with the proboscises in a spear-like fashion. predatory leeches feed on small invertebrates such as snails, earthworms and insect larvae. the prey is usually sucked in and swallowed whole. some rhynchobdellida however suck the soft tissues from their prey, making them intermediate between predators and blood-suckers. leech attacking a slug blood-sucking leeches use their anterior suckers to connect to hosts for feeding. once attached, they use a combination of mucus and suction to stay in place while they inject hirudin into the hosts' blood. in general, blood-feeding leeches are non host-specific, and do little harm to their host, dropping off after consuming a blood meal. some marine species however remain attached until it is time to reproduce. if present in great numbers on a host, these can be debilitating, and in extreme cases, cause death. leeches are unusual in that they do not produce certain digestive enzymes such as amylases, lipases or endopeptidases. a deficiency of these enzymes and of b complex vitamins is compensated for by enzymes and vitamins produced by endosymbiotic microflora. in hirudo medicinalis, these supplementary factors are produced by an obligatory mutualistic relationship with the bacterial species, aeromonas veronii. non-bloodsucking leeches, such as erpobdella octoculata, are host to more bacterial symbionts. in addition, leeches produce intestinal exopeptidases which remove amino acids from the long protein molecules one by one, possibly aided by proteases from endosymbiotic bacteria in the hindgut. this evolutionary choice of exopeptic digestion in hirudinea distinguishes these carnivorous clitellates from oligochaetes, and may explain why digestion in leeches is so slow. nervous system a leech's nervous system is formed of a few large nerve cells. their large size makes leeches convenient as model organisms for the study of invertebrate nervous systems. the main nerve centre consists of the cerebral ganglion above the gut and another ganglion beneath it, with connecting nerves forming a ring around the pharynx a little way behind the mouth. a nerve cord runs backwards from this in the ventral coelomic channel, with 21 pairs of ganglia in segments six to 26. in segments 27 to 33, other paired ganglia fuse to form the caudal ganglion. several sensory nerves connect directly to the cerebral ganglion; there are sensory and motor nerve cells connected to the ventral nerve cord ganglia in each segment. leeches have between two and ten pigment spot ocelli, arranged in pairs towards the front of the body. there are also sensory papillae arranged in a lateral row in one annulation of each segment. each papilla contains many sensory cells. some rhynchobdellids have the ability to change colour dramatically by moving pigment in chromatophore cells; this process is under the control of the nervous system but its function is unclear as the change in hue seems unrelated to the colour of the surroundings. leeches can detect touch, vibration, movement of nearby objects, and chemicals secreted by their hosts; freshwater leeches crawl or swim towards a potential host standing in their pond within a few seconds. species that feed on warm-blooded hosts move towards warmer objects. many leeches avoid light, though some blood feeders move towards light when they are ready to feed, presumably increasing the chances of finding a host. gas exchange leeches live in damp surroundings and in general respire through their body wall. the exception to this is in the piscicolidae, where branching or leaf-like lateral outgrowths from the body wall form gills. some rhynchobdellid leeches have an extracellular haemoglobin pigment, but this only provides for about half of the leech's oxygen transportation needs, the rest occurring by diffusion. movement leeches move using their longitudinal and circular muscles in a modification of the locomotion by peristalsis, self-propulsion by alternately contracting and lengthening parts of the body, seen in other annelids such as earthworms. they use their posterior and anterior suckers (one on each end of the body) to enable them to progress by looping or inching along, in the manner of geometer moth caterpillars. the posterior end is attached to the substrate, and the anterior end is projected forward peristaltically by the circular muscles until it touches down, as far as it can reach, and the anterior end is attached. then the posterior end is released, pulled forward by the longitudinal muscles, and reattached; then the anterior end is released, and the cycle repeats. leeches explore their environment with head movements and body waving. the hirudinidae and erpobdellidae can swim rapidly with up-and-down or sideways undulations of the body; the glossiphoniidae in contrast are poor swimmers and curl up and fall to the sediment below when disturbed. leeches move by looping using their front and back suckers. interactions with humans leeches can be removed by hand, since they do not burrow into the skin or leave the head in the wound. bites leech bites are generally alarming rather than dangerous, though a small percentage of people