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Originally appearing in Volume V13, Page 426 of the 1911 Encyclopedia Britannica.
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INTERNAL ORGANS Nervous System.—The nervous system in the Hexapoda is built up on the'typical arthropodan plan of a double ventral nerve-cord with a pair of ganglia in each segment, the cords passing on either side of the gullet and connecting with an anterior nerve-centre or brain (fig. 7) in the head. The brain innervates the eyes and feelers, and must be regarded as a " syncerebrum " representing the ganglia of the three foremost limb-bearing somites united with the primitive cephalic lobes. Behind the gullet lies the sub-oesophageal nerve-centre (fig. 7, sb), composed of the ganglia of the four hinder headsomites and sending nerves to the jaws. A pair of ganglia in each thoracic segment is usual (fig. 8), and as many as eight distinct pairs of abdominal ganglia may sb often be distinguished, the hindmost of which repre- sents the fused ganglia of the last four segments. But in many highly organized insects a remark-able concentration of the trunk-ganglia takes place, all the nerve-centres of the thorax and abdomen in the chafers and in the Hemiptera, for instance, being represented by a single mass situated in the thorax. The legs, wings and other organs of the trunk receive their nerves from the thoracic and abdominal ganglia, and the fusion of several pairs of these ganglia may be regarded as corresponding to a centralization of individuality. A special " sympathetic " system arises by paired nerves from the oesophageal connectives; these nerves unite, and send back a median recurrent nerve associated with ganglia on the gullet and crop. whence proceed cords to various parts of the digestive system. In connexion with the central nervous system there are usually numerous organs of special sense. Most insects possess a pair of compound eyes, and many have, in addition, three simple eyes or ocelli on the vertex. The nature of these organs is described in the article ARTHROPODA. The surface of a compound eye is seen to be covered with a large number of hexagonal corneal facets, each of which over-lies an ommatidium or series of cell elements (fig. 9, A, B). There are over 25,000 ommatidia in the eye of a hawk moth. Auditory organs of a simple type are present in most insects. These consist of fine rods suspended between two points of the cuticle, and connected with nerve-fibres; they are known as chordotonal organs. In many cases a more complex ear is developed, which may be situated in strangely diverse regions of the insect's body. In locusts (Acridiidae) a the complex antennae of a male cockchafer. Organs of similar type on large ovate, tympanic membrane (fig. 9, G) is conspicuous on l the maxillae and epipharynx appear to exercise the function of taste, From Miall and Denny (after Newton), The Cock-roach, Lovell Reeve & Co. C F From Ridley, Insect Life, vol. 7 (U.S. Dept. Agr.). After Wall and Denny, The Cockroach, Lovell Reeve & Co. FIG. 8.—Ventral Muscles and Nerve Cord of Cockroach. ¢22 Muscular System.—The muscles in the Hexapoda are striated, as in Arthropods generally, the large fibres being associated in bundles which are at- tached from point to point of the cuticle, so as to move adjacent scler- ites with respect to one another (see figs. 8, to). For example, the con- traction of the tergo- sternal muscles, connect- ing the dorsal with the ventral sclerites of the abdomen, lessens the capacity of the abdo- minal region, while the contraction of the power- ful muscles arising from the thoracic walls, and inserted into the proxi- mal ends of the thighs, flexes or extends the legs. Circulatory System.— Insects afford an excel- lent illustration of the remarkable type of blood- system characterizing the Arthropoda. The dorsal vessel is an elongate tube, whose abdominal portion is usually c h a m here d, forming a contractile heart (fig. Io). At the constrictions between the chambers are paired slits, through which the blood passes from the surround- ing pericardial sinus. The dorsal vessel is prolonged anteriorly into an aorta, through which the blood is propelled into the great After Miall aad Denny, The Cockroach, Lovell body-cavity or haemo-Reeve & Co. coel. After bathing the dorsalwards into the peri- cardial sinus through fine perforations of its floor, and so makes its way into the heart again. Some water-bugs, e.g. of the families Belostomatidae, Nepidae, Corixidae and Hydrometridae have a pulsating sac at each knee-joint to assist the flow of blood through the legs, while in dragon-flies and locusts (Acridiidae) there is a ventral pulsating diaphragm, which forms the roof of a sinus enclosing the nerve-cords. Respiratory System.