Hemiptera

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.  
  
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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 animal tissue from which the insect is sucking. The point of the rostrum is pressed against the surface to be pierced; then the stylets come into play and the fluid food is believed to pass into the mouth by capillary attraction.  
  
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After Marlatt, Bull. 14 (N.S.) Div. Ent. U.S. Dept. Agr.  
Fig. 2.—Head and Prothorax of Cicad, parts separated.  
  
I., a, frons; b, clypeus; c, labrum; d, epipharynx.

I’., Same from behind.

II., Mandible.

III., 1st maxillae, a, base; b, sheath; c, stylet; c′, muscle.

IV., 2nd maxillae, a, sub-mentum; b, mentum; c, ligula, forming beak; d, hypopharynx (shown also from front d′, and behind d″).

V., Prothorax, b, haunch; a, trochanter.  
  
The prothorax (figs. 1 and 2, V.) in Hemiptera is large and free, and the mesothoracic scutellum is usually extensive. The number of tarsal segments is reduced; often three, two or only one may be present instead of the typical insectan number five. The wings will be described in connexion with the various sub-orders, but an interesting peculiarity of the Hemiptera is the occasional presence of winged and wingless races of the same species. Eleven abdominal segments can be recognized, at least in the early stages; as the adult condition is reached, the hinder segments become reduced or modified in connexion with the external reproductive organs, and show, in some male Hemiptera, a marked asymmetry. The typical insectan ovipositor with its three pairs of processes, one pair belonging to the eighth and two pairs to the ninth abdominal segment, can be distinguished in the female.  
  
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After Marlatt, Bull. 4 (N.S.) Div. Ent. U.S. Dept. Agr.  
Fig. 3.—a, Cast-off nymphal skin of Bed-bug (Cimex lectularius); b, Second instar after emergence from a; c, The same after a meal.  
  
In the nervous system the concentration of the trunk ganglia into a single nerve-centre situated in the thorax is remarkable. The digestive system has a slender gullet, a large crop and no gizzard; in some Hemiptera the hinder region of the mid-gut forms a twisted loop with the gullet. Usually there are four excretory (Malpighian) tubes; but there are only two in the Coccidae and none in the Aphidae. “Stink glands,” which secrete a nauseous fluid with a defensive function, are present in many Hemiptera. In the adult there is a pair of such glands opening ventrally on the hindmost thoracic segment, or at the base of the abdomen; but in the young insect the glands are situated dorsally and open to the exterior on a variable number of the abdominal terga.

Development.—In most Hemiptera the young insect (fig. 3) resembles its parents except for the absence of wings, and is active through all stages of its growth. In all Hemiptera the wing-rudiments develop externally on the nymphal cuticle, but in some families—the cicads for example—the young insect (fig. 10) is a larva differing markedly in form from its parent, and adapted for a different mode of life, while the nymph before the final moult is sluggish and inactive. In the male Coccidae (Scale-insects) the nymph (fig. 4) remains passive and takes no food. The order of the Hemiptera affords, therefore, some interesting transition stages towards the complete metamorphosis of the higher insects.  
  
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After Riley and Howard, Insect Life, vol. i. (U.S. Dept. Agr.).  
Fig. 4.—Passive Nymph or “Pupa” of male scale-insect (Icerya).  
  
Distribution and Habits.—Hemiptera are widely distributed, and are plentiful in most quarters of the globe, though they probably have not penetrated as far into remote and inhospitable regions as have the Coleoptera, Diptera and Aptera. They feed entirely by suction, and the majority of the species pierce plant tissues and suck sap. The leaves of plants are for the most part the objects of attack, but many aphids and scale-insects pierce stems, and some go underground and feed on roots. The enormous rate at which aphids multiply under favourable conditions makes them of the greatest economic importance, since the growth of immense numbers of the same kind of plant in close proximity—as in ordinary farm-crops—is especially advantageous to the insects that feed on them. Several families of bugs are predaceous in habit, attacking other insects—often members of their own order—and sucking their juices. Others are scavengers feeding on decaying organic matter; the pond skaters, for example, live mostly on the juices of dead floating insects. And some, like the bed-bugs, are parasites of vertebrate animals, on whose bodies they live temporarily or permanently, and whose blood they suck.

