Chapter XVII: Tracheata (2)

In the median ventral part of the procephalic region there arises the labrum (fig. 187, _ls_). It is formed by the coalescence of a pair of prominences very similar to true appendages, though it is probable that they have not this value[172].

[172] If these structures are equivalent to appendages, they may
correspond to one of the pairs of antennæ of Crustacea. From a
figure by Fritz Müller of the larva of Calotermes (_Jenaische
Zeit_. Vol. XI. pl. 11, fig. 12) it would appear that they lie in
front of the true antennæ, and would therefore on the above
hypothesis correspond to the first pair of antennæ of Crustacea.
Bütschli (No. 405) describes in the Bee a pair of prominences
immediately in front of the mandibles which eventually unite to
form a kind of underlip; they in some ways resemble true
appendages.

[FIG. 187. TWO STAGES IN THE DEVELOPMENT OF HYDROPHILUS PICEUS.
(From Gegenbaur, after Kowalevsky.)

_ls._ labrum; _at._ antenna; _md._ mandible; _mx._ maxilla I.; _li._
maxilla II.; _p´ p´´ p´´´._ feet; _a._ anus.]

The antennæ themselves can hardly be considered to have the same morphological value as the succeeding appendages. They are rather equivalent to paired processes of the præ-oral lobes of the Chætopoda.

From the first three post-oral segments there grow out the mandibles and two pairs of maxillæ, and from the three following segments the three pairs of thoracic appendages. In many Insects (cf. Hydrophilus) a certain number of appendages of the same nature as the anterior ones are visible in the embryo on the abdominal segments, a fact which shews that Insects are descended from ancestors with more than three pairs of ambulatory appendages.

In Apis according to Bütschli (No. 405) all the abdominal segments are provided with appendages, which always remain in a very rudimentary condition. All trace of them as well as of the thoracic appendages is lost by the time the embryo is hatched. In the phytophagous Hymenoptera the larva is provided with 9-11 pairs of legs.

In the embryo of Lepidoptera there would appear from Kowalevsky's figures to be rudiments of ten pairs of post-thoracic appendages. In the caterpillar of this group there are at the maximum five pairs of such rudimentary feet, viz. a pair on the 3rd, 4th, 5th, and 6th, and on the last abdominal segment. The embryos of Hydrophilus (fig. 187), Mantis, etc. are also provided with additional appendages. In various Thysanura small prominences are present on more or fewer of the abdominal segments (fig. 192), which may probably be regarded as rudimentary feet.

Whether all or any of the appendages of various kinds connected with the hindermost segments belong to the same category as the legs is very doubtful. Their usual absence in the embryo or in any case their late appearance appears to me against so regarding them; but Bütschli is of opinion that in the Bee the parts of the sting are related genetically to the appendages of the penultimate and antepenultimate abdominal segments, and this view is to some extent supported by more recent observations (Kraepelin, etc.), and if it holds true for the Bee must be regarded as correct for other cases also.

As to the order of the appearance of the appendages observations are as yet too scanty to form any complete scheme. In many cases all the appendages appear approximately at the same moment, _e.g._ Hydrophilus, but whether this holds good for all Coleoptera is by no means certain. In Apis the appendages are stated by Bütschli to arise simultaneously, but according to Kowalevsky the two mouth appendages first appear, then the antennæ, and still later the thoracic appendages. In the Diptera the mouth appendages are first formed, and either simultaneously with these, or slightly later, the antennæ. In the Hemiptera and Libellulidæ the thoracic appendages are the first to be formed, and the second pair of maxillæ makes its appearance before the other cephalic appendages.

The history of the changes in the embryonic appendages during the attainment of the adult condition is beyond the scope of this treatise, but it may be noted that the second pair of maxillæ are relatively very large in the embryo, and not infrequently (Libellula, etc.) have more resemblance to the ambulatory than to the masticatory appendages.

[FIG. 188. FIGURES ILLUSTRATING AQUATIC RESPIRATION IN INSECTS.
(After Gegenbaur.)

A. Hinder portion of the body of Ephemera vulgata. _a._ longitudinal
tracheal trunks; _b._ alimentary canal; _c._ tracheal gills.
B. Larva of Æschna grandis. _a._ superior longitudinal tracheal
trunks; _b._ their anterior end; _c._ portion branching on
proctodæum; _o._ eyes.
C. Alimentary canal of the same larva from the side. _a, b,_ and
_c._ as in B; _d._ inferior tracheal trunk; _e._ transverse branches
between upper and lower tracheal trunks.]