—As mentioned above, respiration by means of air-tubes (tracheae) is a most characteristic feature of the Hexapoda. An air-tube consists of an epithelium of large polygonal cells with a thin basement-membrane externally and a chitinous layer internally, the last-named being continuous with the outer cuticle. The chitinous layer is usually strengthened by thread-like thickenings which, in the region close to the outer opening of the tube, form a network enclosing polygonal areas, but which, through most of the tracheal system, are arranged spirally, the strengthening thread not forming a continuous spiral, but being interrupted after a few turns around the tube. The tracheal system in Hexapods is very complex, forming a series of longitudinal trunks with transverse anastomosing connexions (fig. II), and extending by Cock- the finest sub-division and by re-After roach, Lovell Miall and Reeve & DCo.enny, The pouted branching into all parts of of Air-Tubes ral Portion the tubes swell out into numerous o air-sacs, by which the breathing capacity is much increased. Atmospheric air gains access to the air-tubes through paired spiracles or stigmata, which usually occur laterally on most of the[INTERNAL ORGANS body-segments. These spiracles have firm chitinous edges, and can be closed by valves moved by special muscles. When the spiracles are open and the body contracts, air is expired. The subsequent expansion of the body causes fresh air to enter the tracheal system, and if the spiracles be then closed and the body again contracted, this air is driven to the finest branches of the air-tubes, where a direct oxygenation of the tissues takes place. The physiology of respiration has been carefully studied by F. Plateau (1884). In aquatic insects various devices for obtaining or entangling air are found; these modifications are described In the special articles on the various orders of insects (CoLEOPTERA, HEMIPTERA, &c.). Many insects have aquatic larvae, some of which take in atmospheric air at intervals, while others breathe dissolved air by means of tracheal gills. These modifications are mentioned below in the section on metamorphosis. Digestive System.—A striking feature in the food-canal of the Hexapoda, as in other Arthropods, is the great extent of the " fore-gut " and " hind-gut," lined with a chitinous cuticle, continuous with the exoskeleton. The fore-gut is composed of a tubular gullet, a large sac-like crop (fig. 12, c) and a proventriculus or " gizzard," whose function is to strain the food-substances before they pass on into the tubular stomach, which has no chitinous lining. This organ, usually regarded as a mid-gut, gives off a number of secretory caecal tubes (fig. 12, coe). At its hinder end it is continuous with the hind-gut, which is usually differentiated into a tubular coiled intestine (fig. 12, i) and a swollen rectum (fig. 12, r). From the fore-end of the hind-gut arise the slender Malpighian tubes (fig. 12, k), which have a renal function. On either side of the gullet are from one to ten pairs of salivary glands (fig. 12, s) whose ducts open into the mouth. Some of these glands may be modified for special purposes—as silk-producing glands in caterpillars or as poison-glands in blood-sucking flies and bugs. The food passing into the crop is there acted on by the saliva and also by an s, Salivary glands and reservoir. acid gastric juice which c, Crop (the gizzard below it). passes forwards from the coe, Caecal tubes (below them the stomach through the pro- stomach). ventriculus. As the k, Kidney tubes. various portions of the i, Intestine. food undergo digestion, r, Rectum. they are allowed to pass through the proventriculus into the stomach, where the nutrient substances are absorbed. Excretory System.—Nitrogenous waste-matter is removed from the body by the Malpighian tubes which open into the food-canal, usually where the hind-gut joins the stomach. These tubes vary in number from four to over a hundred in different orders of insects. The cells which line them and also the cavities of the tubes contain urates, which are excreted from the blood in the surrounding body-cavity. This cavity contains an irregular mass of whitish tissue, the fat-body, consisting of fat-cells which undergo degradation and become more or less filled with urates. When the worn-out cells are broken down, the urates are carried dissolved in the blood to the Malpighian tubes for excretion. The fat-body is therefore the seat of important metabolic processes in the hexapod body. Reproductive System.