The Hemiptera are especially interesting as an order from the variety of aquatic insects included therein. Some of these—the Hydrometridae or pond-skaters, for example—move over the surface-film, on which they are supported by their elongated, slender legs, the body of the insect being raised clear of the water. They are covered with short hairs which form a velvet-like pile, so dense that water cannot penetrate. Consequently when the insect dives, an air-bubble forms around it, a supply of oxygen is thus secured for breathing and the water is kept away from the spiracles. In many of these insects, while most individuals of the species are wingless, winged specimens are now and then met with. The occasional development of wings is probably of service to the species in enabling the insects to reach new fresh-water breeding-grounds. This family of Hemiptera (the Hydrometridae) and the Saldidae contain several insects that are marine, haunting the tidal margin. One genus of Hydrometridae (Halobates) is even oceanic in its habit, the species being met with skimming over the surface of the sea hundreds of miles from land. Probably they dive when the surface becomes ruffled. In these marine genera the abdomen often undergoes excessive reduction (fig. 5).

Other families of Hemiptera—such as the “Boatmen” (Notonectidae) and the “Water-scorpions” (fig. 6) and their allies (Nepidae) dive and swim through the water. They obtain their supply of air from the surface. The Nepidae breathe by means of a pair of long, grooved tail processes (really outgrowths of the abdominal pleura) which when pressed together form a tube whose point can pierce the surface film and convey air to the hindmost spiracles which are alone functional in the adult. The Notonectidae breathe mostly through the thoracic spiracles; the air is conveyed to these from the tail-end, which is brought to the surface, along a kind of tunnel formed by overlapping hairs.  
  
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After Carpenter, Proc. R. Dublin Soc., vol. viii. | Fig. 6.—Water-scorpion (Nepa cinerea) with raptorial fore-legs, heteropterous wings, and long siphon for conveying air to spiracles. Somewhat magnified. sc, scutellum; co, cl, m, corium, clavus and membrane of forewing.  
Fig. 5.—A reef-haunting hemipteron (Hermatobates haddonii) with excessively reduced abdomen. Magnified.  
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From Marlatt, Bull. 14 (N.S.) Div. Ent. U.S. Dept. Agr.  
Fig. 7.  
  
a, Body of male Cicad from below, showing cover-plates of musical organs;

b, From above showing drums, natural size;

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c, Section showing muscles which vibrate drum (magnified);

d, A drum at rest;

e, Thrown into vibration, more highly magnified.  
  
Sound-producing Organs.—The Hemiptera are remarkable for the variety of their stridulating organs. In many genera of the Pentatomidae, bristle-bearing tubercles on the legs are scraped across a set of fine striations on the abdominal sterna. In Halobates a comb-like series of sharp spines on the fore-shin can be drawn across a set of blunt processes on the shin of the opposite leg. Males of the little water-bugs of the genus Corixa make a shrill chirping note by drawing a row of teeth on the flattened fore-foot across a group of spines on the haunch of the opposite leg. But the loudest and most remarkable vocal organs of all insects are those of the male cicads, which “sing” by the rapid vibration of a pair of “drums” or membranes within the metathorax. These drums are worked by special muscles, and the cavities in which they lie are protected by conspicuous plates visible beneath the base of the abdomen (see fig. 7).

Fossil History.—The Heteroptera can be traced back farther than any other winged insects if the fossil Protocimex silurica Moberg, from the Ordovician slates of Sweden is rightly regarded as the wing of a bug. But according to the recent researches of A. Handlirsch it is not insectan at all. Both Heteropterous and Homopterous genera have been described from the Carboniferous, but the true nature of some of these is doubtful. Eugereon is a remarkable Permian fossil, with jaws that are typically hemipterous except that the second maxillae are not fused and with cockroach-like wings. In the Jurassic period many of the existing families, such as the Cicadidae, Fulgoridae, Aphidae, Nepidae, Reduviidae, Hydrometridae, Lygaeidae and Coreidae, had already become differentiated.

Classification.—The number of described species of Hemiptera must now be nearly 20,000. The order is divided into two sub-orders, the Heteroptera and the Homoptera. The Anoplura or lice should not be included among the Hemiptera, but it has been thought convenient to refer briefly to them at the close of this article.

