THE ORGANS OF REPRODUCTION
Insects are without exception unisexual, the male and female organs existing in different individuals, no insects being normally hermaphroditic. The reproductive organs are situated in the hind-body or abdomen, especially near the end, the genital glands opening externally either in the space between the 7th and 8th, or 8th and 9th, or 9th and 10th abdominal segments, but as a rule between the 8th and 9th segments (Fig. 299).
The primary or essential male organs are the testes, those of the female being the ovaries. As we shall see, the primitive number of seminal ducts and oviducts was two, this number being still retained in Lepisma and the Ephemeridæ. The reproductive organs of both sexes are at first, in their embryonic condition, of the same shape and structure, becoming differentiated in form and function before sexual maturity. These glands and ducts have a paired mesodermal genital rudiment, the ends of the ducts being often connected with corresponding ectodermal invaginations of the cuticle.
The secondary sexual organs mainly comprise the external genital armature of the male, and the egg-laying organs, or ovipositor of the female. Besides these structures there are other more superficial secondary sexual characters, such as differences in the size and ornamentation as well as coloring of the body, or of parts of it.
The primary sexual organs of insects have been conveniently tabulated by Kolbe, thus:—
I. Male reproductive organs.
1. Two testes, with testicular follicles.
2. Seminal ducts (vasa deferentia).
3. Seminal vesicle.
4. Accessory glands.
5. The common seminal outlet, with the penis.
6. The copulatory apparatus.
II. Female reproductive organs.
1. Two ovaries, with the egg-tubes.
2. Two oviducts.
3. Receptaculum seminis; bursa copulatrix.
4. Accessory sac.
5. The common oviduct, vagina, uterus.
6. The ovipositor.
The ducts of the sexual glands in Peripatus being transformed nephridia or segmental organs, it has been inferred that this is also the case with those of insects, though, as Lang states, there is a considerable difference in the two cases, as the greater part of the ducts in Peripatus arises out of the ectoderm, while in the Myriopoda and insects they come from the mesoderm; but he adds that in the Annelids the greater part of the nephridial duct is of mesodermal origin.
While in insects there is but a single pair of genital outlets, the serial arrangement of the testicular (Fig. 458) and egg-tubes (Fig. 459) in some Thysanura (Campodea, Japyx, and Lepisma), where the tubes (5 to 7 on each side) open singly one behind the other in segmental succession, indicates that in their ancestors these egg-tubes opened out on different segments situated one behind the other. Each egg-tube independently opens into one of the two oviducts, which extend through the abdomen as straight canals. The two oviducts open externally by a short unpaired terminal portion, which in Machilis is said to be wanting, only the outer aperture of the two oviducts being in this case common to both. In Campodea and in the Collembola the ovaries and testes on each side are simply tubes. It is to be observed that in the young Lepisma Nassonow found that the external openings of the two ejaculatory ducts are paired (Fig. 458 B, ed.).
Fig. 458.—Male genital organs of Thysanura: A, Lepisma in which the testes are segmentally arranged.—After Grassi. B, Lepisma saccharina, young ♂: vd, vas deferens; ed, ejaculatory duct; ga, external appendages,—After Nassonow. C, Machilis, the testis lateral and separate, but not corresponding to the segments. D, Japyx, with an undivided testicular tube on each side; tt, testes; cd, vas deferens; vs, seminal vesicle; ce, ejaculatory duct.—After Grassi, from Perrier.
In the Stylopidæ, also, though this may be the result of adaptation to the singular parasitic habits of the females whose bodies are mostly situated in the abdomen of their host, the ends of the oviducts are formed by the invagination of the integument of the 2d, 3d, and 4th abdominal segments. In the 2d to 5th segments are situated tubes which open in the cavity of the body with funnel-like ends, so that the ducts have a close resemblance to the segmental organs of worms. (Nassonow.)
Fig. 459.—Ovaries of Thysanura: A, of Campodea. B, of Japyx.—After Grassi. C, of Machilis.—After Oudemans, from Sharp.
Fig. 460.—Female genital organs of Lepisma saccharina, adult: ov, ovaries; a, part of the oviduct, corresponding to the calyx of winged insects; od, oviduct; vg, vagina; rs, copulatory pouch; gg, accessory glands; m, muscles; n, nervous cord.—After Nassonow, from Perrier.
Among the winged insects the reproductive organs of the cricket (Fig. 466) are perhaps as simple as any. The testes are separate, and the vasa deferentia very long. The seminal vesicles bear numerous large and short utricles (utriculi majores and breviores), the penis being simple and dilated at the end; while in Phyllodromia germanica the testes are functional throughout life, and consist of four lobes each. In the common cockroach (P. orientalis) (Fig. 461) the testes are functional only in the young male; they afterwards shrivel and are functionally replaced by the vesiculæ seminales and their appendages, when the later transformations of the sperm-cells are effected. The accessory glands are numerous and differ both in function and insertion. Two sets of these glands (utriculi majores and breviores) are attached to the vesiculæ seminales and the fore end of the ejaculatory duct, while another appendage, called by Miall and Denny the conglobate gland, opens separately on the exterior upon a double hook, which forms a part of the external genital armature. The so-called penis is long, slender, and dilated at the end, but is not perforated.
