Stages of PlatygasterFig. 190. A series of stages in the development of Platygaster.(From Lubbock; after Ganin.)
Fig. 190. A series of stages in the development of Platygaster.(From Lubbock; after Ganin.)
The very first stages are unfortunately but imperfectly known, and the interpretations offered by Ganin do not in all cases appear quite satisfactory. In the earliest stage after being laid the egg is enclosed in a capsule produced into a stalk (fig. 190A). In the interior of the egg there soon appears a single spherical body, regarded by Ganin as a cell (fig. 190B). In the next stage three similar bodies appear in the vitellus, no doubt derived from the first one (fig. 190C). The central one presents somewhat different characters to the two others, and, according to Ganin, gives rise tothe whole embryo. The two peripheral bodies increase by division, and soon appear as nuclei imbedded in a layer of protoplasm (fig. 190D, E, F). The layer so formed serves as a covering for the embryo, regarded by Ganin as equivalent to the amnion (? serous membrane) of other Insect embryos. In the embryo cell new cells are stated to be formed by a process of endogenous cell formation (fig. 190D, E). It appears probable that Ganin has mistaken nuclei for cells in the earlier stages, and that a blastoderm is formed as in other Insects, and that this becomes divided in a way not explained into a superficial layer which gives rise to the serous envelope, and a deeper layer which forms the embryo. However thismay be, a differentiation into an epiblastic layer of columnar cells and a hypoblastic layer of more rounded cells soon becomes apparent in the body of the embryo. Subsequently to this the embryo grows rapidly, till by a deep transverse constriction on the ventral surface it becomes divided into an anterior cephalothoracic portion and a posterior caudal portion (fig. 190F). The cephalothorax grows in breadth, and near its anterior end an invagination appears, which gives rise to the mouth and œsophagus. On the ventral side of the cephalothorax there is first formed a pair of claw-like appendages on each side of the mouth, then a posterior pair of appendages near the junction of the cephalothorax and abdomen, and lastly a pair of short conical antennæ in front.
At the same time the hind end of the abdomen becomes bifid, and gives rise to a fork-like caudal appendage; and at a slightly later period four grooves make their appearance in the caudal region, and divide this part of the embryo into successive segments. While these changes have been taking place in the general form of the embryo, the epiblast has given rise to a cuticle, and the hypoblastic cells have become differentiated into a central hypoblastic axis—the mesenteron—and a surrounding layer of mesoblast, some of the cells of which form longitudinal muscles.
With this stage closes what may be regarded as the embryonic development of Platygaster. The embryo becomes free from the amnion, and presents itself as a larva, which from its very remarkable characters has been spoken of as the Cyclops larva by Ganin.
The larvæ of three species have been described by Ganin, which are represented infig. 191A, B, C. These larvæ are strangely dissimilar to the ordinary Hexapod type, whether larval or adult. They are formed of a cephalothoracic shield with the three pairs of appendages (a,kf,lfg), the development of which has already been described, and of an abdomen formed of five segments, the last of which bears the somewhat varying caudal appendages. The nervous system is as yet undeveloped.
The larvæ move about in the tissues of their hosts by means of their claws.
The first larval condition is succeeded by a second with very different characters, and the passage from the first to the second is accompanied by an ecdysis.
The ecdysis commences at the caudal extremity, and the whole of the last segment is completely thrown off. As the ecdysis extends forwards the tail loses its segmentation and becomes strongly compressed, the appendages of the cephalothorax are thrown off, and the whole embryo assumes an oval form without any sharp distinction into different regions and without theslightest indication of segmentation(fig. 191D). Of the internal changes which take place during the shedding of the cuticle, the first is the formation of a proctodæum (gh) by an invagination, which ends blindly in contact with the mesenteron. Shortly after this a thickening of the epiblast (bsm) appears along the ventral surface, which gives rise mainly to the ventral nerve cord; this thickening is continuous behind with theepiblast which is invaginated to form the proctodæum, and in front is prolonged on each side into two procephalic lobes, in which there are also thickenings of the epiblast (gsae), which become converted into supra-œsophageal ganglia, and possibly other parts.