The exact nature of the wings and their relation to the other segments is still very obscure. They appear as dorsal leaf-like appendages on the 2nd and 3rd thoracic segments, and are in many respects similar to the tracheal gills of the larvæ of Ephemeridæ and Phryganidæ (fig. 188 A), of which they are supposed by Gegenbaur and Lubbock to be modifications. The undoubtedly secondary character of the _closed_ tracheal system of larvæ with tracheal gills tells against this view. Fritz Müller finds in the larvæ of Calotermes rugosus (one of the Termites) that peculiar and similar dorsal appendages are present on the two anterior of the thoracic segments. They are without tracheæ. The anterior atrophies, and the posterior acquires tracheæ and gives rise to the first pair of wings. The second pair of wings is formed from small processes on the third thoracic segment like those on the other two. Fritz Müller concludes from these facts that the wings of Insects are developed from dorsal processes of the body, not equivalent to the ventral appendages. What the primitive function of these appendages was is not clear. Fritz Müller suggests that they may have been employed as respiratory organs in the passage from an aqueous to a terrestrial existence, when the Termite ancestors lived in moist habitations--a function for which processes supplied with blood-channels would be well adapted. The undoubted affinity of Insects to Myriapods, coupled with the discovery by Moseley of a tracheal system in Peripatus, is however nearly fatal to the view that Insects can have sprung directly from aquatic ancestors not provided with tracheæ. But although this suggestion of Fritz Müller cannot be accepted, it is still possible that the processes discovered by him may have been the earliest rudiments of wings, which were employed first as organs of propulsion by a water-inhabiting Insect ancestor which had not yet acquired the power of flying.

The nervous system. The nervous system arises entirely from the epiblast; but the development of the præ-oral and post-oral sections may be best considered separately.

The post-oral section, or ventral cord of the adult, arises as two longitudinal thickenings of the epiblast, one on each side of the median line (fig. 189 B, _vn_), which are subsequently split off from the superficial skin and give rise to the two lateral strands of the ventral cord. At a later period they undergo a differentiation into ganglia and connecting cords.

Between these two embryonic nerve cords there is at first a shallow furrow, which soon becomes a deep groove (fig. 189 C). At this stage the differentiation of the lateral elements into ganglia and commissures takes place, and, according to Hatschek (No. 414), the median groove becomes in the region of the ganglia converted into a canal, the walls of which soon fuse with those of the ganglionic enlargements of the lateral cords, and connect them across the middle line. Between the ganglia on the other hand the median groove undergoes atrophy, becoming first a solid cord interposed between the lateral strands of the nervous system, and finally disappearing without giving rise to any part of the nervous system. It is probable that Hatschek is entirely mistaken about the entrance of a median element into the ventral cord, and that the appearances he has described are due to shrinkage. In Spiders the absence of a median element can be shewn with great certainty, and, as already stated, this element is not present in Peripatus. Hatschek states that in the mandibular segment the median element is absorbed, and that the two lateral cords of that part give rise to the oesophageal commissures, while the sub-oesophageal ganglion is formed from the fusion of the ganglia of the two maxillary segments.

[FIG. 189. THREE TRANSVERSE SECTIONS THROUGH THE EMBRYO OF
HYDROPHILUS. (After Kowalevsky.)

A. Transverse section through the larva represented in fig. 187 A.
B. Transverse section through a somewhat older embryo in the region
of one of the stigmata.
C. Transverse section through the larva represented in fig. 187 B.
_vn._ ventral nerve cord; _am._ amnion and serous membrane; _me._
mesoblast; _me.s._ somatic mesoblast; _hy._ hypoblast(?);
_yk._ yolk cells (true hypoblast); _st._ stigma of trachea.]

The præ-oral portion of the nervous system consists entirely of the supra-oesophageal ganglion. It is formed, according to Hatschek, of three parts. Firstly and mainly, of a layer separated from the thickened inner part of the cephalic lobe on each side; secondly, of an anterior continuation of the lateral cords; and thirdly, of a pit of skin invaginated on each side close to the dorsal border of the antennæ. This pit is at first provided with a lumen, which is subsequently obliterated; while the walls of the pit become converted into true ganglion cells. The two supra-oesophageal ganglia remain disconnected on the dorsal side till quite the close of embryonic life.

