. Animal parasites and human disease. Medical parasitology; Insects as carriers of disease. CLASSIFICATION 407 a tiny puncture, and then the serrated mandibles enlarge the hole by an up and down sawing motion. As these organs are sunk into the flesh of the host the labial palpi bend back like a bow under the flea's head. The two grooved mandibles, placed in apposition, form a tube for the outflow of saliva, while the epipharynx, which is also grooved, forms a tube with the man- dibles for the inflow of blood. The digestive tract is provided with a pharynx which acts like a suction pump, and a

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. Animal parasites and human disease. Medical parasitology; Insects as carriers of disease. CLASSIFICATION 407 a tiny puncture, and then the serrated mandibles enlarge the hole by an up and down sawing motion. As these organs are sunk into the flesh of the host the labial palpi bend back like a bow under the flea's head. The two grooved mandibles, placed in apposition, form a tube for the outflow of saliva, while the epipharynx, which is also grooved, forms a tube with the man- dibles for the inflow of blood. The digestive tract is provided with a pharynx which acts like a suction pump, and a very large and distensible stomach. Classification. — Several hundred species of fleas have already been described and it is probable that many more species will be found. Although some authors split the fleas into a consider- able number of families, it is more usual to recognize only two. Fig. 179. Heads of common fleas, showing distribution of ctenidia or " combs''; A, human flea, Pulex irritans, without combs; B, dog flea, Ctcnocephalus canis, with combs on both head and pronotum; C, rat flea, Ceratophyllus fasciafus, with only pronotal combs. well-defined famiUes or groups — the Pulicidse and the Sarcopsyl- lidae. The former family includes all the "ordinary" fleas, whereas the Sarcopsyllidse is a very specialized group of fleas with a much shortened thorax, which appears as if mashed between the head and abdomen, with slender anterior and middle legs, and with feeble labial palpi of only three segments. Whereas all of the Pulicidse lay their eggs singly, or in small groups, and develop- ment of the embryos occurs after the eggs are deposited, in some of the Sarcopsyllidse the eggs, during their early development, are retained in the abdomen of the female, which swells up to such a size that the head and thorax appear as a small append- age at one end of it.. Please note that these images are extracted from scanned page images that may have been digitall

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