Development of PlatygasterFig. 191. A series of stages in the development of Platygaster. (From Lubbock; after Ganin.)A. B. C. Cyclops larvæ of three species of Platygaster.D. Second larval stage. E. Third larval stage.mo.mouth;a.antenna;hf.hooked feet;lfg.lateral feet;f.branches of tail;ul.lower lip;slkf.œsophagus;gsae.supra-œsophageal ganglion;bsm.ventral epiblastic plate;lm.lateral muscles (the letters also point in D to the salivary glands);gh.proctodæum;ga.generative organs;md.mandibles;ag.ducts of salivary glands;sp.(in E) salivary glands;mls.stomach;ed.intestine;ew.rectum;ao.anus;tr.tracheæ;fk.fat body.
Fig. 191. A series of stages in the development of Platygaster. (From Lubbock; after Ganin.)
A. B. C. Cyclops larvæ of three species of Platygaster.D. Second larval stage. E. Third larval stage.
mo.mouth;a.antenna;hf.hooked feet;lfg.lateral feet;f.branches of tail;ul.lower lip;slkf.œsophagus;gsae.supra-œsophageal ganglion;bsm.ventral epiblastic plate;lm.lateral muscles (the letters also point in D to the salivary glands);gh.proctodæum;ga.generative organs;md.mandibles;ag.ducts of salivary glands;sp.(in E) salivary glands;mls.stomach;ed.intestine;ew.rectum;ao.anus;tr.tracheæ;fk.fat body.
Towards the close of the second larval period the muscles (lm) become segmentally arranged, and give indications of the segmentation which becomes apparent in the third larval period. The third and last larval stage (fig. 191E) of Platygaster, during which it still remains in the tissues of its host, presents no very peculiar features. The passage from the second to the third form is accompanied by an ecdysis.
Remarkable as are the larvæ just described, there can I think be no reason, considering their parasitic habits, for regarding them as ancestral.
Metamorphosis and heterogamy.
Metamorphosis.The majority of Insects are born in a condition in which they obviously differ from their parents. The extent of this difference is subject to very great variations, but as a rule the larvæ pass through a very marked metamorphosis before reaching the adult state. The complete history of this metamorphosis in the different orders of Insects involves a far too considerable amount of zoological detail to be dealt with in this work; and I shall confine myself to a few observations on the general characters and origin of the metamorphosis, and of the histological processes which take place during its occurrence[176].
In the Aptera the larva differs from the adult only in the number of facets in the cornea and joints in the antennæ.
In most Orthoptera and Hemiptera the larvæ differ from the adult in the absence of wings and in other points. The wings, etc., are gradually acquired in the course of a series of successive moultings. In the Ephemeridæ and Libellulidæ, however, the metamorphosis is more complicated, in that the larvæ have provisional tracheal gills which are exuviated before the final moult. In the Ephemeridæ there are usually a great number of moultings; the tracheal gills appear after the second moult, and the rudiments of the wings when the larva is about half grown. Larval life may last for a very long period.
In all the other groups of Insects,viz.the Diptera, Neuroptera, Coleoptera, Lepidoptera, and Hymenoptera, the larva passes—with a few exceptions—through a quiescent stage, in which it is known as a pupa, before it attains the adult stage. These forms are known as theHolometabola.
In the Diptera the larvæ are apodous. In the true flies (Muscidæ) they are without a distinct head and have the jaws replaced by hooks. In the Tipulidæ there is on the other hand a well-developed head with the normal appendages. The pupæ of the Muscidæ are quiescent, and are enclosed in the skin of the larva which shrinks and forms a firm oval case. In theTipulidæ the larval skin is thrown off at the pupa stage, and in some cases the pupæ continue to move about.
The larvæ of the Neuroptera are hexapodous voracious forms. When the larva becomes a pupa all the external organs of the imago are already established. The pupa is often invested in a cocoon. It is usually quiescent, though sometimes it begins to move about shortly before the imago emerges.
In the Coleoptera there is considerable variety in the larval forms. As a rule the larvæ are hexapodous and resemble wingless Insects. But some herbivorous larvæ (e.g.the larva of Melolontha) closely resemble true caterpillars, and there are also grub-like larvæ without feet (Curculio) which resemble the larvæ of Hymenoptera. The pupa is quiescent, but has all the parts of the future beetle plainly visible. The most interesting larvæ among the Coleoptera are those of Sitaris, one of the Meloidæ (Fabre,No.409). They leave the egg as active hexapodous larvæ which attach themselves to the bodies of Hymenoptera, and are thence transported to a cell filled with honey. Here they eat the ovum of the Hymenopterous form. They then undergo an ecdysis, in which they functionally lose their appendages, retaining however small rudiments of them, and become grubs. They feed on the honey and after a further ecdysis become pupæ.
