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Photo#75824
Bombyliid fly on the beach - Villa - female

Bombyliid fly on the beach - Villa - Female
Woods Hole, near Nobska lighthouse, Barnstable County, Massachusetts, USA
July 2, 2006
Size: ~12 mm
There were a couple of these flies on the beach. Are they ovipositing?

Bio note
She is not ovipositing. She is filling her "sand chamber" or "psammophore", a ventral abdominal pocket into which the eggs are laid, and a feature unique to "higher" Bombyliidae. It is said that eggs are coated with sand in the chamber to fascilitate release later. I think that, given the relative size of sand grains and Bombyliid eggs, the reverse is true: the tiny eggs coat sand grains in the chamber, which are heavy and easier to toss with accuracy, kind of like sticking a piece of gum to a baseball. Ovipositing looks like little aerial dipping movements accompanied by flicks of the keester. Eggs are thrown through the air.

 
Maybe sand provides crypsis too?
I'm just speculating here, but if a single egg were covered by sand grains, then it would blend in better with the surroundings and perhaps be less likely to be recognized and removed by the adult mothers of the bombyliid's larval hosts. Also, provided the relative sizes and stickiness could work out, attaching many sand grains to a single egg (rather than many eggs to a single sand grain) would provide more ballast for flicking.

 
Similar behavior observed
Aha! I saw the same behavior and thought it was oviposition:

http://www.flickr.com/photos/pcoin/7466279718/

Thanks for this excellent information.

Moved
Moved from Bee Flies.

Villa sp.
moving to genus.

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