Fish louse (Argulus foliaceus), dark-field illumination light micrograph. This flat freshwater crustacean parasite is equipped with two suckers (upper left and upper right) that enable it to attach itself to its fish host. Between the suckers is the needle-like proboscis (tip at top centre) that it uses for feeding. This crustacean is an ectoparasite of freshwater fish, found throughout the temperate world. When attached to its host, it pierces the fish's skin and sucks its blood. Magnification: x40 when printed at 10 centimetres wide.