In the Lepidoptera the larva has the well-known form of a caterpillar. The caterpillars have strong jaws, adapted for biting vegetable tissues, which are quite unlike the oral appendages of the adult. They have three pairs of jointed thoracic legs, and a variable number (usually five) of pairs of rudimentary abdominal legs—the so-called pro-legs. The larva undergoes numerous ecdyses, and the external parts of the adult such as the wings, etc., are formed underneath the chitinous exoskeleton before the pupa stage. The pupa is known as a chrysalis and in some Lepidoptera is enveloped in a cocoon.
The Hymenoptera present considerable variations in the character of the larvæ. In the Aculeata, many Entomophaga, the Cynipidæ, etc., the larvæ are apodous grubs, incapable of going in search of their food; but in the Siricidæ they are hexapodous forms like caterpillars, which are sometimes even provided with pro-legs. In some of the Entomophaga the larvæ have very remarkable characters which have already been described in a special section,videpp.418,419.
Before proceeding to the consideration of the value of the various larval forms thus shortly enumerated, it is necessary to say a few words as to the internal changes which take place during the occurrence of the above metamorphosis. In the simplest cases, such as those of the Orthoptera and Hemiptera, where the metamorphosis is confined to the gradual formation of the wings, etc. in a series of moults, the wings first appear as two folds of the epidermis beneath the cuticle on the two posterior thoracic segments. At the next moult these processesbecome covered by the freshly formed cuticle, and appear as small projections. At every successive moult these projections become more prominent owing to a growth in the epidermis which has taken place in the preceding interval. Accompanying the formation of such organs as the wings, internal changes necessarily take place in the arrangement of the muscles, etc. of the thorax, which proceedpari passuwith the formation of the organs to which they belong. The characters of the metamorphosis in such forms as the Ephemeridæ only differ from the above in the fact that provisional organs are thrown off at the same time that the new ones are formed.
In the case of the Holometabola the internal phenomena of the metamorphosis are of a very much more remarkable character. The details of our knowledge are largely due to Weismann (Nos.430and431). The larvæ of the Holometabola have for the most part a very different mode of life to the adults. A simple series of transitions between the two is impossible, because intermediate forms would be for the most part incapable of existing. The transition from the larval to the adult state is therefore necessarily a more or less sudden one, and takes place during the quiescent pupa condition. Many of the external adult organs are however formed prior to the pupa stage, but do not become visible on the surface. The simplest mode of Holometabolic metamorphosis may be illustrated by the development of Corethra plumicornis, one of the Tipulidæ. This larva, like that of other Tipulidæ, is without thoracic appendages, but before the last larval moult, and therefore shortly before the pupa stage, certain structures are formed, which Weismann has calledimaginal discs. These imaginal discs are in Corethra simply invaginations of the epidermis. There are in the thorax six pairs of such structures, three dorsal and three ventral. The three ventral are attached to the terminations of the sensory nerves, and the limbs of the imago are formed as simple outgrowths of them, which as they grow in length take a spiral form. In the interior of these outgrowths are formed the muscles, tracheæ, etc., of the limbs; which are believed by Weismann (it appears to me without sufficient ground) to be derived from a proliferation of the cells of the neurilemma. The wings are formed from the two posterior dorsal imaginaldiscs. The hypodermis of the larva passes directly into that of the imago.
The pupa stage of Corethra is relatively very short, and the changes in the internal parts which take place during it are not considerable. The larval abdominal muscles pass for the most part unchanged into those of the imago, while the special thoracic muscles connected with the wings, etc., develop directly during the latest larval period from cords of cells already formed in the embryo.
In the Lepidoptera the changes in the passage from the larval to the adult state are not very much more considerable than those in Corethra. Similar imaginal discs give rise during the later larval periods to the wings, etc. The internal changes during the longer pupa period are somewhat more considerable. Important modifications and new formations arise in connection with the alimentary tract, the nervous and muscular systems.
The changes which take place in the true flies (Muscidæ) are far more complicated than either those in Corethra or in the Lepidoptera. The abdomen of the larva of Musca becomes bodily converted into the abdomen of the imago as in the above types, but the whole epidermis and appendages of the head and thorax are derived from imaginal discs which are formed within and (so far as is known) independently of the epidermis of the larva or embryo. These imaginal discs are simple masses of apparently indifferent cells, which for the most part appear at the close of embryonic life, and are attached to nerves or tracheæ. They grow in size during larval life, but during the relatively long pupa stage they unite together to give rise to a continuous epidermis, from which the appendages grow out as processes. The epidermis of the anterior part of the larva is simply thrown off, and has no share in forming the epidermis of the adult.
