Polarised light micrograph of cross section of porcupine (Erethizon dorsatum) quill. These quills, like hair and horns, are made of keratin. The quills are not hollow, but have a honeycomb structure. Porcupines produce a pungent warning odour when their quills are erected. Quills also contain a fluorescent material that brightens the quills at night, when the porcupine is most likely to meet predators. Magnification: x40 at 10cm wide.