The tracheæ and salivary glands. The tracheæ, as was first shewn by Bütschli (No. 405), arise as independent segmentally arranged paired invaginations of the epiblast (fig. 189 B and C, _st_). Their openings are always placed on the outer sides of the appendages of their segments, where such are present.

Although in the adult stigmata are never found in the space between the prothorax and head[173], in the embryo and the larva tracheal invaginations may be developed in all the thoracic (and possibly in the three jaw-bearing segments) and in all the abdominal segments except the two posterior.

[173] In Smynthurus, one of the Collembola, there are, according
to Lubbock, only two stigmata, which are placed on the head.

In the embryo of the Lepidoptera, according to Hatschek (No. 414), there are 14 pairs of stigmata, belonging to the 14 segments of the body behind the mouth; but Tichomiroff states that Hatschek is in error in making this statement for the foremost post-oral segments. The last two segments are without stigmata. In the larvæ of Lepidoptera as well as those of many Hymenoptera, Coleoptera and Diptera, stigmata are present on all the postcephalic segments except the 2nd and 3rd thoracic and the two last abdominal. In Apis there are eleven pairs of tracheal invaginations according to Kowalevsky (No. 416), but according to Bütschli (No. 405) only ten, the prothorax being without one. In the Bee they appear simultaneously, and before the appendages.

The blind ends of the tracheal invaginations frequently (_e.g._ Apis) unite together into a common longitudinal canal, which forms a longitudinal tracheal stem. In other cases (_e.g._ Gryllotalpa, _Dohrn_, No. 408) they remain distinct, and each tracheal stem has a system of branches of its own.

The development of the tracheæ strongly supports the view, arrived at by Moseley from his investigations on Peripatus, that they are modifications of cutaneous glands.

The salivary and spinning glands are epiblastic structures, which in their mode of development are very similar to the tracheæ, and perhaps have a similar origin. The salivary glands arise as paired epiblastic invaginations, not, as might be expected, of the Stomodæum, but of the ventral plate behind the mouth on the inner side of the mandibles. At first independent, they eventually unite in a common duct, which falls into the mouth. The spinning glands arise on the inner side of the second pair of maxillæ in Apis and Lepidoptera, and form elongated glands extending through nearly the whole length of the body. They are very similar in their structure and development to salivary glands, and are only employed during larval life. They no doubt resemble the mucous glands of the oral papillæ of Peripatus, with which they have been compared by Moseley. The mucous glands of Peripatus may perhaps be the homologous organs of the first pair of maxillæ, for the existence of which there appears to be some evidence amongst Insects.

Mesoblast. It has been stated that the mesoblast becomes divided in the region of the body into two lateral bands (fig. 189 A). These bands in many, if not all forms, become divided into a series of somites corresponding with the segments of the body. In each of them a cavity appears--the commencing perivisceral cavity--which divides them into a somatic plate in contact with the epiblast, and a splanchnic plate in contact with the hypoblast (fig. 189). In the interspaces between the segments the mesoblast is continuous across the median ventral line. The mesoblast is prolonged into each of the appendages as these are formed, and in the appendages there is present a central cavity. By Metschnikoff these cavities are stated to be continuous, as in Myriapods and Arachnida, with those of the somites; but by Hatschek (No. 414) they are stated to be independent of those in the somites and to be open to the yolk.

The further details of the history of the mesoblast are very imperfectly known, and the fullest account we have is that by Dohrn (No. 408) for Gryllotalpa. It would appear that the mesoblast grows round and encloses the dorsal side of the yolk earlier than the epiblast. In Gryllotalpa it forms a pulsating membrane. As the epiblast extends dorsalwards the median dorsal part of the membrane is constricted off as a tube which forms the heart. At the same time the free space between the pulsating membrane and the yolk is obliterated, but transverse passages are left at the lines between the somites, through which the blood passes from the ventral part of the body to corresponding openings in the wall of the heart. The greater part of the membrane gives rise to the muscles of the trunk.

Ventrally the mesoblastic bands soon meet across the median line. The cavities in the appendages become obliterated and their mesoblastic walls form the muscles, etc. The cavities in the separate mesoblastic somites also cease to be distinctly circumscribed.

The splanchnic mesoblast follows the hypoblast in its growth, and gives rise to the connective tissue and muscular parts of the walls of the alimentary tract. The mesoblastic wall of the proctodæum is probably formed independently of the mesoblastic somites. In the head the mesoblast is stated to form at first a median ventral mass, which does not pass into the procephalic lobe; though it assists in forming both the antennæ and upper lip.

