Fly attacking Spider Metepeira labyrinthea !!

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Home » Guide » Arthropods (Arthropoda) » Insects (Insecta) » Flies (Diptera) » Calyptratae » Oestroidea » Flesh Flies (Sarcophagidae)

Photo#340229

Copyright © 2009 S. E. Seater

Fly attacking Spider Metepeira labyrinthea !!

Raleigh, near Rts 50 and US 70, Wake County, North Carolina, USA
September 30, 2009

Please identify the fly and explain the behavior I observed. I came to take a photo of these egg sacs that are different from the common spheres of Mecynogea lemniscata, Basilica Orbweaver. This web in a Snow Azalea is the only one I have found that has what I call sacs with "hats" in a linear vertical chain. The spider is Metepeira labyrinthea.

I had never seen a spider on these egg sacs in the past. Before I realized that the spider was present at the end of the egg sac chain, I saw this fly (only 2 wings) about the size of a housefly sitting on the outside of the "messy" web (no regular structure apparent). The fly started picking its way carefully into the center of the web and then flew directly at the spider, over and over. The spider responded by moving to the other side of the chain which may have been to get away from the fly or to move its mass so the chain started turning around. This made it difficult for the fly to aim at the spider or to land on a sac. After 3 or 4 darts by the fly in the cramped inner space, the fly would pause within (to rest?) and the spider would continue circling the chain of sacs. Then the fly picked its way out and sat on the outside again for several minutes. The spider went around and around the sac chain from bottom to top and down again as though it were wrapping silk around the egg sacs but I could not see any silk. After a couple minutes the fly repeated the attack. I watched for 15 minutes, 3 attack sequences until the fly left.

It was 3:40-3:53pm on a sunny afternoon. The spider body length and the fly were similar. I never saw the underside or mouth parts of the fly.

Hypothesis: The fly wanted to #1 parasitize an egg sac; #2 parasitize the spider. Or?

Images of this individual: tag all

Fly attacking Spider Metepeira labyrinthea !!

Fly attacking Spider?! - Metepeira labyrinthea - female

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Contributed by S. E. Seater on 4 October, 2009 - 3:39am
Last updated 30 October, 2009 - 12:20pm

Moved

Moved from Labyrinth Orbweaver.

… Abigail M. Parker, 30 October, 2009 - 12:20pm

Thank you, Abigail

By the way, the spider is still there, Oct 29, with a chain of egg sacs (one more than when the fly attacked a month ago), sometimes on the bottom of the chain and sometimes in the center of the orbweb.

… S. E. Seater, 30 October, 2009 - 1:10pm

Moved

Moved from ID Request.

… Abigail M. Parker, 16 October, 2009 - 12:53pm

Abigail, How do I continue to ask for a fly ID?

Abigail, How do I continue to ask for a fly ID if all the photos are filed under the spider? Susan

… S. E. Seater, 16 October, 2009 - 9:44pm

Belated response

I was on vacation and didn't see this comment till now. Tag the fly images, navigate to Sarcophagidae, click the "Images" tab, and click the "Move tagged images" link. Linked photos don't all have to be in the same section of the guide.

… Abigail M. Parker, 30 October, 2009 - 12:17pm

I wrote to Dr George Uetz...

Dr. Seater - Thank you for the interesting link and excellent photos. The spider is definitely Metepeira labyrinthea, which is the only species of that genus in the Eastern U.S. Must have been a good year, as there are quite a few egg sacs in that string. Although M. labyrinthea is typically solitary, it will occasionally aggregate (mentioned in the works of McCook in the 1890's, and reported more recently by David Wise in the 1990's) in areas of high prey or concentrated vegetation. Unfortunately, I cannot identify the fly for several reasons: 1) species ID on sarcophagids requires examination of genitalia under a microscope; 2) that is apparently very difficult, even for sarcophagid experts; and 3) I am not a dipterologist. The behavior of the fly and spider that you describe is certainly very similar to that of the Arachnidomyia - Metepeira interaction we observed in Mexico. I have seen a similar attack on colonial M. spinipes in California. I suspect that the egg sac was the intended target, as there are a number of sarcophagid species that parasitize spider egg sacs.

Thanks for sharing.
Best regards,
George Uetz

… S. E. Seater, 6 October, 2009 - 9:28am

I think

Metepeira labyrinthea for the spider

… John R. Maxwell, 4 October, 2009 - 7:59am

a tachnid?

i wonder if it's a Tachnid fly looking to lay an egg on the spider? It looks similar to one I observed doing the same thing to a caterpillar.

I'll be interested in hearing what an expert has to say.

… Michael Battenberg, 4 October, 2009 - 7:38am

Sarcophagid?

My bet for the fly would be something in the family Sarcophagidae. They collectively seem to parasitize nearly everything! The spider I am a lot less certain about, though obviously an orb weaver of some sort.

… Eric R. Eaton, 5 October, 2009 - 12:20pm

Egg sacs

look right for labyrinth spider--a chain of lenticular sacs in contrast to the chain of spherical egg sacs made by a basilica spider. Labyrinth spiders build an orb web with an elaborate messy "labyrinth" web next to it, in which the spider makes its retreat and hangs its egg sacs.

… Charley Eiseman, 5 October, 2009 - 12:31pm

Labyrinth spider fits with Sarcophagidae

Excellent spider pics. Too bad the fly wouldn't sit still (but then she wouldn't have got her job done, I guess). Great info on the egg sacs. Like Eric, I want the fly to be a sarcophagid, because members of that family are known parasitoids on labyrinth spiders (see, for example, this report or right-click to download this video of a sarchophagid [Arachnidomyia] attempting to deposit larvae on a Metepeira egg sac). But I sure wish I could clearly see what Ron Hemberger calls "three black racing stripes and a taillight" on the fly. Still waiting, with Mike, for an expert on Oestroidea. Till then, Tachinid looks possible and Sarcophagid seems likely to me.

… Don Patterson, 5 October, 2009 - 1:41pm

The racing stripes and the taillight...

I went to the library and got Eric Eaton's Guide before I read any of today's posts about the fly, just looking for flies that resemble a housefly. I immediately picked out the Sarcophaga sp. picture on p307. and said Aha! because of the stripes. (Graphomya maculata stripes weren't right.) I saw the live fly much better than my camera did because for the camera I always had to choose focus for either the fly or the spider. From Bugguide photos I get as far as Subfamily Sarcophaginae.

Now I went back to the dozens of original photos and can definitely see the three stripes even when the fly is out of focus. As for looking for a "taillight," I worked on photo 340234 in Photo Shop to sharpen and saturate color and there is a dot at the end of the abdomen of more yellow than red, but I couldn't say it was the marking. The only other abdominal pic is too out of focus to see anything other than black at the end of the fly. I didn't know that mark was what I would need -- my lack of experience. I will add these 2 photos, really frass, just in case they help.

The link to George Uetz's work and his description of behavior was helpful; certainly fits what I saw.

… S. E. Seater, 5 October, 2009 - 8:05pm

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