—All the Hexapoda are of separate sexes. The ovaries (fig. 13) in the female are paired, each ovary.consisting of a variable number of tubes (one in the bristle-tail Carepodea and fifteen hundred in a queen termite) in which the eggs are developed. From each ovary an oviduct (fig. 13, od) leads, and in some of the more primitive insects (bristle-tails, earwigs, may-flies) the two oviducts open separately direct to the exterior. Usually they open into a median vagina, formed by an ectodermal inpushing and lined with chitin. The vagina usually opens in front of the eighth abdominal sternite. Behind it is situated a spermatheca (fig. 14, sb) From Miall and Denny, The Cockroach. Lovell Reeve & Co. and the ovipositor previously mentioned, with its three pairs of ejaculatory duct. The male opening is on the ninth abdominal processes (Fig. 14, G, g). segment, to which belong the processes that form the claspers or The paired testes of the male consist of a variable number of seminal genital armature. Accessory glands are commonly present in connexion both with the male and the female reproductive organs. The poison-glands of the sting in wasps and bees are well-known examples of these. EMBRYOLOGY The Egg.—Among the Hexapoda, as in Arthropods generally, the egg is large, containing an accumulation of yolk for the nourishment of the growing embryo. Most insect eggs are of an elongate oval shape; some are globular, others flattened, while others again are flask-shaped, and the outer envelope (chorion) is often beautifully sculptured (figs. 2o, d; 21, a, b). Various devices are adopted for the protection of the eggs from mechanical injury or from the attacks of enemies, and for fixing them in appropriate situations. For example, the egg may be raised above the surface on which it is laid by an elongate stalk; the eggs may be protected by a secretion, which in some cases forms a hard protective capsule or " purse "; or they may be covered with shed hairs of the mother, while among water-insects a gelatinous envelope, often of rope-like form, is common. In various groups of the Hexapoda—aphids and some flesh-flies (Sarcophaga), for example—the egg undergoes development within the body of the mother, and the young insect is born in an active state; such insects are said to be " viviparous." Parthenogenesis.—A number of cases are known among the Hexapoda of the development of young from the eggs of virgin females. In insects so widely separated as bristle-tails and moths this occurs occasionally. In certain gall-flies (Cynipidae) no males are known to exist at all, and the species seems to be preserved entirely by successive parthenogenetic generations. In other gall-flies and in aphids we find that a sexual generation alternates with one or with many virgin generations. The offspring of the virgin females are in most of these instances females; but among the bees and wasps parthenogenesis occurs normally and always results in the development of males, the " queen " insect laying either a fertilized or unfertilized egg at will. Maturation, Fertilization and Segmentation.—Polar bodies were first observed in the eggs of Hexapoda by F. Blochmann in 1887. The two nuclei are successively divided from the egg nucleus in the usual way, but they frequently become absorbed in the peripheral protoplasm instead of being extruded from the egg-cell altogether. It appears that in parthenogenetic eggs two polar nuclei are formed. According to A. Petrunkevich (1901–1903), the second polar nucleus uniting with one daughter-nucleus of the first polar body gives rise to the germ-cells of the parthenogenetically-produced male. There is no reunion of the second polar nucleus with the female pronucleus, but, according to the recent work of L. Doncaster (1906–1907) on the eggs of sawflies, the number of chromosomes is not 'reduced in parthenogenetic egg-nuclei, while, in eggs capable of fertilization, the usual reduction-divisions occur. Fertilization takes place as the egg is laid, the spermatozoa being ejected from the spermatheca of the female and making their way to the protoplasm of the egg through openings (micropyles) in its firm envelope. The segmentation of the fertilized nucleus results in the formation of a number of nuclei which arrange themselves around the periphery of the egg and, the protoplasm: surrounding them becoming constricted, a blastoderm or layer of cells, enclosing the central yolk, is formed. Within the yolk the nuclei of some " yolk cells " can be distinguished, Germinal Layers and Food-Canal.—The embryo begins to develop as an elongate, thickened, ventral region of the blastoderm which is known as the ventral plate or germ band. Along this band a median furrow appears, and a mass of cells sinks within, the one-layered germ band thus becoming transformed into a band of two cell-layers (fig. 15). In some cases the inner layer is formed not by in- vagination but by n' ~onaovo proliferation or by de- ry OQV lamination. The From Nussbaum in Miall and Denny's, Cockroach, Looelh outer of these two Reeve & Co. the ectoderm. (fig. 15, With FIG. I5.