In Bulbophyllum involutum and B. ipanemense (Orchidaceae), two closely related species, shortly after removal the pollinarium has a diameter of approximately twice that of the entrance of the stigmatic cavity, requiring a mean time of 105 to 135 minutes to shrink in width and allow pollination. Because the pollinators of these species remain for some minutes in the same flower after removing the pollinia this mechanism, previously unknown in Orchidaceae, may be very important in preventing self-pollination. This mechanism does not occur in B. weddellii, and the pollinator does not remain in the flower after removing the pollinia. The smaller diameter of the stigmatic cavity in B. involutum reduces by 50% the chances of interspecific pollination with B. weddellii, and interspecific crossing is strictly unidirectional. This is important in maintaining isolation between these sympatric species, which share the same pollinators and have synchronized flowering.21741732197204