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HEXAPODA (Gr. 4, six, and gobs, foot)

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Originally appearing in Volume V13, Page 421 of the 1911 Encyclopedia Britannica.
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HEXAPODA (Gr. 4, six, and gobs, foot), a term used in systematic zoology for that class of the ARTHROPODA, popularly known as insects. Linnaeus in his Systema nalurae (1735) grouped under the class Insecta all segmented animals with firm exoskeleton and jointed limbs—that is to say, the insects, centipedes, millipedes, crustaceans, spiders, scorpions and their allies. This assemblage is now generally regarded as a great division (phylum or sub-phylum) of the animal kingdom and known by K. T. E. von Siebold's (1848) name of Arthropoda. For the class of the true insects included in this phylum, Linnaeus's old term Insecta, first used in a restricted sense by M. J. Brisson (1756), is still adopted by many zoologists, while others prefer the name Hexapoda, first used systematically in its modern sense by P. A. Latreille in 1825 (Families naturelles du regne animal), since it has the advantage of expressing, in a single word, an important characteristic of the group. The terms "Hexapoda " and " hexapod " had already been used by F. Willughby, J. Ray and others in the late lath century to include the active larvae of beetles, as well as bugs, lice, fleas and other insects with undeveloped wings. Ckaracters. A true insect, or member of the class Hexapoda, may be known by the grouping of its body-segments in three distinct regions—a head, a thorax and an abdomen—each of which consists of a definite number of segments. In the terminology proposed by E. R. Lankester the arrangement is " nomomer- istic " and " nomotagmic." The head of an insect carries usually four pairs of conspicuous appendages — feelers, mandibles and two pairs of maxillae, so that the presence of four primitive somites is immediately evident. The compound eyes of insects resemble so closely the similar organs in Crustaceans that there can hardly be reasonable doubt of their homology, and the primitively appendicular nature of the eyes in the latter class suggests that in the Hexapoda also they represent the appendages of an anterior (protocerebral) segment. Behind the antennal (or deutocerebral) segment an " intercalary " or tritocerebral segment has been demonstrated by W. M. Wheeler (1893) and others in various insect embryos, while in the lowest insect order—the Aptera—a pair of minute jaws—the maxillulae—in close association with the tongue are present, as has been shown by H. J. Hansen (1893) and J. W. Folsom (1900). Distinct vestiges of the maxillulae exist also in the earwigs and booklice, according to G. Enderlein and C. BOrner (1904), and they are very evident in larval may-flies. The number of limb-bearing somites in the insectan head is thus seen to be seven. All of these are to be regarded as primitively post-oral, but in the course of development the mouth moves back to the mandibular segment, so that the first three somitesocular, antennal and intercalary—lie in front of it. In Lankester's terminology, therefore, the head of an insect is" triprosthomerous." The maxillae of the hinder pair become more or less fused together to form a " lower lip " or labium, and the segment of these appendages is, in some insects, only imperfectly united with the head-capsule. The thorax is composed of three segments; each bears a pair of jointed legs, and in the vast majority of insects the two hind-most bear each a pair of wings. From these three pairs of thoracic legs comes the name—Hexapoda—which distinguishes the class. And the wings, though not always present, are highly characteristic of the Hexapoda, since no other group of the Arthropoda has acquired the power of flight. In the more generalized insects the abdomen evidently consists of ten segments, the hindmost of which often carries a pair of tail-feelers, (cerci or cercopods) and a terminal anal segment. In some cases, however, it can be shown that the cerci really belong to an eleventh abdominal segment which usually becomes fused with the tenth. With very few exceptions the abdomen is without loco-motor limbs. Paired processes on the eighth and ninth abdominal segments may be specialized as external organs of reproduction, but these are probably not appendages. The female genital opening usually lies in front of the eighth abdominal segment, the male duct opens on the ninth. In all main points of their internal structure the Hexapoda agree with other Arthropoda. Specially characteristic of the class, however, is the presence of a complex system of air-tubes (tracheae) for respiration, usually opening to the exterior by a series of paired spiracles on certain of the body segments. The possession of a variable number of excretory tubes (Malpighian tubes), which are developed as outgrowths of the hind-gut and pour their excretion into the intestine,is also a distinctive character of the Hexapoda. The wings of insects are, in all cases, developed after hatching, the younger stages being wingless, and often unlike the parent in other respects. In such cases the development of wings and the attainment of the adult form depend upon a more or less profound transformation or metamorphosis. With this brief summary of the essential characters of the Hexapoda, we may pass to a more detailed account of their structure. EXOSKELETON The outer cellular layer (ectoderm or " hypodermis ") of insects as of other Arthropods, secretes a chitinous cuticle which has to be periodically shed and renewed during the growth of the animal. The regions of this cuticle have a markedly segmental arrangement„ and the definite hardened pieces (sclerites) of the exoskeleton are in close contact with one another along linear sutures, or are united by regions of the cuticle which are less chitinous and more membranous, so as to permit freedom of movement. Head.