The nature of feather construction
Feather Anatomy 101, continued
Barbules may or may not have attached to them structures collectively referred to as barbicels. Barbicels allow adjacent barbules to interlock or marry. They differ on the barbule in shape and function by location.
Distal barbules (those extending off the barb toward the feather tip) with barbicels have projecting structures at the base of the whip-like pennulum (distal half of the barbule) on the ventral (under) surface that are long and hooked, hooklets (hamuli,) with the remainder of the pennulum having shorter spines (ventral processes).
Proximal barbules (those extending off the barb toward the quill) tend to be more twisted than the distal barbules, and have a trough-shaped dorsal flange (groove) on the anterior (front) edge. As the hooklets of a distal barbule overlap the adjacent proximal barbule, the hooklets attach to the grooved edge while the spines stop the hooklets from sliding too far.
The diagonal cross-over of barbules creates a visible herringbone pattern. Both distal and proximal barbules have other lesser processes on the underside of the ribbon-like lamella referred to as ventral teeth and on the upper side of the whip-like pennulum referred to as dorsal cilium and spines. The hooklets and spines create the marriage of the adjacent barbs while the dorsal processes and ventral teeth catch the barbs and barbules of overlaying feathers to help maintain a solid, air-tight surface in flight. In turn the feather vane is maintained as not only air tight, but with some birds, a water tight structure. Barbicels is a collective term referring to all the processes that interlock to create the vane.
The shaft gives support while the vane (vexillum) or the web of a feather (which includes all the flat, expanded barbs, as well as any attached barbules, and barbicels) provides the surface for an airfoil in flight feathers and for covering and insulation in contour feathers. At the typical feather's base, the vane is downy and provides some insulation. This part of the vane is referred to as the plumulaceous vane.