Longlegged Flies

Our crape myrtle tree (Lagerstroemia) is home to numerous Longlegged Flies (family Dolichopodidae). I don't know enough about them to identify the species, but my guess would be Dactylomyia lateralis.

While photographing them, I observed some interesting behavior. When a fly would come upon a stationary one, it would place its front legs on the latter's back. This could be a preliminary courtship maneuver, though courtship never did take place, or a way of recognizing a fellow fly, as when ants touch each other as they pass by. 

One leg...

...and two.

However, before I could get to know these flies better, the Orchard Orbweaver (Leucauge venusta) that lives in the tree caught most of them in her web.

References:
http://bugguide.net/node/view/100 (Dolichopodidae)
http://www.diptera.info/articles.php?article_id=13 (Neurigona courtship)
http://beheco.oxfordjournals.org/content/14/4/526.full.pdf (courtship; caution: evolutionary bent)
http://www.nadsdiptera.org/Doid/Dolichar/Dolichar.htm (characteristics & natural history)