There are a pair of cephalic imaginal discs and six pairs of thoracic discs. Two pairs, a dorsal and a ventral, give rise to each thoracic ring, and the appendages attached to it.
Though, as mentioned above, no evidence has yet been produced to shew that the imaginal discs of Musca are derived from the embryonic epiblast, yet their mode of growth andeventual fate proves beyond the shadow of a doubt that they are homologous with the imaginal discs of Corethra. Their earliest origin is well worth further investigation.
The metamorphosis of the internal organs is still more striking than that of the external. There is a disruption, total or partial, of all the internal organs except the generative organs. In the case of the alimentary tract, the Malpighian vessels, the heart and the central nervous system, the disruption is of a partial kind, which has been called by Weismann histolysis. The cells of these organs undergo a fatty degeneration, the nuclei alone in some cases remaining. The kind of plasma resulting from this degeneration retains the shape of the organs, and finally becomes built up again into the corresponding organs of the imago. The tracheæ, muscles and peripheral nerves, and an anterior part of the alimentary tract, are entirely disrupted. They seem to be formed again from granular cells derived from the enormous fat body.
The phenomena of the development of the Muscidæ are undoubtedly of rather a surprising character. Leaving for the moment the question of the origin of the pupa stage to which I return below, it will be admitted on all hands that during the pupa stage the larva undergoes a series of changes which, had they taken place by slow degrees, would have involved, in such a case as Musca, a complete though gradual renewal of the tissues. Such being the case, the cells of the organs common to the larva and the imago would, in the natural course of things, not be the same cells as those of the larva but descendants of them. We might therefore expect to find in the rapid conversion of the larval organs into those of the adult some condensation, so to speak, of the process of ordinary cell division. Such condensations are probably represented in the histolysis in the case of the internal organs, and in the formation of imaginal discs in the case of the external ones, and I think it probable that further investigation will shew that the imaginal discs of the Muscidæ are derivatives of the embryonic epiblast. The above considerations by no means explain the whole of Weismann’s interesting observations, but an explanation is I believe to be found by following up these lines.
More or less parallel phenomena to those in Insects are found in the development of the Platyelminthes and Echinoderms. The four disc-like invaginations of the skin in many larval Nemertines (videp.198), which give rise to the permanent body wall of the Nemertine, may be compared to the imaginal discs. The subsequent throwing off of the skin of Pilidium or larva of Desor is a phenomenon like the absorption of part of the larval skin of Musca. The formation of an independent skin within the first larvalform in the Distomeæ and in the Cestoda may be compared to the apparently independent formation of the imaginal discs in Musca.
The fact that in a majority of instances it is possible to trace an intimate connection between the surroundings of a larva and its organization proves in the clearest waythat the characters of the majority of existing larval forms of Insects have owed their origin to secondary adaptations. A few instances will illustrate this point.
Half of Campodea FragilisFig. 192. Anterior half of Campodea Fragilis.(From Gegenbaur; after Palmen.)a.antennæ;p.feet;p´.post-thoracic rudimentary feet;s.stigma.
Fig. 192. Anterior half of Campodea Fragilis.(From Gegenbaur; after Palmen.)
a.antennæ;p.feet;p´.post-thoracic rudimentary feet;s.stigma.
In the simplest types of metamorphosis,e.g.those of the Orthoptera genuina, the larva has precisely the same habits as the adult. We find that a caterpillar form is assumed by phytophagous larvæ amongst the Lepidoptera, Hymenoptera and Coleoptera. Where the larva has not to go in search of its nutriment the grub-like apodous form is assumed. The existence of such an apodous larva is especially striking in the Hymenoptera, in that rudiments of thoracic and abdominal appendages are present in the embryo and disappear again in the larva. The case of the larva of Sitaris, already described (p. 421), affords another very striking proof that the organization of the larva is adapted to its habits.
It follows from the above that the development of such forms as the Orthoptera genuina is more primitive than that of the holometabolous forms; a conclusion which tallies with the fact that both palæontological and anatomical evidence shew the Orthoptera to be a very primitive group of Insects.
