The white contrasting "throat circle" in a  flower of Lobivia backebergii

The throat circle or annulus colour often differs from and contrasts to the portion of the distal throat  below it and to the colour of the flower petals situated adjacent to the circle. The throat-circle apparently serves as an attractant for day-flying pollinators to an opened flower and/or to orient a pollinator, already on a blossom, to the throat itself. The throat-circle is often set upon a slight bulge of the throat wall that circles the tip of the distal throat division.

The annulus is variably developed in Echinopsis, s.l., but is most prominent in lobivias. It apparently functions to make the throat-circle more evident to a pollinator. In some cases, though,  an annulus is greatly reduced or apparently absent.

In some species (e.g. Lobivia jajoiana ) the throat-circle, is covered by a thin, stiff, transparent layer which is highly reflective. An insect looking down at a flower would see a  gorgeous, iridescent terminals that flashed on and off as the flower fluttered in the breezes. This apparently is a great signal to attract pollinators.