—Diagram showing Formation of regard to the inner Germinal Layers. E, ectoderm; M, inner layer (endoblast of layer. Magnified. some authors, fig. 15, M) much difference of opinion has prevailed. It has usually been regarded as representing both endoderm and mesoderm, and the groove which usually leads to its formation has been compared to the abnormally elongated blastopore of a typical gastrula. No doubt can be entertained that the greater part of the inner layer corresponds to the mesoderm of more ordinary embryos, for the coelomic pouches, the germ-cells, the musculature and the vascular system all arise from it. Further, there is general agreement that the chitin-lined fore-gut and hind-gut, which form From Miall and Denny, The Cockroach, Lovell Reeve & Co. tubes, those of each testis opening into a vas deferens. In some bristle-tails and may-flies, the two vasa deferentia open separately, but usually they lead into a sperm-reservoir, whence issues a median From Miall and Denny, The Cockroach, Lovell Reeve & Co. Ts &c. Tergites. Od, Vagina. Sr, 7th Sternite. sp, Spermatheca. 9, Sclerite between 7th and 8th G, Anterior, and g, pos-S°, 8th Sclerite. [sterna. terior gonapophyses. Embryonic Membranes.—A remarkable feature in the embryonic development of most Hexapoda is the formation of a protective S From Nussbaum in Miell and Denny, The Cockroach, Lovell Reeve & Co. Fin. 16.—Cross section of Embryo of German Cockroach (Phyllodromia). S, serosa; A, amnion; E, ectoderm; N, rudiment of nerve-cord; M, mesodermal pouches. - , of the germ band a double fold in the undifferentiated blastoderm, which grows over the surface of the embryo, so that its inner and outer layers become continuous, forming respectively the amnion and the serosa (fig. 16, A, S). The embryo of a moth, a dragon-fly or a bug is invaginated into the yolk at the head end, the portion of the blastoderm necessarily pushed in with it forming the amnion. The embryo thus becomes transferred to the dorsal face of the egg, but at a later stage it undergoes reversion to its original ventral position. In some parasitic Hymenoptera there is only a single embryonic membrane formed by delamination from the blastoderm, while in a few insects, including the wingless spring-tails, the embryonic membranes are vestigial or entirely wanting. In the bristle-tails Lepisma and Machilis, an interesting transitional condition of the embryonic membranes has lately been shown by Heymons. The embryo is invaginated into the yolk, but the surface edges of the blastoderm do not close over, so that a groove or pore puts the insunken space that represents the amniotic cavity into communication with the outside. Heymons believes that the " dorsal organ " in the embryos of the lower Arthropoda corresponds with the region invaginated to form the serosa of the hexapod embryo. Wheeler, however, compares with the " dorsal organ " the peculiar extra embryonic membrane or indusium which he has observed between serosa and amnion in the embryo of the grasshopper Xiphidium. Metameric Segmentation. The segments are perceptible at a very early stage of the development as a number of transverse bands arranged in a linear sequence. The first segmentation of the ventral plate is not, however, very definite, and the segmentation does not make its appearance simultaneously throughout the whole length of the plate; the anterior parts are segmented before the posterior. In Orthoptera and Thysanura, as well as some others of the lower insects, twenty-one of these divisions—not, however, all similar—may be readily distinguished, six of which subsequently enter into the formation of the head, three going to the thorax and twelve to the abdomen. In Hemiptera only eleven and in Collembola only six abdominal segments have been detected. The first and last of these twenty-one divisions are so different from the others that they can scarcely be considered true segments. Head Segments.—In the adult insect the head is insignificant in size compared with the thorax or abdomen, but in the embryo it forms a much larger portion of the body than it does in the adult. Its composition has been the subject of prolonged difference of opinion. Formerly it was said that the head consisted of four divisions, viz. three segments and the procephalic or prae-oral lobes. It is now ascertained that the procephalic lobes consist of three divisions, so that the head must certainly be formed from at least six segments. The first of these, according to the nomenclature of Heymons (see fig. 17), is the mouth or oral piece; the second, the antennal segment; the third, the intercalary or prae-mandibular segment; while the fourth, fifth, and sixth are respectively the segments of the mandibles and of the first and second maxillae. These six divisions of the head are diverse in kind, and subsequently undergo so much change that the part each of them takes in the formation of the head-capsule is not finally determined. The labrum and clypeus are developed as a single prolongation of the