The above argument probably applies with still greater force to the case of the Thysanura; and it seems to be probable that this group is more nearly related than any other to the primitive wingless ancestors of Insects[177]. The characters of the oralappendages in this group, the simplicity of their metamorphosis, and the presence of abdominal appendages (fig. 192), all tell in favour of this view, while the resemblance of the adult to the larvæ of the Pseudoneuroptera, etc., points in the same direction. The Thysanura and Collembola are not however to be regarded as belonging to the true stock of the ancestors of Insects, but as degenerated relations of this stock; much as Amphioxus and the Ascidians are degenerate relations of the ancestral stock of Vertebrates, and Peripatus of that of the Tracheata. It is probable that all these forms have succeeded in retaining their primitive characters from their degenerate habits, which prevented them from entering into competition in the struggle for existence with their more highly endowed relatives. While in a general way it is clear that the larval forms of Insects cannot be expected to throw much light on the nature of Insect ancestors, it does nevertheless appear to me probable that such forms as the caterpillars of the Lepidoptera are not without a meaning in this respect. It is easy to conceive that even a secondary larval form may have been produced by the prolongation of one of the embryonic stages; and the general similarity of a caterpillar to Peripatus, and the retention by it of post-thoracic appendages, are facts which appear to favour this view of the origin of the caterpillar form.
The two most obscure points which still remain to be dealt with in the metamorphosis of Insects are (1) the origin of the quiescent pupa stage; (2) the frequent dissimilarity between the masticatory apparatus of the larva and adult.
These two points may be conveniently dealt with together, and some valuable remarks about them will be found in Lubbock (No.420).
On grounds already indicated it may be considered certain that the groups of Insects without a pupa stage, and with a larva very similarly organised to the adult, preceded the existing holometabolic groups. The starting point in the metamorphosis of the latter groups was therefore something like that of the Orthoptera. Suppose it became an advantage to a species that the larva and adult should feed in a somewhat different way, a difference in the character of their mouth parts would soon make itself manifest; and, since an intermediate type of mouth partswould probably be disadvantageous, there would be a tendency to concentrate into a single moult the transition from the larval to the adult form of mouth parts. At each ordinary moult there is a short period of quiescence, and this period of quiescence would naturally become longer in the important moult at which the change in the mouth parts was effected. In this way a rudimentary pupa stage might be started. The pupa stage, once started, might easily become a more important factor in the metamorphosis. If the larva and imago diverged still more from each other, a continually increasing amount of change would have to be effected at the pupa stage. It would probably be advantageous to the species that the larva should not have rudimentary functionless wings; and the establishment of the wings as external organs would therefore become deferred to the pupa stage. The same would probably apply to other organs.
Insects usually pass through the pupa stage in winter in cold climates and during the dry season in the tropics, this stage serving therefore apparently for the protection of the species during the inclement season of the year. These facts are easily explained on the supposition that the pupa stage has become secondarily adapted to play a part in the economy of the species quite different from that to which it owes its origin.
Heterogamy.The cases of alternations of generations amongst Insects all fall under the heading already defined in the introduction as Heterogamy. Heterogamy amongst Insects has been rendered possible by the existence of parthenogenesis, which, as stated in the introduction, has been taken hold of by natural selection, and has led to the production of generations of parthenogenetic forms, by which a clear economy in reproduction is effected. Parthenogenesis without heterogamy occurs in a large number of forms. In Bees, Wasps, and a Sawfly (Nematus ventricosus) the unfertilized ova give rise to males. In two Lepidopterous genera (Psyche and Solenobia) the unfertilized ova give rise mainly, if not entirely, to females. Heterogamy occurs in none of the above types, but in Psyche and Solenobia males are only occasionally found, so that a series of generations producing female young from unfertilized ova are followed by a generation producing young of both sexes from fertilized ova. Itwould be interesting to know if the unimpregnated female would not after a certain number of generations give rise to both males and females; such an occurrence might be anticipated on grounds of analogy. In the cases of true heterogamy parthenogenesis has become confined to special generations, which differ in their character from the generations which reproduce themselves sexually. The parthenogenetic generations generally flourish during the season when food is abundant; while the sexual generations occur at intervals which are often secondarily regulated by the season, supply of food, etc.