—The head-capsule of an insect (figs. 1, 2) is composed of a number of sclerites firmly sutured together, so that the primitive segmentation is masked. Above is the crown (vertex or epicranium), on which or on the " front may be seated three simple eyes(ocelli). Below this comes the front, and then the face or clypeus, to which a very distinct upper lip (labrum) is usually jointed. Behind the lain-urn arises a process—the epipharynx—which in some blood-sucking insects becomes a formidable piercing-organ. On either side a variable amount of convex area is occupied by the compound eye; in many insects of acute sense and accurate flight these eyes are very large and sub-globular, almost meeting on the middle line of the head. Below each eye is a cheek area (gene), often divided into an anterior and a posterior part, while a distinct chin-sclerite (gula) is often developed behind the mouth. Feelers.—Most conspicuous among the appendages of the head are the feelers or antennae, which correspond to the anterior feelers (antennules) of Crustacea. In their simpler condition they are long and many-jointed, the segments bearing numerous olfactory and tactile nerve-endings. Elaboration in the form of the feelers, often a secondary sexual character in Inale insects, may result from a distal broadening of the segments, so that the appendage becomes serrate, or from the development of processes bearing sensory organs, so that the structure is pinnate or feather-like. On the other hand, the number of segments may be reduced, certain of them often becoming highly modified in form. Jaws.—The mandibles of the Hexapoda are usually strong jaws with one or more teeth at the apex (fig. 1, A,• B, mn)), articulating at their bases with the head-capsule by sub-globular condyles, and provided with abductor and adductor muscles by means of which they can be separated or drawn together so as to bite solid food, or seize objects which have to be carried about. They never bear seg- mented limbs (palps) and only exception- ally (as in the chafers) is the skeleton com- posed of more than one sclerite. The mandibles often furnish a good example of "secondary sexual characters,' being more strongly developed in the male than in the female of the same species. In most insects that feed by suction the mandi- bles are modified. In bugs (Heteroptera) and many flies, for example, they are changed into needle-like piercers (fig. 2, II), while in moths and caddis-flies they are reduced to mere vestiges or altogether After Marlatt Entom. Bull. r4, n. s. (D. S. Dept. Agric.). suppressed.. s pprev ar of ntee frons; b, clypeus (the pointed labrum tinned, a Q,n of minute beneath it);II, mandible; 111, first laws—ths maxillulathe maxilla; (a, base ; b, sheath ; c, piercer), are present in the lowest order of insects, III', inner view of sheath; IV, second between the mandibles maxillae forming rostrum (b, mentum; c, and the first maxillae. ligula). They usually consist of an inner and an outer lobe arising from a basal piece, which bears'also in some genera a small palp (see APTERA). In their typical state of development, the first maxillae offer a striking contrast to the mandibles, being composed of a two-segmented basal piece (cardo and stipes, fig. 1, C, ca, st) bearing a distinct inner and outer lobe (lacinia and galea, fig. 1, C, la, ga) and externally ajointed limb or palp (fig i, C, pa). Such maxillae are found in most biting insects. In insects whose mouths are adapted for sucking and piercing, remarkable modifications may occur. In many blood-sucking flies, for example, the galea is absent, while the lacinia becomes a strong knife-like piercer and the palp is well developed. In bugs and aphids the lacinia is a slender needle-like piercer (fig. 2, III), while the palp is wanting. In butterflies and moths the lacinia is absent while the galea becomes i flexible process, grooved on its inner face, so as to make with its fellow a hollow sucking-trunk, and the palp is usually very small. The second pair of maxillae are more or less completely fused together to form what is known as the labium or " lower lip." In generalized biting insects, such as cockroaches and locusts (Orthoptera), the parts of a typical maxilla can be easily recognized in the labium. The fused cardines form a broad basal plate (sub-mentum) and the stipites a smaller plate (mentum)—see fig. 1, C, sm, me-jointed on to the submentum, while the galeae, laciniae and palps remain distinct. In specialized biting insects, such