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The nature of feather construction
Fly tiers somewhat incorrectly refer to this part of the feather as being the web or webby portion of the feather. Web is a term synonymous with the whole vane.
Some feathers have barbules without barbicels. Examples would include peacock upper tail covert feather barbs below the eye as well as down feathers from any bird. (When considering peacock herl, the barbs are often mistakenly referred to as quills. The barbules also are confusingly referred to as herl for example where the fly pattern for a Quill Gordon calls for a quill body that requires the removal of the herl from the barb. In actuality, the quill is a barb and the herl referred to are barbules on the herl or barb.) Barbicels are found on flight feathers (i.e., turkey tail feathers, peacock secondary wing feathers, etc.) with the exception of flightless birds (i.e., emu, ostrich, kiwi, etc.) Just as a barb does not necessarily have barbules, barbules do not necessarily have barbicels. Turkey marabou (semi-plume) is an example of a feather with barbules, but no barbicels. Body (contour) feathers of most pheasants are examples of feathers having barbules without barbicels (plumulacious vane) on some barbs and barbules with barbicels (pennaceous vane) on others.