Penguin feather. Coloured scanning electron micrograph (SEM) of a fractured quill (calamus) from a penguin feather. The pithy material (keratin) contains air cells to make the quill light yet strong. The penguin (family Spheniscidae) has the most densely packed feathers of any bird. The central shaft (rachis) runs down the centre and rows of filaments (barbs) project from either side of the rachis. Along the length of each barb are smaller filaments that are hooked, known as barbules. The hooks prevent the feathers from sliding relative to one another, increasing their effectiveness as insulators, as well as allowing the birds to become more streamlined in the water. A layer of oil is secreted onto the feathers to make them waterproof. Magnification: x 1000 when printed 10 cm wide.