The alimentary canal. The alimentary tract of Insects is formed of three distinct sections (fig. 181)--a mesenteron or middle section (_me_), a stomodæum (_st_) and a proctodæum (_an_). The stomodæum and proctodæum are invaginations of the epiblast, while the mesenteron is lined by the hypoblast. The distinction between the three is usually well marked in the adult by the epiblastic derivatives being lined by chitin. The stomodæum consists of mouth, oesophagus, crop, and proventriculus or gizzard, when such are present. The mesenteron includes the stomach, and is sometimes (Orthoptera, etc.) provided at its front end with pyloric diverticula--posteriorly it terminates just in front of the Malpighian bodies. These latter fall into the proctodæum, which includes the whole of the region from their insertion to the anus.

The oral invagination appears nearly coincidently with the first formation of segments at the front end of the groove between the lateral nerve cords, and the anal invagination appears slightly later at the hindermost end of the ventral plate.

The Malpighian bodies arise as _two pairs of outgrowths of the epiblast of the proctodæum_, whether solid at first is not certain. The subsequent increase which usually takes place in their number is due to sproutings (at first solid) of the two original vessels.

The glandular walls of the mesenteron are formed from the hypoblast; but the exact origin of the layer has not been thoroughly worked out in all cases. In Hydrophilus it is stated by Kowalevsky (No. 416) to appear as two sheets split off from the lateral masses of mesoblast, which gradually grow round the yolk, and a similar mode of formation would seem to hold good for Apis. Tichomiroff (No. 420) confirms Kowalevsky on this point, and further states that these two masses meet first ventrally and much later on the dorsal side. In Lepidoptera, on the other hand, Hatschek finds that the hypoblast arises as a median mass of polygonal cells in the anterior part of the ventral plate. These cells increase by absorbing material from the yolk, and then gradually extend themselves and grow round the yolk.

Dohrn (No. 408) believes that the yolk cells, the origin of which has already been spoken of, give rise to the hypoblastic walls of the mesenteron, and this view appears to be shared by Graber (No. 412), though the latter author holds that some of the yolk cells are derived by budding from the blastoderm[174].

[174] Graber's view on this point may probably be explained by
supposing that he has mistaken a passage of yolk cells into the
blastoderm for a passage of blastoderm cells into the yolk. The
former occurrence takes place, as I have found, largely in
Spiders, and probably therefore also occurs in Insects.

From the analogy of Spiders I am inclined to accept Dohrn's and Graber's view. It appears to me probable that Kowalevsky's observations are to be explained by supposing that the hypoblast plates which he believes to be split off from the mesoblast are really separated from the yolk.

It will be convenient to add here a few details to what has already been stated as to the origin of the yolk cells. As mentioned above, the central yolk breaks up at a period, which is not constant in the different forms, into polygonal or rounded masses, in each of which a nucleus has in many instances been clearly demonstrated although in others such nuclei have not been made out. It is probable however that nuclei are in all cases really present, and that these masses must be therefore regarded as cells. They constitute in fact the yolk cells. The periphery of the yolk breaks up into cells while the centre is still quite homogeneous.

The hypoblastic walls of the mesenteron appear to be formed in the first instance laterally (fig. 189 B and C, _hy_). They then meet ventrally (fig. 185 A and B), and finally close in the mesenteron on the dorsal side.

The mesenteron is at first a closed sack, independent of both stomodæum and proctodæum; and in the case of the Bee it so remains even after the close of embryonic life. The only glandular organs of the mesenteron are the not unfrequent pyloric tubes, which are simple outgrowths of its anterior end. It is possible that in some instances they may be formed _in situ_ around the lateral parts of the yolk.

In many instances the whole of the yolk is enclosed in the walls of the mesenteron, but in other cases, as in Chironomus and Simulia (Weismann, No. 430; Metschnikoff, No. 423), part of the yolk may be left between the ventral wall of the mesenteron and the ventral plate. In Chironomus the mass of yolk external to the mesenteron takes the form of a median and two lateral streaks. Some of the yolk cells either prior to the establishment of the mesenteron, or derived from the unenclosed portions of the yolk, pass into the developing organs (Dohrn, 408) and serve as a kind of nutritive cell. They also form blood corpuscles and connective-tissue elements. Such yolk cells may be compared to the peculiar bodies described by Reichenbach in Astacus, which form the secondary mesoblast. Similar cells play a very important part in the development of Spiders.

