong hosts, if you had cuts on your hand or touched your eye. It would be pretty hard to get this bot in you, and would not be able to complete development in you at any rate. So easy to get removed if you found it trying to use you as a host. I am surprised you were able to get all the shots this way, as some bots make a buzzing noise like a bee that scares predators into dropping it. Maybe this species doesn't buzz.
Jeff Boettner
Plant, Soil and Insect Sciences

Thanks for the ID/info!
I can assure you this species does buzz very loudly. So far there are no maggots in my flesh, so I think I'm safe.

Brave soul
Glad you got to experience the buzz! Thanks for hanging on. There are very few pictures of the female of this species out there. So a great find. You should get BugGuide bonus points for finding a new species for BugGuide. Not that there are any bonus points, but in your case there should be.
Sabrosky 1986 looked at 262 museum specimens (133 females) of this fly from British Columbia, California, Colorado, Montana, Utah, Washington, Idaho, North Dakota and Nevada. But from his records there have only been 2 females and 7 males ever found in CA. So not a common critter in your area.

Re: Brave soul
Fascinating... Why is it that the female bot flies seem to be much less common than the males? Would the male of this species look a lot different than the insect I found? It's also interesting that the host for this species is wood rats... I've been living here for over 10 years and never seen a wood rat or one of their nests in the area. We have tons of all the other forest creatures - rabbits, squirrels, raccoons, bobcats, foxes, etc. - but I've never seen any signs of wood rats.

The male does look all black like the female in this species. The other black bot females have males that look very different with white patches. The eggs are laid in a 50/50 sex ratio, ie same number of males as females, so they should be even in nature. But the males of this species aggregate at "leks" (ie males hang out together and wait for females to come pick a mate...kind of like speed dating in a singles bar). This leads to bias in insect collections as collectors will sometimes collect a number of males at these leks, whereas the females are present only long enough to mate. And these sites are used year after year --often the lek is the highest point in the area- (if you had a bare topped mountain near you, this would be the place to look for males) many species play king of the hill with the holder of the top of the hill getting to mate with the next female to arrive.) So this leads to some of the bias in collections. Leks are somewhat easy to find, lone females laying eggs are rare to uncover.
Dr. Craig Baird wrote some really nice papers on your species. See: Baird, C. R. 1973 Biology of Cuterebra tenebrosa, a botfly causing cutaneous myiasis in Neotoma woodrats. Doctoral Dissertation, Washington State University 108 pgs. (1973 Diss Abst. B. Sci Eng. 34(6): 2675-B.
Baird, C. R. 1974. Field behavior and seasonal activity of the rodent botfly, Cuterebra tenebrosa, in central Washington. Great Basin Naturalist 34:247-253
Baird, C. R. 1975. Larval development of the rodent botfly, Cuterebra tenebrosa in bushy-tailed wood rats and its relationship to pupal diapause. Canadian Journal of Zoology. 53: 1788-1798.
Baird, C. R. 1979. Incidence of infection and host specificity of Cuterebra tenebrosa in bushy-tailed wood rats (Neotoma cinerea) from Central Washington. Journal of Parasitology 65: 639-644.
There is also a lek paper on this species by:
Hunter, D. M. and J. M. Webster 1973. Aggregation behavior of adult Cuterebra grisea and C. tenebrosa. Canadian Entomologist 105: 1301-1307.
If you would like copies of any of these let me know and I can send them to you. If you send your snail mail address to me at I can send it out to you. I have about 500 papers on bots so I guess you could say I am into them (which is better than them "into" me, I guess). In exchange, I would love copies of your photos to study.
Wood rats can be somewhat secretive. But my guess is you do have them around you somewhere. While these guys are strong fliers, the female is not likely too far from the woodrat nest. Sometimes they emerge in houses when a cat catches an infected animal and brings it home and the bot drops out of the host and emerges later as an adult.

Moved from Flies.

No question it is a bot fly in the genus Cuterebra. Could easily be a new species for Bugguide, too. I'll get Jeff Boettner to take a look at these. Nice work getting all the angles!

I still have a few more photos in case any more are needed... I tried to cover all areas of the fly's body since I don't know which parts are important for identification.

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