Heteroptera

In this sub-order are included the various families of bugs and their aquatic relations. The front of the head is not in contact with the haunches of the fore-legs. There is usually a marked difference between the wings of the two pairs. The fore-wing is generally divided into a firm coriaceous basal region, occupying most of the area, and a membranous terminal portion, while the hind-wing is delicate and entirely membranous (see fig. 6). In the firm portion of the fore-wing two distinct regions can usually be distinguished; most of the area is formed by the corium (fig. 6, co), which is separated by a longitudinal suture from the clavus (fig. 6, cl) on its hinder edge, and in some families there is also a cuneus (fig. 9 cu) external to and an embolium in front of the corium.  
  
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After Marlatt, Bull. 4 (N.S.) Div. Ent. U.S. Dept. Agr.  
Fig. 8.—Bed-bug (Cimex lectularius, Linn.).  
  
a, Female from above;

b, From beneath;

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c, Vestigial wing;

d, Jaws, very highly magnified (tips of mandibles and 1st maxillae still more highly magnified).  
  
Most Heteroptera are flattened in form, and the wings lie flat, or nearly so, when closed. The young Heteropteron is hatched from the egg in a form not markedly different from that of its parent; it is active and takes food through all the stages of its growth. It is usual to divide the Heteroptera into two tribes—the Gymnocerata and the Cryptocerata.  
  
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After M. V. Slingerland, Cornell Univ. Ent. Bull. 58.  
Fig. 9.—Capsid Leaf-bug (Poecilocapsus lineatus) N. America. Magnified—, cu cuneus.  
  
Gymnocerata.—This tribe includes some eighteen families of terrestrial, arboreal and marsh-haunting bugs, as well as those aquatic Heteroptera that live on the surface-film of water. The feelers are elongate and conspicuous. The Pentatomidae (shield-bugs), some of which are metallic or otherwise brightly coloured, are easily recognized by the great development of the scutellum, which reaches at least half-way back towards the tip of the abdomen, and in some genera covers the whole of the hind body, and also the wings when these are closed. The Coreidae have a smaller scutellum, and the feelers are inserted high on the head, while in the Lygaeidae they are inserted lower down. These three families have the foot with three segments. In the curious little Tingidae, whose integuments exhibit a pattern of network-like ridges, the feet are two-segmented and the scutellum is hidden by the pronotum. The Aradidae have two segmented feet, and a large visible scutellum. The Hydrometridae are a large family including the pond-skaters and other dwellers on the surface-film of fresh water, as well as the remarkable oceanic genus Halobates already referred to. The Reduviidae are a family of predaceous bugs that attack other insects and suck their juices; the beak is short, and carried under the head in a hook-like curve, not—as in the preceding families—lying close against the breast. The Cimicidae have the feet three-segmented and the forewings greatly reduced; most of the species are parasites on birds and bats, but one—Cimex lectidarius (figs. 3, 8)—is the well-known “bed-bug” which abounds in unclean dwellings and sucks human blood (see Bug). The Anthocoridae are nearly related to the Cimicidae, but the wings are usually well developed and the forewing possesses cuneus and embolium as well as corium and clavus. The Capsidae are a large family of rather soft-skinned bugs mostly elongate in form with the two basal segments of the feelers stouter than the two terminal. The forewing in this family has a cuneus (fig. 9 cu), but not an embolium. These insects are often found in large numbers on plants whose juices they suck.