as beetles (Coleoptera), the labium tends to become a hard transverse plate bearing the pair of palps, a median structure—known as the ligula—formed of the conjoined laciniae, and a pair of small rounded processes—the reduced galeae—often called the " paraglossae," a term better avoided since it has been applied also to the maxillulae of Aptera, entirely different structures. The long sucking " tongue " of bees is probably a modification of the ligula. In bugs and aphids (Hemiptera), the fused second maxillae form a jointed grooved beak or rostrum (fig. 2, IV) in which the slender piercers (mandibles and first maxillae) work to and fro. This second pair of maxillae (or labium) form then the hinder or lower boundary of the mouth. In front or above the mouth is bounded by the labrum, while the mandibles and first maxillae lie on either side of it. A median process, known as the hypopharynx or tongue, arises from the floor of the mouth in front of the labium, and becomes most variously developed or specialized in different insects. The salivary duct opens on its hinder surface. It does not appear to represent a pair of appendages, but the maxillulae of the Aptera become closely associated with it. According to the view of R. Heymons, the hypopharynx represents the sterna of all the jaw-bearing somites, but other students consider that it belongs to the mandibular and first maxillary segments, or entirely to the segment of the first maxillae. Neck.—The head is usually connected with the thorax by a distinct membranous neck, strengthened in the more generalized orders with small chitinous plates (cervical sclerites). These have been interpreted as indicating one or more primitive segments-between' the head and thorax. Probably, however, as suggested by T. H. Huxley (Anat. Invert. Animals, 1877), they really belong to the labial segment which has not become completely fused with the head-capsule. It has been shown by C. Janet (1889), from careful studies of the musculature, that the greater part of the head-capsule is built up of the four anterior head-segments, the hindmost of which has the mandibles for its appendages, and this conclusion is in the main supported by the recent work on the head skeleton of J. H. Comstock and C. Kochi (1902) and W. A. Riley (1904). Thorax. The three segments which make up the thorax or fore-trunk are known as the prothorax, mesothorax and metathorax (see fig. 3). The dorsal area of the prothorax is occupied by a single sclerite, the ptonotum (fig. 3, d), which is large and conspicuous in those insects, such as cockroaches, bugs (Heteroptera) and beetles, which have the prothorax free--i.e. readily movable on the segment (mesothorax) immediately behind—smaller and of less importance where the prothorax is fixed to the mesothorax, as in bees and flies. The dorsal area of the mesothorax, and also of the metathorax, may be made up of a series of sclerites arranged one-behind the other —prescutum, scutum, scutellum and post-scutellum (fig. 3, e, f, g, h)'the scutellum of the mesothorax being often especially conspicuous. Ventrally, each segment of the thorax has a sternum with which a median pre-sternum and paired episterna and epimera are often associated (see figs. 3, 4). The recent suggestion of K. W. Verhoeff (1904) that the hexapodan thorax in reality contains six primitive segments is entirely without embryological support. Legs.—Each segment of the thorax carries a pair of legs. In most insects the leg is built up of nine segments: (I) a broad triangular, sub-globular, conical or cylindrical haunch (coxa) ; (2) a small trochanter; (3) an elongate stout thigh (femur); (4) a more slender shin (tibia) ; and (5-9) a foot consisting of five tarsal segments. The fifth (distal) tarsal segment carries a median adhesive pad—the pulvillus—on either side of which is a claw. The pulvillus is lbr A From Miall and Denny, The Cockroach, Lovell Reeve & Co. probably to be regarded as a true terminal (tenth) segment of the leg, while the claws are highly modified bristles. Numerous bristles are II - IV Sifter Marian, Fat. Bull. 3, n. s. (U.S. Dept. Agr.). I, Dorsal view. d, Pronotum. 1, Coxa of middle leg. II, Ventral view. Mesothorax: Metathorax: IV, Lateral view with f, Scutum. o, Epimeron. segments separated. g, Scutellum. p, Coxa of hind leg. Prothorax: h, Post-scutellum. n, First Abdominal a, Episternum. i, Mesophragma. Segment. b, Sternum. j. Epimeron. t, Tegula at base of c, Coxa of fore-leg. k, Episternum. fore-wing. usually present on the thighs, shins and feet of insects, some of them so delicate as to be termed " hairs " others so stout and hard that variety, dependent on the order to which the insect belongs, and the special function—walking, running, climbing, digging or swimming—for which the limb is adapted. The walking of insects has been carefully studied by V. Graber (1877) and J. Demoor (189o), who find that the legs are usually moved in two sets of three, the first and third legs of one side moving with the second leg of the other. One tripod thus affords a firm base of support while the legs of the other tripod are brought forward to their new positions. Wings.