How do bees attach comb to topbars?

A. The top bar hive does not use wax foundation as used in Langstroth hives. The bees draw out their own comb using the top bar as a guide. The bees attach themselves on the triangle edge of the top bar and begin to slowly deposit bits of wax in a pattern on that bar.  Using gravity, they hang their bodies and excrete the wax from glands on their abdomen to create comb on the top bar. The nectar they collect is used to create this wax.  It is such an amazing process and it is incredible how quickly  the bees can build new comb.  You may have noticed that the cells on the combs are geometrically uniform.  To create this shape, the bees first make the cell a circle.  Then they detach their wings flexing their muscles creating heat to shape the cell into a circular shape around their bodies. When the wax reaches a certain temperature, the molecules in the wax "click" into a hexagonal shape. Just like all the processes in the hive, I feel this is absolutely magical.  To find out more information on the inner workings of the bee colony, check out a book written by Jurgen Tautz called The Buzz About Bees.  If you are interested, this is an amazing easy read, explaining new scientific breakthoughs about the bee, that will leave you in awe!!