Cryptocerata.—In this tribe are included five or six families of aquatic Heteroptera which spend the greater part of their lives submerged, diving and swimming through the water. The feelers are very small and are often hidden in cavities beneath the head. The Naucoridae and Belostomatidae are flattened insects, with four-segmented feelers and fore-legs inserted at the front of the prosternum. Two species of the former family inhabit our islands, but the Belostomatidae are found only in the warmer regions of the globe; some of them, attaining a length of 4 to 5 in., are giants among insects. The Nepidae (fig. 6) or water-scorpions (q.v.)—two British species—are distinguished by their three-segmented feelers, their raptorial fore-legs (in which the shin and foot, fused together, work like a sharp knife-blade on the grooved thigh), and their elongate tail-processes formed of the abdominal pleura and used for respiration. The Notonectidae, or “water-boatmen” (q.v.) have convex ovoid bodies admirably adapted for aquatic life. By means of the oar-like hind-legs they swim actively through the water with the ventral surface upwards; the fore-legs are inserted at the hinder edge of the prosternum. The Corixidae are small flattened water-bugs, with very short unjointed beak, the labrum being enclosed within the second maxillae, and the foot in the fore and intermediate leg having but a single segment. The hinder abdominal segments in the male show a curious asymmetrical arrangement, the sixth segment bearing on its upper side a small stalked plate (strigil) of unknown function, furnished with rows of teeth. On account of the reduction and modification of the jaws in the Corixidae, C. Börner has lately suggested that they should form a special sub-order of Hemiptera—the Sandaliorrhyncha.  
  
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From Mariatt, Bull. 14 (N. S.), Div. Ent. U.S. Dept. Agr.  
Fig. 10.—a, Nymph (4th stage) of Cicad, magnified; c, d, inner and outer faces of front leg, magnified—; b, teeth on thigh, more highly magnified.  
  
Homoptera

This sub-order includes the cicads, lantern-flies, frog-hoppers, aphids and scale-insects. The face has such a marked backward slope (see fig. 1) as to bring the beak into close contact with the haunches of the fore-legs. The feelers have one or more thickened basal segments, while the remaining segments are slender and thread-like. The fore-wings are sometimes membranous like the hind-wings, usually they are firmer in texture, but they never show the distinct areas that characterize the wings of Heteroptera. When at rest the wings of Homoptera slope roofwise across the back of the insect. In their life-history the Homoptera are more specialized than the Heteroptera; the young insect often differs markedly from its parent and does not live in the same situations; while in some families there is a passive stage before the last moult.  
  
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After Weed, Riley and Howard, Insect Life, vol iii.  
Fig. 11.—Cabbage Aphid (Aphisbrassicae). a, Male; c, female (wingless). Magnified. b and d, Head and feelers of male and female, more highly magnified.  
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After Howard, Year Book U.S. Dept. Agr., 1894.  
Fig. 12.—Apple Scale Insect (Mytilaspis pomorum). a, Male; e, female; c, larva magnified—; b, foot of male; d, feeler of larva, more highly magnified.  
  
The Cicadidae are for the most part large insects with ample wings; they are distinguished from other Homoptera by the front thighs being thickened and toothed beneath. The broad head carries, in addition to the prominent compound eyes, three simple eyes (ocelli) on the crown, while the feeler consists of a stout basal segment, followed by five slender segments. The female, by means of her serrated ovipositor, lays her eggs in slits cut in the twigs of plants. The young have simple feelers and stout fore-legs (fig. 10) adapted for digging; they live underground and feed on the roots of plants. In the case of a North American species it is known that this larval life lasts for seventeen years. The “song” of the male cicads is notorious and the structures by which it is produced have already been described (see also Cicada). There are about 900 known species, but the family is mostly confined to warm countries; only a single cicad is found in England, and that is restricted to the south.  
  
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After Howard, Year Book U.S. Dept. Agr., 1894.  
Fig. 13.—Apple Scale Insect (Mytilaspis pomorum). a, Scale from beneath showing female and eggs; b, from above, magnified—; c and e, female and male scales on twigs, natural size; d, male scale magnified.  
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From Osborn (after Denny), Bull. 5 (N.S.), Div. Ent. U.S. Dept. Agr.  
Fig. 14.—Louse (Pediculus vestimenti). Magnified.  
  
The Fulgoridae and Membracidae are two allied families most of whose members are also natives of hot regions. The Fulgoridae have the head with two ocelli and three-segmented feelers; frequently as in the tropical “lantern-flies” (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.”  
  
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From Osborn (after Schiödte), Bull. 5; (N.S.), Div. Ent. U.S. Dept. Agr.  
Fig. 15.—Proboscis of Pediculus. Highly magnified.  
  
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 Aphides and Phylloxera. 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 Economic Entomology, Scale-Insect.

Anoplura

The Anoplura or lice (see Louse) 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.

Bibliography.—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).

(G. H. C.)

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