Generative organs. The observations on the development of the generative organs are somewhat scanty. In Diptera certain cells--known as the pole cells--are stated by both Metschnikoff (No. 423) and Leuckart to give rise to the generative organs. The cells in question (in Chironomus and Musca vomitoria, Weismann, No. 430) appear at the hinder end of the ovum before any other cells of the blastoderm. They soon separate from the blastoderm and increase by division. In the embryo, produced by the viviparous larva of Cecidomyia, there is at first a single pole cell, which eventually divides into four, and the resulting cells become enclosed within the blastoderm. They next divide into two masses, which are stated by Metschnikoff (No. 423) to become surrounded by indifferent embryonic cells[175]. Their protoplasm then fuses, and their nuclei divide, and they give rise to the larval ovaries, for which the enclosing cells form the tunics.

[175] This point requires further observation.

In _Aphis_ Metschnikoff (No. 423) detected at a very early stage a mass of cells which give rise to the generative organs. These cells are situated at the hind end of the ventral plate; and, except in the case of one of the cells which gives rise by division to a green mass adjoining the fat body, the protoplasm of the separate cells fuses into a syncytium. Towards the close of embryonic life the syncytium assumes a horse-shoe form. The mass is next divided into two, and the peripheral layer of each part gives rise to the tunic, while from the hinder extremity of each part an at first solid duct--the egg-tube--grows out. The masses themselves form the germogens. The oviduct is formed by a coalescence of the ducts from each germogen.

Ganin derives the generative organs in Platygaster (_vide_ p. 347) from the hind end of the ventral plate close to the proctodæum; while Suckow states that the generative organs are outgrowths of the proctodæum. According to these two sets of observations the generative organs would appear to have an epiblastic origin--an origin which is not incompatible with that from the pole cells.

In Lepidoptera the genital organs are present in the later periods of embryonic life as distinct paired organs, one on each side of the heart, in the eighth postcephalic segment. They are elliptical bodies with a duct passing off from the posterior end in the female or from the middle in the male. The egg-tubes or seminal tubes are outgrowths of the elliptical bodies.

In other Insects the later stages in the development of the generative organs closely resemble those in the Lepidoptera, and the organs are usually distinctly visible in the later stages of embryonic life.

It may probably be laid down, in spite of some of Metschnikoff's observations above quoted, that the original generative mass gives rise to both the true genital glands and their ducts. It appears also to be fairly clear that _the genital glands of both sexes have an identical origin_.

_Special types of larvæ._

Certain of the Hymenopterous forms, which deposit their eggs in the eggs or larvæ of other Insects, present very peculiar modifications in their development. Platygaster, which lays its egg in the larvæ of Cecidomyia, undergoes perhaps the most remarkable development amongst these forms. It has been studied especially by Ganin (No. 410), from whom the following account is taken.

[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. 190 A). In the interior of the egg there soon appears a single spherical body, regarded by Ganin as a cell (fig. 190 B). In the next stage three similar bodies appear in the vitellus, no doubt derived from the first one (fig. 190 C). The central one presents somewhat different characters to the two others, and, according to Ganin, gives rise to _the whole embryo_. The two peripheral bodies increase by division, and soon appear as nuclei imbedded in a layer of protoplasm (fig. 190 D, 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. 190 D, 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 this may 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. 190 F). The cephalothorax grows in breadth, and near its anterior end an invagination appears, which gives rise to the mouth and oesophagus. 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 in fig. 191 A, 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 the _slightest indication of segmentation_ (fig. 191 D). 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 the epiblast 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-oesophageal ganglia, and possibly other parts.

[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._ oesophagus;
_gsae._ supra-oesophageal 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. 191 E) 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].

[176] For a systematic account of this subject the reader is
referred to Lubbock (No. 420) and to Graber (No. 411). He will
find in Weismann (Nos. 430 and 431) a detailed account of the
internal changes which take place.

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 the Holometabola.

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 the Tipulidæ 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, _vide_ pp. 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 processes become 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 proceed _pari passu_ with 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. 430 and 431). 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 called imaginal 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 imaginal discs. 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 and eventual 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 (_vide_ p. 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 larval form 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 way _that 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.

[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 oral appendages 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.

[177] Brauer and Lubbock (No. 421) have pointed out the primitive
characters of these forms, especially of Campodea.

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 parts would 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. It would 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 of nature 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.

[178] _Petites Nouvelles Entomologiques_, May, 1878.

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. and XXVIII. 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 moeurs des Méloïdes." _An. Sci. Nat._ Series IV. Vol. VII. 1857.