but the relationship to the Chordata expressed in the designation Hemichordata cannot be regarded as more than an attractive theory with certain arguments in its favour. (P. C. M.)
HEMICYCLE (Gr. ἡμι-, half, and κύκλος, circle), a semicircular recess of considerable size which formed one of the most
conspicuous features in the Roman Thermae, where it was
always covered with a hemispherical vault. A small example
exists in Pompeii, in the street of tombs, with a seat round inside,
where those who came to pay their respects to the departed
could rest. An immense hemicycle was designed by Bramante
for the Vatican, where it constitutes a fine architectural effect
at the end of the great court.
HEMIMERUS, an Orthopterous or Dermapterous insect, the sole representative of the family Hemimeridae, which has affinities with both the Forficulidae (earwigs) and the Blattidae (cockroaches). Only two species have been discovered, both from West Africa. The better known of these (H. hanseni) lives upon a large rat-like rodent (Cricetomys gambianus) feeding perhaps upon its external parasites, perhaps upon scurf and other dermal products. Like many epizoic or parasitic insects, Hemimerus is wingless, eyeless and has relatively short and strong legs. Correlated also with its mode of life is the curious fact that it is viviparous, the young being born in an advanced stage of growth.
HEMIMORPHITE, a mineral consisting of hydrous zinc
silicate, H2Zn2SiO5, of importance as an ore of the metal, of
which it contains 54.4%. It is interesting crystallographically
by reason of the hemimorphic development of its orthorhombic
crystals, these are prismatic in habit and are
differently terminated at the two ends. In
the figure, the faces at the upper end of the
crystal are the basal plane k and the domes
o, p, l, m, whilst at the lower end there are
only the four faces of the pyramid P. Connected
with this polarity of the crystals is
their pyroelectric character—when a crystal
is subjected to changes of temperature it
becomes positively electrified at one end and
negatively at the opposite end. There are perfect
cleavages parallel to the prism faces (d in the
figure). Crystals are usually colourless, sometimes
yellowish or greenish, and transparent;
they have vitreous lustre. The hardness is 5, and the specific
gravity 3.45. The mineral also occurs as stalactitic or botryoidal
masses with a fibrous structure, or in a massive, cellular or
granular condition intermixed with calamine and clay. It is
decomposed by hydrochloric acid with gelatinization; this
property affords a ready means of distinguishing hemimorphite
from calamine (zinc carbonate), these two minerals being, when
not crystallized, very like each other in appearance. The water
contained in hemimorphite is expelled only at a red heat, and
the mineral must therefore be considered as a basic metasilicate,
(ZnOH)2SiO3.
The name hemimorphite was given by G. A. Kenngott in 1853 because of the typical hemimorphic development of the crystals. The mineral had long been confused with calamine (q.v.) and even now this name is often applied to it. On account of its pyroelectric properties, it was called electric calamine by J. Smithson in 1803.
Hemimorphite occurs with other ores of zinc (calamine and blende), forming veins and beds in sedimentary limestones. British localities are Matlock, Alston, Mendip Hills and Leadhills; at Roughten Gill, Caldbeck Fells, Cumberland, it occurs as mammillated incrustations of a sky-blue colour. Well-crystallized specimens have been found in the zinc mines at Altenberg near Aachen in Rhenish Prussia, Nerchinsk mining district in Siberia, and Elkhorn in Montana. (L. J. S.)
HEMINGBURGH, WALTER OF, also commonly, but erroneously,
called Walter Hemingford, a Latin chronicler of the
14th century, was a canon regular of the Austin priory of Gisburn
in Yorkshire. Hence he is sometimes known as Walter of Gisburn
(Walterus Gisburnensis). Bale seems to have been the first to
give him the name by which he became more commonly known.
His chronicle embraces the period of English history from the
Conquest (1066) to the nineteenth year of Edward III., with
the exception of the years 1316–1326. It ends with the title of a
chapter in which it was proposed to describe the battle of Creçy
(1346); but the chronicler seems to have died before the required
information reached him. There is, however, some controversy
as to whether the later portions which are lacking in some of the
MSS. are by him. In compiling the first part, Hemingburgh
apparently used the histories of Eadmer, Hoveden, Henry of
Huntingdon, and William of Newburgh; but the reigns of the
three Edwards are original, composed from personal observation
and information. There are several manuscripts of the history
extant—the best perhaps being that presented to the College of
Arms by the earl of Arundel. The work is correct and judicious,
and written in a pleasing style. One of its special features is the
preservation in its pages of copies of the great charters, and
Hemingburgh’s versions have more than once supplied deficiencies
and cleared up obscurities in copies from other sources.
The first three books were published by Thomas Gale in 1687, in his Historiae Anglicanae scriptores quinque, and the remainder by Thomas Hearne in 1731. The first portion was again published in 1848 by the English Historical Society, under the title Chronicon Walteri de Hemingburgh, vulgo Hemingford nuncupati, de gestis regum Angliae, edited by H. C. Hamilton.
HEMIPTERA (Gr. ἡμι-, half and πτερόν, a wing), the name applied in zoological classification to that order of the class Hexapoda (q.v.) which includes bugs, cicads, aphids and scale insects. The name was first used by Linnaeus (1735), who derived it from the half-coriaceous and half-membranous condition of the forewing in many members of the order. But the wings vary considerably in different families, and the most distinctive feature is the structure of the jaws, which form a beak-like organ with stylets adapted for piercing and sucking. Hence the name Rhyngota (or Rhynchota), proposed by J. C. Fabricius (1775), is used by many writers in preference to Hemiptera.
| |
| After Marlatt, Bull. 14 (N.S.) Div. Ent. U.S. Dept. Agr. | |
| Fig. 1.—Head and Prothorax of Cicad from side. | |
| I., | Frons. |
| II., | Base of mandible. |
| III., | Base of first maxillae. |
| IV., | Second maxillae forming rostrum. |
| V., | Pronotum. |
Structure.—The head varies greatly in shape, and the feelers have usually but few segments—often only four or five. The arrangement of the jaws is remarkably constant throughout the order, if we exclude from it the lice (Anoplura). Taking as our type the head of a cicad, we find a jointed rostrum or beak (figs. 1 and 2, IV. b, c) with a deep groove on its anterior face; this organ is formed by the second pair of maxillae and corresponds therefore to the labium or “lower lip” of biting insects. Within the groove of the rostrum two pairs of slender piercers—often barbed at the tip—work to and fro. One of these pairs (fig. 2, II. a, b, c) represents the mandibles, the other (fig. 2, III. a, b, c) the first maxillae. The piercing portions of the latter—representing their inner lobes or laciniae—lie median to the mandibular piercers in the natural position of the organs. These homologies of the hemipterous jaws were determined by J. C. Savigny in 1816, and though disputed by various subsequent writers, they have been lately confirmed by the embryological researches of R. Heymons (1899). Vestigial palps have been described in various species of Hemiptera, but the true nature of these structures is doubtful. In front of the rostrum and the piercers lies the pointed flexible labrum and within its base a small hypopharynx (fig. 2, IV. d) consisting of paired conical processes which lie dorsal to the “syringe” of the salivary glands. This latter organ injects a secretion into the plant or
