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Rh have the head with two ocelli and three-segmented feelers; frequently as in the tropical “” (q.v.) the head is prolonged into a conspicuous bladder, or trunk-like process. The Membracidae are remarkable on account of the backward prolongation of the pronotum into a process or hood-like structure which may extend far behind the tail-end of the abdomen. Two other allied families, the Cercopidae and Jassidae, are more numerously represented in our islands. The young of many of these insects are green and soft-skinned, protecting themselves by the well-known frothy secretion that is called “cuckoo-spit.”

In all the above-mentioned families of Homoptera there are three segments in each foot. The remaining four families have feet with only two segments. They are of very great zoological interest on account of the peculiarities of their life-history—parthenogenesis being of normal occurrence among most of them. The families Psyllidae (or “jumpers”) with eight or ten segments in the feeler and the Aleyrodidae (or “snowy-flies”) distinguished by their white mealy wings, are of comparatively slight importance. The two families to which special attention has been paid are the Aphidae or plant-lice (“green fly”) and the Coccidae or scale-insects. The aphids (fig. 11) have feelers with seven or fewer distinct segments, and the fifth abdominal segment usually carries a pair of tubular processes through which a waxy secretion is discharged. The sweet “honey-dew,” often sought as a food by ants, is secreted from the intestines of aphids. The peculiar life-cycle in which successive generations are produced through the summer months by virgin females—the egg developing within the body of the mother—is described at length in the articles and. The Coccidae have only a single claw to the foot; the males (fig. 12 a) have the fore-wings developed and the hind-wings greatly reduced, while in the female wings are totally absent and the body undergoes marked degradation (figs. 12, e, 13, a, b). In the Coccids the formation of a protective waxy secretion—present in many genera of Homoptera—reaches its most extreme development. In some coccids—the “mealy-bugs” (Dactylopius, &c.) for example—the secretion forms a white thread-like or plate-like covering which the insect carries about. But in most members of the family, the secretion, united with cast cuticles and excrement, forms a firm “scale,” closely attached by its edges to the surface of the plant on which the insect lives, and serving as a shield beneath which the female coccid, with her eggs (fig. 13 a) and brood, finds shelter. The male coccid passes through a passive stage (fig. 4) before attaining the perfect condition. Many scale-insects are among the most serious of pests, but various species have been utilized by man for the production of wax (lac) and red dye (cochineal). See , .

The Anoplura or lice (see ) are wingless parasitic insects (fig. 14) forming an order distinct from the Hemiptera, their sucking and piercing mouth-organs being apparently formed on quite a different plan from those of the Heteroptera and Homoptera. In front of the head is a short tube armed with strong recurved hooks which can be fixed into the skin of the host, and from the tube an elongate more slender sucking-trunk can be protruded (fig. 15). Each foot is provided with a single strong claw which, opposed to a process on the shin, serves to grasp a hair of the host, all the lice being parasites on different mammals. Although G. Enderlein has recently shown that the jaws of the Hemiptera can be recognized in a reduced condition in connexion with the louse’s proboscis, the modification is so excessive that the group certainly deserves ordinal separation.

—A recent standard work on the morphology of the Hemiptera by R. Heymons (Nova Acta Acad. Leop. Carol. lxxiv. 3, 1899) contains numerous references to older literature. An excellent survey of the order is given by D. Sharp (Cambridge Nat. Hist. vol. vi., 1898). For internal structure of Heteroptera see R. Dufour, ''Mem. savans étrangers'' (Paris, iv., 1833); of Homoptera, E. Witlaczil (Arb. Zool. Inst. Wien, iv., 1882, Zeits. f. wiss. Zool. xliii., 1885). The development of Aphids has been dealt with by T. H. Huxley (Trans. Linn. Soc. xxii., 1858) and E. Witlaczil (Zeits. f. wiss. Zool. xl., 1884). Fossil Hemiptera are described by S. H. Scudder in K. Zittel’s Paléontologie (French translation, vol. ii. Paris, 1887, and English edition, vol. i., London, 1900), and by A. Handlirsch (Verh. zool. bot. Gesell. Wien, lii., 1902). Among general systematic works on Heteroptera may be mentioned J. C. Schiödte (Ann. Mag. Nat. Hist. (4) vi., 1870); C. Stal’s Enumeratio Hemipterorum (K. Svensk. Vet. Akad. Handl. ix.-xiv., 1870–1876); L. Lethierry and G. Severin’s Catalogue générale des hémiptères (Brussels 1893, &c.); G. C. Champion’s volumes in the Biologia Centrali-Americana; W. L. Distant’s Oriental Cicadidae (London, 1889–1892), and many other papers; M. E. Fernald’s Catalogue of the Coccidae (Amherst, U.S.A., 1903). European Hemiptera have been dealt with in numerous papers by A. Puton. For British species we have E. Saunders’s Hemiptera-Heteroptera of the British Isles (London, 1892); J. Edwards’s Hemiptera-Homoptera of the British Isles (London, 1896); J. B. Buckton’s British Aphidae (London, Ray Society, 1875–1882); and R. Newstead’s British Coccidae (London, Ray Society, 1901–1903). Aquatic Hemiptera are described by L. C. Miall (Nat. History Aquatic Insects; London, 1895), and by G. W. Kirkaldy in numerous recent papers (Entomologist, &c.). For marine Hemiptera (Halobates) see F. B. White (Challenger Reports, vii., 1883); J. J. Walker (Ent. Mo. Mag., 1893); N. Nassonov (Warsaw, 1893), and G. H. Carpenter (Knowledge, 1901, and Report, Pearl Oyster Fisheries, Royal Society, 1906). Sound-producing organs of Heteroptera are described by A. Handlirsch (Ann. Hofmus. Wien, xv. 1900), and G. W. Kirkaldy (Journ. Quekett Club (2) viii. 1901); of Cicads by G. Carlet (Ann. Sci. Nat. Zool. (6) v. 1877). For the Anoplura see E. Piaget’s Pediculines (Leiden, 1880–1905), and G. Enderlein (Zool. Anz. xxviii., 1904).

HEMLOCK (in O. Eng. hemlic or hymlice; no cognate is found in any other language, and the origin is unknown), the Conium maculatum of botanists, a biennial umbelliferous plant, found wild in many parts of Great Britain and Ireland, where it occurs in waste places on hedge-banks, and by the borders of fields, and also widely spread over Europe and temperate Asia, and naturalized in the cultivated districts of North and South America. It is an erect branching plant, growing from 3 to 6 ft. high, and emitting a disagreeable smell, like that of mice. The stems are hollow, smooth, somewhat glaucous green, spotted with dull dark purple, as alluded to in the specific name, maculatum. The root-leaves have long furrowed footstalks, sheathing the stem at the base, and are large, triangular in outline, and repeatedly divided or compound, the ultimate and very numerous segments being small, ovate, and deeply incised at the edge. These leaves generally perish after the growth of the flowering stem, which takes place in the second year, while the leaves