oral piece, not as a pair of appendages. The antennal segment apparently entirely disappears, with the exception of a pair of appendages it bears; these become the antennae; it is possible that the original segment, or some part of it, may even become a portion of the actual antennae. The intercalary segment has no appendages, nor rudiments thereof, except, according to H. Uzel (1897), in the thysanuran Campodea, and probably entirely disappears, though J. H. Comstock and C. Kochi believe that the labrum belongs to it. The appendages of the posterior three or trophal segments become the parts of the mouth. The appendages of the two maxillary segments arise as treble instead of single projections, thus differing from other appendages. From these facts it appears that the anterior three divisions of the head differ strongly from the posterior three, which greatly resemble thoracic segments; hence it has been thought possible that the anterior divisions may represent a primitive head, to which three segments and their leg-like appendages were subsequently added to form the head as it now exists. This is, how-ever, very doubtful, and an entirely different inference is possible. Besides the five limb-bearing somites just enumerated, two others must now be recognized in the head. One of these is the ocular segment, in front of the antennal, and behind the primitive pre-oral segment. The other is the segment of the maxillulae (sep above, under Jaws), behind the mandibular somite; the presence of this in the embryo of the collembolan Anurida has been lately shown (1900) by J. W. Folsom (fig. 18, v. 5), who terms the maxillulae " superlinguae " on account of their close association with the hypopharynx or lingua. In reference to the structure of the head-capsule in the imago, it appears that the clypeus and labrum represent, as already said, an unpaired median outgrowth of the oral piece. According to W. A. Riley (1904) the epicranium or " vertex," the compound eyes and the front divisions of the genae are formed by the cephalic lobes of the embryo (belonging membrane analogous to the amnion of higher Vertebrates and to the ocular segment), while the mandibular and maxillary segments known by the same term. Usually there arises around the edge ( form the hinder parts of the genae and the hypopharynx. 424 HEXAPODA [EMBRYOLOGY Great difference of opinion exists as to the hypopharynx, which has even been thought to represent a distinct segment, or the pair of appendages of a distinct segment. Heymons considers that it represents the sternites of the three trophal segments, and that the gula is merely a secondary development. Felsom looks on the hypopharynx as a secondary development. Riley holds that the hypopharynx be- longs to the mandibular and maxillary segments, while the cervical sclerites or gula represent the ster- .__ . Ant num of the labial segment. The ganglia of the nervous -Tre system offer some important e ~j evidence as to the mor- bid--- phology of the head, and are alluded to below. Thoracic Segments.—These are always three in number. The three pairs of legs appear very early as rudiments. Though the thoracic segments bear the wings, no trace of these appendages exists till the close of the embryonic life, nor even, in many cases, till much later. The thoracic segments, as seen in an early stage of the ventral plate, display in a . well-marked manner the essential elements of the insect segment. These elements are a central piece or sternite, and a lateral field on each side bearing the leg-rudiment. The external part of the lateral field subsequently grows up, and by coalescence with its fellow forms the tergite or dorsal part of the segment. Abdominal Segments and Appendages.—We have al-ready seen that in numerous lower insects the abdomen is formed from twelve divisions placed in linear fashion. Eleven of these may perhaps be considered as true segments, but the twelfth or terminal one is different, and After Hermon., is called by Heymons a telson; 17.-Morphology of an Insect; lson; in it is placed the anal orifice, and the mass the embryo of Gryllotalpa, somewhat subsequently becomes the diagrammatic. The longitudinal seg- upper and lower laminae mented band along the middle line re- anales. In Hemiptera this presents the early segmentation of the telson is absent, and the nervous system and the subsequent anal orifice is placed quite median field of each sternite ; the lateral at the termination of the transverse unshaded bands are the eleventh, segment. More-lateral fields of each segment; the over, in this order the abshaded areas indicate the more inter- domen shows at first a Wally placed mesoderm layer. The seg- division into only nine segments are numbered 1-21 ; 1-6 will form ments and a terminal mass, the head, 7-9 the thorax,10-21 the abdo- which last subsequently be-men. A, anus; Abx1 