The orchid, the spider and the fly seem like they are playing "cat and mouse",

Last week a Bulbophyllum Elizabeth Ann 'Jean' bloomed in my garden.  It started attracting flies. All the fly activity got the attention of a spider that moved to the flowers.  What I find most remarkable of this is that the flies don't seem to do anything on the flower, they certainly are not pollinating it, or drinking nectar, or even laying eggs, they just sit on the flowers and defends them from other flies with remarkable vigor.  What stimulus the flower is using on the fly to elicit this behavior is a mystery to me, although I suspect it is the fragrance.  The presence of a crab spider make staying on the flower a perilous thing.  The fly seems aware of the spider, however to my surprise, sometimes it gets dangerously close to it.  The spider in question is a crab spider, this spider doesn't make a web, instead, it is build up like a wrestler, with large front legs for grabbing and overpowering prey.  Getting near this spider is the last thing a fly should do, but in this case the flies seems almost desperate to sit where the spider is.  The spider, which preyed on a less alert fly last thursday, was noticeably fatter on friday.  Unfortunately I left home for the weekend and by the time I am back the flowers will have fallen and I will never know if these particular flies survived its obssesion with the flowers.