A very simple case of this kind occurs, if we may trust the recent researches of Lichtenstein[178], in certain Gall Insects (Cynipidæ). He finds that the female of a form known as Spathegaster baccarum, of which both males and females are plentiful, pricks a characteristic gall in certain leaves, in which she deposits the fertilized eggs. The eggs from these galls give rise to a winged and apparently adult form, which is not, however, Spathegaster, but is a species considered to belong to a distinct genus known as Neuroterus ventricularis. Only females of Neuroterus are found, and they lay unfertilized ova in peculiar galls which develop into Spathegaster baccarum. Here we have a true case of heterogamy, the females which produce parthenogenetically having become differentiated from those which produce sexually. Another interesting type of heterogamy is that which has been long known in the Aphides. In the autumn impregnated eggs are deposited by females, which give rise in the course of the spring to females which produce parthenogenetically and viviparously. The viviparous females always differ from the females which lay the fertilized eggs. The generative organs are of course differently constituted, and the ova of the viviparous females are much smaller than those of the oviparous females, as is generally the case in closely allied viviparous and oviparous forms; but in addition the former are usually without wings, while the latter are winged. The reverse is however occasionally the case. An indefinite number of generations of viviparous females may be produced if they are artificially kept warm and supplied with food; but in the course ofnature the viviparous females produce in the autumn males and females which lay eggs with firm shells, and so preserve the species through the winter. The heterogamy of the allied Coccidæ is practically the same as that of the Aphidæ. In the case of Chermes and Phylloxera the parthenogenetic generations lay their eggs in the normal way.
The complete history of Phylloxera quercus has been worked out by Balbiani (No.401). The apterous females during the summer lay eggs developing parthenogenetically into apterous females, which continue the same mode of reproduction. In the autumn, however, the eggs which are laid give rise in part to winged forms and in part to apterous forms. Both of these forms lay smaller and larger eggs, which develop respectively into very minute males and females without digestive organs. The fertilized eggs laid by these forms probably give rise to the parthenogenetic females.
A remarkable case of heterogamy accompanied by pædogenesis was discovered by Wagner to take place in certain species of Cecydomyia (Miastor), a genus of the Diptera. The female lays a few eggs in the bark of trees, etc. These eggs develop in the winter into larvæ, in which ovaries are early formed. The ova become detached into the body cavity, surrounded by their follicles, and grow at the cost of the follicles. They soon commence to undergo a true development, and on becoming hatched they remain for some time in the body cavity of the parent, and are nourished at the expense of its viscera. They finally leave the empty skin of their parent, and subsequently reproduce a fresh batch of larvæ in the same way. After several generations the larvæ undergo in the following summer a metamorphosis, and develop into the sexual form.
Another case of pædogenesis is that of the larvæ of Chironomus, which have been shewn by Grimm (No.413) to lay eggs which develop exactly in the same way as fertilized eggs into larvæ.
Bibliography.