Abx11, appendage comes divided into two. of 1st and of I ith abdominal segments; The appendages of the A ns, anal piece = telson or 12th abdo- abdomen are called cerci, urinal segment; Ant, antenna; Dc, stylets and gonapophyses. deuterencephalon; Md, mandible; They differ much according lix1, first maxilla; Mx2, second to the kind of insect, and maxilla or labium; 0, mouth; Obcl, in the adult according to rudimentary labrum and clypeus; sex. Difference of opinion Pre, protencephalon; St1, St1a, stig- as to the nature of the mata 1 and to; Terg, tergite; Thx1, abdominal appendages pre- appendage of first thoracic segment ; vails. The cerci, when Tre, tritencephalon; a thickening present, appear in the at hinder margin of the mouth. mature insect to be attached to the tenth segment, but according to Heymons they are really appendages of the eleventh segment, their connexion with the tenth being secondary and the result of considerable changes that take place in the terminal segments. It has been disputed whether any true cerci exist in the higher insects, but they are probably represented in the Diptera and in the scorpion-flies (Mecaptera). In those insects in which a median terminal appendage exists between the two cerci this is considered to be a A. After \\'heeler, Journ. Morph. vol. viii., and Folsom, Bull. Mats. Harvard, xxxvi. B. After Folsom. 1, Ocular segment. the appendages of the ninth segment are 2, Antennal. the stylets, and that the gonapophyses 3, Trito-cerebral. cannot therefore be appendicular. The 4, Mandibular. pseudopods that exist on the abdomen of 5, Maxillular. numerous caterpillars may possibly arise 6, Maxillary. from the embryonic pseudopods, but this 7, Labial, also is far from being established. 8, Prothoracic. Nervous System.—The nervous system is 9 Mesothoracic. ectodermal in origin, and is developed and lo, Metathoracic. segmented to a large extent in connexion with the outer part of the body, so that it affords important evidence as to the segmentation thereof. The continuous layer of cells front which the nervous system is developed undergoes a segmentation analogous with that we have described as occurring in the ventral plate; there is thus formed a pair of contiguous ganglia for each segment of the body, but there is no ganglion for the telson. The ganglia become greatly changed in position during the later life, and it is usually said that there are only ten pairs of abdominal ganglia even in the embryo. In Orthoptera, Heymons has demonstrated the existence of eleven pairs, the terminal pair becoming, however, soon united with the tenth. The nervous system of the embryonic head exhibits three ganglionic masses, anterior to the thoracic ganglionic masses; these three masses subsequently amalgamate and form the sub-oesophageal ganglion, which supplies the` trophal segments. In front of the three masses that will form the sub-oesophageal ganglion the mass of cells that is to form the nervous system is very large, and projects on each side; this anterior or " brain " mass consists of three lobes (the prot-, deut-, and tritencephalon of Viallanes and others), each of which might be thought to represent a segmental ganglion. But the protocerebrum contains the ganglia of the ocular segment in addition to those of the procephalic lobes. These three divisions subsequently form the supra-oesophageal ganglion or brain proper. There are other ganglia in addition to those of the ventral chain, and Janet supposes that the ganglia of the sympathetic system indicate the existence of three anterior head-segments; the remains of the segments themselves are, in accordance with this view, to be sought in the Obel prolongation of the eleventh tergite. The stylets, when present, are placed on the ninth segment, and in some Thysanura exist also on the eighth segment; their development takes place later in life than that of the cerci. The gonapophyses are the projections near the extremity of the body that surround the sexual orifices, and vary extremely according to the kind of insect. They have chiefly been studied in the female, and form the sting and ovipositor, organs peculiar to this sex. They are developed on the ventral surface of the body and are six in number, one pair arising from the eighth ventral plate and two pairs from the ninth. This has been found to be the case in insects so widely different as Orthoptera and Aculeate Hymenoptera. The genital armature of the male is formed to a considerable extent by modifications of the segments them-selves. The development of the armature has been little studied, and the question whether there may be present gonapophyses homologous with those of the female is open. In the adult state no insect possesses more than six legs, and they are always attached to the thorax; in many Thysanura there are, however, processes on the abdomen that, as