have severe allergic or anaphylactic reactions and require urgent medical care. symptoms of these reactions include red blotches or an itchy rash over the body, swelling around the lips or eyes, a feeling of faintness or dizziness, and difficulty in breathing. an externally attached leech will detach and fall off on its own accord when it is satiated on blood, which may take from twenty minutes to a few hours; bleeding from the wound may continue for some time. internal attachments, such as inside the nose, are more likely to require medical intervention. bacteria, viruses, and protozoan parasites from previous blood sources can survive within a leech for months, so leeches could potentially act as vectors of pathogens. nevertheless, only a few cases of leeches transmitting pathogens to humans have been reported. leech saliva is commonly believed to contain anaesthetic compounds to numb the bite area, but no evidence for this has been published. although morphine-like substances have been found in leeches, they have been found in the neural tissues, not the salivary tissues. they are used by the leeches in modulating their own immunocytes and not for anaesthetising bite areas on their hosts. depending on the species and size, leech bites can be barely noticeable or they can be fairly painful. medical use further information: hirudotherapy and hirudo medicinalis § current the medicinal leech hirudo medicinalis, and some other species, have been used for clinical bloodletting for at least 2,500 years: ayurvedic texts describe their use for bloodletting in ancient india. in ancient greece, bloodletting was practised according to the theory of humours found in the hippocratic corpus of the fifth century bc, which maintained that health depended on a balance of the four humours: blood, phlegm, black bile and yellow bile. bloodletting using leeches enabled physicians to restore balance if they considered blood was present in excess. pliny the elder reported in his natural history that the horse leech could drive elephants mad by climbing up inside their trunks to drink blood. pliny also noted the medicinal use of leeches in ancient rome, stating that they were often used for gout, and that patients became addicted to the treatment. in old english, lǣce was the name for a physician as well as for the animal, though the words had different origins, and lǣcecraft, leechcraft, was the art of healing. "leech finders" from the costume of yorkshire by george walker, 1814, engraved by robert havell pharmacy leech jar with airholes in the lid. england, 1830–1870. three leech doctors decide on bloodletting for their grasshopper patient. lithograph by f.-j.-v. broussais from a cartoon by j. j. grandville, c. 1832 william wordsworth's 1802 poem "resolution and independence" describes one of the last of the leech-gatherers, people who travelled britain catching leeches from the wild, and causing a sharp decline in their abundance, though they remain numerous in romney marsh. by 1863, british hospitals had switched to imported leeches, some seven million being imported to hospitals in london that year. in the nineteenth century, demand for leeches was sufficient for hirudiculture, the farming of leeches, to become commercially viable. leech usage declined with the demise of humoral theory, but made a small-scale comeback in the 1980s after years of decline, with the advent of microsurgery, where venous congestion can arise due to inefficient venous drainage. leeches can reduce swelling in the tissues and promote healing, helping in particular to restore circulation after microsurgery to reattach body parts. other clinical applications include varicose veins, muscle cramps, thrombophlebitis, and joint diseases such as epicondylitis and osteoarthritis. leech secretions contain several bioactive substances with anti-inflammatory, anticoagulant and antimicrobial effects. one active component of leech saliva is a small protein, hirudin. it is widely used as an anticoagulant drug to treat blood-clotting disorders, and manufactured by recombinant dna technology. in 2012 and 2018, ida schnell and colleagues trialled the use of haemadipsa leeches to gather data on the biodiversity of their mammalian hosts in the tropical rainforest of vietnam, where it is hard to obtain reliable data on rare and cryptic mammals. they showed that mammal mitochondrial dna, amplified by the polymerase chain reaction, can be identified from a leech's blood meal for at least four months after feeding. they detected annamite striped rabbit, small-toothed ferret-badger, truong son muntjac, and serow in this way. water pollution exposure to synthetic estrogen as used in contraceptive medicines, which may enter freshwater ecosystems from municipal wastewater, can affect leeches' reproductive systems. although not as sensitive to these compounds as fish, leeches showed physiological changes after exposure, including longer sperm sacs and vaginal bulbs, and decreased epididymis weight. notes the caption below the lithograph reads "there's redundancy of blood and humours, we'll bleed you to-morrow, till then, very little food."