(401)M. Balbiani.“Observations s. la reproduction d. Phylloxera du Chêne.”An. Sc. Nat.Ser.V.Vol.XIX.1874.(402)E. Bessels.“Studien ü. d. Entwicklung d. Sexualdrüsen bei den Lepidoptera.”Zeit. f. wiss. Zool.Bd.XVII.1867.(403)Alex. Brandt.“Beiträge zur Entwicklungsgeschichte d. Libellulida u. Hemiptera, mit besonderer Berücksichtigung d. Embryonalhüllen derselben.”Mém. Ac. Pétersbourg,Ser.VII.Vol.XIII.1869.(404)Alex. Brandt.Ueber das Ei u. seine Bildungsstätte. Leipzig, 1878.(405)O. Bütschli.“Zur Entwicklungsgeschichte d. Biene.”Zeit. f. wiss. Zool.Bd.XX.1870.(406)H. Dewitz.“Bau u. Entwicklung d. Stachels, etc.”Zeit. f. wiss. Zool.Vols.XXV.andXXVIII.1875 and 1877.(407)H. Dewitz.“Beiträge zur Kenntniss d. Postembryonalentwicklung d. Gliedmassen bei den Insecten.”Zeit. f. wiss. Zool.XXX.Supplement. 1878.(408)A. Dohrn.“Notizen zur Kenntniss d. Insectenentwicklung.”Zeitschrift f. wiss. Zool.Bd.XXVI.1876.(409)M. Fabre.“l’hypermétamorphose et les mœurs des Méloïdes.”An. Sci. Nat.SeriesIV.Vol.VII.1857.(410)Ganin.“Beiträge zur Erkenntniss d. Entwicklungsgeschichte d. Insecten.”Zeit. f. wiss. Zool.Bd.XIX.1869.(411)V. Graber.Die Insecten.München, 1877.(412)V. Graber.“Vorlauf. Ergeb. üb. vergl. Embryologie d. Insecten.”Archiv f. mikr. Anat.Vol.XV.1878.(413)O. v. Grimm.“Ungeschlechtliche Fortpflanzung einer Chironomus-Art u. deren Entwicklung aus dem unbefruchteten Ei.”Mém. Acad. Pétersbourg.1870.(414)B. Hatschek.“Beiträge zur Entwicklung d. Lepidopteren.”Jenaische Zeitschrift,Bd.XI.(415)A. Kölliker.“Observationes de primâ insectorum genese, etc.”Ann. Sc. Nat.Vol.XX.1843.(416)A. Kowalevsky.“Embryologische Studien an Würmern u. Arthropoden.”Mém. Ac. imp. Pétersbourg,Ser.VII.Vol.XVI.1871.(417)C. Kraepelin.“Untersuchungen üb. d. Bau, Mechanismus u. d. Entwick, des Stachels d. bienartigen Thiere.”Zeit. f. wiss. Zool.Vol.XXIII.1873.(418)C. Kupffer.“Faltenblatt an d. Embryonen d. Gattung Chironomus.”Arch. f. mikr. Anat.Vol.II.1866.(419)R. Leuckart.Zur Kenntniss d. Generationswechsels u. d. Parthenogenese b. d. Insecten.Frankfurt, 1858.(420)Lubbock.Origin and Metamorphosis of Insects.1874.(421)Lubbock.Monograph on Collembola and Thysanura.Ray Society, 1873.(422)Melnikow.“Beiträge z. Embryonalentwicklung d. Insecten.”Archiv f. Naturgeschichte,Bd.XXXV.1869.(423)E. Metschnikoff.“Embryologische Studien an Insecten.”Zeit. f. wiss. Zool.Bd.XVI.1866.(424)P. Meyer.“Ontogenie und Phylogenie d. Insecten.”Jenaische Zeitschrift,Vol.X.1876.(425)Fritz Müller.“Beiträge z. Kenntniss d. Termiten.”Jenaische Zeitschrift,Vol.IX.1875.(426)A. S. Packard. “Embryological Studies on Diplex, Perithemis, and the Thysanurous genus Isotoma.”Mem. Peabody Acad. Science, 1. 2. 1871.(427)Suckow.“Geschlechtsorgane d. Insecten.”Heusinger’sZeitschrift f. organ. Physik,Bd.II.1828.(428)Tichomiroff.“Ueber die Entwicklungsgeschichte des Seidenwurms.”Zoologischer Anzeiger,II.Jahr.No.20 (Preliminary Notice).(429)Aug. Weismann.“Zur Embryologie d. Insecten.”Archiv f. Anat. und Phys.1864.(430)Aug. Weismann.“Entwicklung d. Dipteren.”Zeit. f. wiss. Zool.Vols.XIII.andXIV.Leipzig, 1863‑4.(431)Aug. Weismann.“Die Metamorphose d. Corethra plumicornis.”Zeit. f. wiss. Zool.Vol.XVI.1866.(432)N. Wagner.“Beitrag z. Lehre d. Fortpflanzung d. Insectenlarven.”Zeit. f. wiss. Zool.Vol.XIII.1860.(433)Zaddach.Untersuchungen üb. d. Bau u. d. Entwicklung d. Gliederthiere.Berlin, 1854.
Arachnida[179].
The development of several divisions of this interesting group has been worked out; and it will be convenient to deal in the first instance with the special history of each of these divisions, and then to treat in a separate section the development of the organs for the whole group.
Ovum of ScorpionFig. 193. Ovum of Scorpion with the already formed blastoderm shewing the partial segmentation.(After Metschnikoff.)bl. blastoderm.
Fig. 193. Ovum of Scorpion with the already formed blastoderm shewing the partial segmentation.(After Metschnikoff.)
bl. blastoderm.
Scorpionidæ.The embryonic development always takes place within the female Scorpion. In Buthus it takes place within follicle-like protuberances of the wall of the ovary. In Scorpio also development commences while the egg is still in the follicle, but when the trunk becomes segmented the embryo passes into the ovarian tube. The chief authority for the development of the Scorpionidæ is Metschnikoff (No.434).
At the pole of the ovum facing the ovarian tube there isformed a germinal disc which undergoes a partial segmentation (fig. 193bl). A somewhat saucer-shaped one-layered blastoderm is then formed, which soon becomes thickened in the centre and then divided into two layers. The outer of these is the epiblast, the inner the mesoblast. Beneath the mesoblast there subsequently appear granular cells, which form the commencement of the hypoblast[180].
