its prey. Such powerful electrogenesis makes use of large electric organs modified from muscles. These consist of a stack of electrocytes, each capable of generating a small voltage; the voltages are effectively added together (in series) to provide a powerful electric organ discharge. Monotremes The monotremes, including the semi-aquatic platypus and the terrestrial echidnas, are the only group of mammals that have evolved electroreception. While the electroreceptors in fish and amphibians evolved from mechanosensory lateral line organs, those of monotremes are based on cutaneous glands innervated by trigeminal nerves. The electroreceptors of monotremes consist of free nerve endings located in the mucous glands of the snout. Among the monotremes, the platypus (Ornithorhynchus anatinus) has the most acute electric sense. The platypus localises its prey using almost 40,000 electroreceptors arranged in front-to-back stripes along the bill. The arrangement is highly directional, being most sensitive off to the sides and below. By making short quick head movements called saccades, platypuses accurately locate their prey. The platypus appears to use electroreception along with pressure sensors to determine the distance to prey from the delay between the arrival of electrical signals and pressure changes in water. The electroreceptive capabilities of the four species of echidna are much simpler. Long-beaked echidnas (genus Zaglossus) have some 2,000 receptors, while short-beaked echidnas (Tachyglossus aculeatus) have around 400, near the end of the snout. This difference can be attributed to their habitat and feeding methods. Western long-beaked echidnas feed on earthworms in leaf litter in tropical forests, wet enough to conduct electrical signals well. Short-beaked echidnas feeds mainly on termites and ants, which live in nests in dry areas; the nest interiors are presumably humid enough for electroreception to work. Experiments have shown that echidnas can be trained to respond to weak electric fields in water and moist soil. The electric sense of the echidna is hypothesised to be an evolutionary remnant from a platypus-like ancestor. Dolphins Dolphins have evolved electroreception in structures different from those of fish, amphibians and monotremes. The hairless vibrissal crypts on the rostrum of the Guiana dolphin (Sotalia guianensis''), originally associated with mammalian whiskers, are capable of electroreception as low as 4.8 μV/cm, sufficient to detect small fish. This is comparable to the sensitivity of electroreceptors in the platypus. Bees Until recently, electroreception was known only in vertebrates. Recent research has shown that bees can detect the presence and pattern of a static