—Two pairs of wings are present in the vast majority of insects, borne respectively on the mesothorax and mtathorax. At the base of the wing, i.e. its attachment to the trunk, we find a ,highly complex series of small sclerites adapted for the varied movements necessary for flight. Those of the dragon-flies (Odonata) have been described in detail by R. von Lendenfeld.(1881). The long axis of the wings, when at rest, lies parallel to the body axis. In this position the outer margin of the wing is the costa, the inner the dorsum, and the hind-margin the termen. The angle between the costa and termen is the apex. When the wing is spread, its long axis is more or less at a right angle to the body axis. A wing is an out-growth from the dorsal and pleural regions of the thoracic segment that bears it, and microscopic examination shows it to consist of a double layer of cuticularized skin, the two layers being in contact except where they are thickened and folded to form the firm tubular nervures, which serve as a supporting framework for the wing membrane, enclose air-tubes, and convey blood. These nervures consist of a series of trunks radiating from the wing-base and usually branching as they approach the wing-margins, the branches being often connected by short transverse nervures, so that the wing-area is marked off into a number o: " cell§ " or areolets. The details of the nervuration vary greatly in the different orders, but J. H. Comstock and J. G. Needham have lately (1898-1899) shown that a common arrangement underlies all, six series of longitudinal or radiating nervures being present in the typical wing (see fig. 5). Along the costa runs a costal nervure. This is followed by a sub-costal which some-times shows two main branches. Then comes the radial—usually the most important nervure pf the wing—typically with five branches, and the median with four. These sets arise from a main trunk towards the front region of the wing-base. From another hinder trunk arise the two-branched cubital nervure and three separate anal nervures. In the hind-wing of many insects the number of e radial branches becomes re- duced, while the anal area is After Quail. Natural Science, vol. xiii., especially well developed and J.-M. Dent & Co. undergoes a fan-like folding FIG. 5.—Wing-Neuration in a when the wings are closed. Cossid Moth. 2, sub-costal; 3, Great diversity exists in the radial; 4, median; 5, cubical', texture and functions of fore 6 7, 8, anal nervures. and hind-wings in different in- ' sects; these differences are discussed in the descriptions of the various orders. The wings often afford secondary sexual characters, being not infrequently absent or reduced in the female when well developed in the male (see fig. 6). Rarely the male is the wingless sex. In addition to the wings there are smaller dorsal outgrowths of the thorax in many insects. Paired erectile plates (patagia) are borne on the prothorax in moths, while in moths, sawflies, wasps, bees and other insects there are small plates (tegulae)—see Fig. 3, t—on the mesothorax at the base of the fore-wings. Abdomen.—In the abdominal exoskeleton the segmental structure is very clearly marked, a series of sclerites—dorsal terga and abdominal sterna—being connected by pale, feebly chitinized cuticle, so that considerable freedom of movement between the segments is possible. The first and second abdominal sterna are often suppressed or reduced, on account of the strong development of the hind-legs. In many insects ten, and in a few eleven, abdominal segments can be clearly distinguished in addition to a small terminal anal segment. The female genital opening usually lies between the seventh and eighth segments, the male on the ninth. Prominent paired limbs are often borne on the tenth segment, the elongate tail-feelers (cerci) of bristle-tails and may-flies, or the forceps of earwigs, for example. In the Embiidae, a family of Isoptera, it has been shown by G. Erfderlein (1901) that these cerci clearly belong to a partially suppressed eleventh segment, and R. Heymons (1895-1896) has proved by embryological study that in all cases they really belong to this eleventh segment, which in the course of development becomes fused with the tenth. Smaller appendages (such as the stylets of male cockroaches) may be carried on the ninth segment. Pairs of processes carried on the eighth and ninth segments often become specialized to form the ovipositor of the female (see €[g. 14) and the genital armature of the male. A marked modification of the hinder abdominal segments may be noticed in most insects, After Miall and Denny, The Cockroach, Lovell Reeve & Co. I, Fore-leg and pro-sternum (S) ta, Tarsal segments. in front of which are the II, Middleleg and mesosternum. ventral cervical sclerites III, Hind leg and metasternum. (c). In IIIa, the episternum (a) and cx, Coxa. tr, Trochanter. epimeron (bare slightly separ- fe, Thigh. lb, Shin. ated. they are named " spines " or " spurs." In the relative development and shape of the various segments of the leg there is almost endless From Miall and Denny, The Cockroach, Lovell Reeve & Co.
End of Article: HEXAPODA (Gr. 4, six, and gobs, foot)
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HEXASTYLE (Gr. gE, six, and ori)Xos, column)

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