During the formation of the blastoderm a cellular envelope is formed round the embryo. Its origin is doubtful, though it is regarded by Metschnikoff as probably derived from the blastoderm and homologous with the amnion of Insects. It becomes double in the later stages (fig. 195).
Views of a developing ScorpionFig. 194. Three surface views of the ventral plate of a developing Scorpion.(After Metschnikoff.)A. Before segmentation.B. After five segments have become formed.C. After the appendages have begun to be formed.
Fig. 194. Three surface views of the ventral plate of a developing Scorpion.(After Metschnikoff.)
A. Before segmentation.B. After five segments have become formed.C. After the appendages have begun to be formed.
During the differentiation of the three embryonic layers the germinal disc becomes somewhat pyriform, the pointed end being the posterior. At this extremity there is a special thickening which is perhaps equivalent to the primitive cumulus of Spiders. The germinal disc continues gradually to spread over the yolk, but the original pyriform area is thicker than the remainder, and is marked off anteriorly and posteriorly by a shallow furrow. It constitutes a structure corresponding with the ventral plate of other Tracheata. It soon becomes grooved by a shallow longitudinal furrow (fig. 194A) which subsequently becomes less distinct. It is then divided by two transverse lines into three parts.[181]
In succeeding stages the anterior of the three parts is clearly marked out as the procephalic lobe, and soon becomes somewhat broader. Fresh segments are added from before backwards, and the whole ventral plate increases rapidly in length (fig. 194B).
When ten segments have become formed, appendages appear as paired outgrowths of the nine posterior segments (fig. 194C). The second segment bears the pedipalpi, the four succeeding segments the four ambulatory appendages, and the four hindermost segments smaller provisional appendages which subsequently disappear, with the possible exception of the second. The foremost segment, immediately behind the procephalic lobes, is very small, and still without a rudiment of the cheliceræ, which are subsequently formed on it. It would appear from Metschnikoff’s figures to be developed later than the other post-oral segments present at this stage. The still unsegmented tail has become very prominent and makes an angle of 180° with the remainder of the body, over the ventral surface of which it is flexed.
Advanced embryo of ScorpionFig. 195. A fairly advanced embryo of the Scorpion enveloped in its membranes.(After Metschnikoff.)ch.cheliceræ;pd.pedipalpi;p1-p4.ambulatory appendages;ab.post-abdomen (tail).
Fig. 195. A fairly advanced embryo of the Scorpion enveloped in its membranes.(After Metschnikoff.)
ch.cheliceræ;pd.pedipalpi;p1-p4.ambulatory appendages;ab.post-abdomen (tail).
By the time that twelve segments are definitely formed, the procephalic region is distinctly bilobed, and in the median groove extending along it the stomodæum has become formed (fig. 196A). The cheliceræ (ch) appear as small rudiments on the first post-oral segment, and thenerve cords are distinctly differentiated and ganglionated. In the embryonic state there is one ganglion for each segment. The ganglion in the first segment (that bearing the cheliceræ) is very small, but is undoubtedly post-oral.
At this stage, by a growth in which all the three germinal layers have a share, the yolk is completely closed in by the blastoderm. It is a remarkable fact with only few parallels, and those amongst the Arthropoda, that the blastopore, or point where the embryonic membranes meet in closing in the yolk, is situated on the dorsal surface of the embryo.
The general relations of the embryo at about this stage are shewn infig. 195, where the embryo enclosed in its double cellular membrane is seen in a side view. This embryo is about the same age as that seen from the ventral surface infig. 196A.
The general nature of the further changes may easily be gathered from an inspection offig. 196B and C, but a few points may be noted.
An upper lip or labrum is formed as an unpaired organ in the line between the procephalic lobes. The pedipalpi become chelate before becoming jointed, and the cheliceræ also early acquire their characteristic form. Rudimentary appendages appear on the six segments behind the ambulatory legs, five of which are distinctly shewn infig. 195; they persist only on the second segment, where they appear to form the comb-like organs or pectines. The last abdominal segment,i.e.that next the tail, is without provisional appendages. The embryonic tail is divided into six segments including the telson (fig. 196C,ab). The lungs (st) are formed by paired invaginations, the walls of which subsequently become plicated, on the four last segments which bear rudimentary limbs, and simultaneously with the disappearance of the rudimentary limbs.