to their position, are similar to legs. In the embryos of many insects there are projections from the segments of the abdomen similar, to a considerable extent, to the rudimentary thoracic legs. The question whether these projections can be considered an indication of former polypody in insects has been raised. They do not long persist in the embryo, but disappear, and the area each one occupied becomes part of the sternite. In some embryos there is but a single pair, of these rudiments (or vestiges) situate on the first abdominal segment, and in some cases they become invaginations of a glandular nature. Whether cerci, stylets and gonapophyses are developed from these rudiments has been much debated. It appears that it is possible to accept cerci and stylets as modifications of the temporary pseudo-pods, but it is more difficult to believe that this is the case with the gonapophyses, for they apparently commence their development considerably later than cerci and stylets and only after the apparently complete disappearance of the embryonic pseudopods. The fact that there are two pairs of gonapophyses on the ninth abdominal segment would be fatal to the view that they are in any way homologous with legs, were it not that there is some evidence that the division into two pairs is secondary and incomplete. But another and apparently in-superable objection may be raised—that stomodaeum. Folsom has detected in the embryo of Anurida a pair of ganglia (fig. t8, 5) belonging to the maxillular (or superlingual) segment, thus establishing seven sets of cephalic ganglia, and sup-porting his view as to the composition of the head. Air-tubes.—The air-tubes, like the food-canal, are formed by invaginations of the ectoderm, which arise close to the developing appendages, the rudimentary spiracles appearing soon after the budding limbs. The pits leading from these lengthen into tubes, and undergo repeated branching as development proceeds. Dorsal Closure.—The germ band evidently marks the ventral aspect of the developing insect, whose body must be completed by the extension of the embryo so as to enclose the yolk dorsally. The method of this dorsal closure varies in different insects. In the Colorado beetle (Doryphora), whose development has been studied by W. M. Wheeler, the amnion is ruptured and turned back from covering the germ band, enclosing the yolk dorsally and becoming finally absorbed, as the ectoderm of the germ band itself spreads to form the dorsal wall. In some midges and in caddis-flies the serosa becomes ruptured and absorbed, while the germ band, still clothed with the amnion, grows around the yolk. In moths and certain saw-flies there is no rupture of the membranes; the Russian zoologists Tichomirov and Kovalevsky have described the growth of both amnion and embryonic ectoderm around the yolk, the embryo being thus completely enclosed until hatching time by both amnion and serosa. V. Graber has described a similar method of dorsal closure in the saw-fly Hylotoma. Mesoderm, Coelom and Blood-System.—From the mesoderm most of the organs of the body—muscular, circulatory, reproductive— take their origin. The mass of cells undergoes segmentation corre- sponding with the outer segmentation of the embryo, and a pair of cavities—the coelomic pouches (fig. i6, M)— are formed in each seg- ment. Each coelomic pouch—as traced by Heymons in his study on the development of the cockroach (Phyllo- dromia)—divides into three parts, of which the most dorsal con- tains the primitive germ-cells, the median disappears, and the ventral loses its boun- daries as it becomes filled up with the grow- ing fat body (fig. 19). This latter, as well as the heart and the walls of the blood spaces, arises by the modification of mesodermal cells, and the body cavity is formed by the enlarge- of ovarian tubes. on, Muscle-rudiment. n, Nerve-chain. f, Fat body. s, Inpushing of ectoderm to form air-tubes. x, Secondary body-cavity. mesodermal in origin. The median vagina, spermatheca and ejaculatory duct are, on the other hand, formed by ectodermal inpushings. The classical researches of J. A. Palmen (1884) on these ducts have shown that in may-flies and in female earwigs the paired mesodermal ducts open directly to the exterior, while in male earwigs there is a single mesodermal duct, due either to the coalescence of the two or to the suppression of one. In the absence of the external ectodermal ducts usual in winged insects, these two groups resemble therefore the primitive Aptera. The presence of rudiments of the genital ducts of both sexes in the embryo of either sex is interesting and suggestive. The ejaculatory duct which opens on the ninth abdominal sternum in the adult male arises in the tenth abdominal embryonic segment and subsequently moves forward.
End of Article: INTERNAL

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