Fig. 461.—A, male organs of the cockroach, ventral view: Ts, testis; VD, vas deferens; DE, ductus ejaculatorius; U, utriculi majores; u, utriculi breviores; 2, dorsal view; 3, CG, conglobate gland and its duct. B, male organs, side view: A, titillator; B, penis; other letters as in A.—After Miall and Denny.
In the locusts (Acrydiidæ) the testes are, unlike those of most other Orthoptera, closely united to each other so as to form a single mass of tubular glands into which penetrate both simple and dilated tracheæ; the entire mass is situated in the 3d, 4th, and 5th abdominal segments, and above the intestine. The anterior end of the testicular mass is rounded and held in place by a broad, thin band, one on each side; two similar bands are situated a little behind the middle of the mass. From the under side, and a little in advance of the middle of the mass, two straight small ducts, as long as the testicular mass, pass obliquely to the sides of the body, at the posterior end of the 7th segment of the abdomen; these are the vasa deferentia. Each vas deferens, with its mate, forms a convoluted mass of tubes, comprising twenty folded bundles (epididymis of Dufour), and two single, long, convoluted tubes, the vesiculæ seminales, which are lobed in the 6th and 7th segments of the abdomen. The two vesiculæ unite over the 5th abdominal ganglion, forming a thick, very short canal (ductus ejaculatorius), which passes into a large spherical muscular mass (præputium), behind which is the large intromittent organ (penis), which forms a short chitinous cylinder, quite complicated in structure, being armed with hooks and projections and affording excellent specific characters. It can be seen in place without dissection by drawing back the orbicular convex piece called the velum penis.
In the Hymenoptera the reproductive system is quite simple, as seen in Fig. 462.
Fig. 462.—Male organs of saw-fly (Athalia centifoliæ): a, a, testes; b, b, epididymis; c, d, vas deferentia; e, vesiculæ seminales; f, ductus ejaculatorius; h, penis (see also p. 180).—After Newport.
The general shape and relations of the female reproductive organs are seen in Fig. 298, of the locust (Acrydiidæ). The ovaries consist of two large bundles of tubes, each bundle tied to the other by slight bands, with air-sacs and tracheæ ramifying among them. These tubes extend along the intestine, passing into the prothorax. The ovarian tubes opening into the oviducts unite to form the vagina, which lies on the floor of the abdomen. (In the cockroach the vagina has a muscular wall and chitinous lining.) Above the opening of the duct, and directly communicating with it, is the copulatory pouch (bursa copulatrix), a capacious pocket lined within with several narrow, longitudinal, chitinous bands. Behind the bursa copulatrix lies, partly resting under the fifth abdominal ganglion, the sebific, cement, or colleterial gland (colleterium; compare Fig. 299, sb), which is flattened, pear-shaped, a little over half as long as a ripe egg of the same insect. From the under side, a little in advance of the middle, arises the sebific duct, which, after making three tight coils next to the ganglion, passes back and empties into the upper side of the bursa copulatrix, dilating slightly before its junction with the latter.
The most primitive type of reproductive organs observed in insects is that of the young Lepisma and the Ephemeridæ, in which the outlets of the oviducts and of the vasa deferentia respectively are double or paired, showing that insects have probably inherited these structures from the segmental organs of their vermian ancestors.
Réaumur had already observed the process of oviposition and seen that the female Ephemera had two openings near the end of the “6th” abdominal segment, from which he saw two masses of eggs pass out at a time (Fig. 463). Eaton afterwards (1871) referred to the oviducts as terminating between the 7th and 8th segments of the abdomen, and after him Joly; but for a detailed monograph on the subject we are indebted to Palmén. He found that the outlets of the sexual glands are paired, not only in the larvæ of all stages, but also in the imagines, and in both sexes. In the males the vasa deferentia pass on the ventral side of the 9th segment through two external appendages, both reproductive organs, at whose tips or sides the openings are situated. In the larvæ the female openings are not formed until after the last moult. In the females the two oviducts open on the ventral side of the hind-body between the 7th and 8th segments.
Fig. 463.—Upturned end of body of Ephemera, with two egg-masses (o) issuing at the same time from the double oviducts; q, anus.—After Réaumur.
Palmén suggests that the Ephemerids represent, in respect to the reproductive system among insects, a very primitive type of organization, and he concludes that the inner sexual organs of insects are built up of two different morphological elements; i.e. (a) internal primitive paired structures (testes with vasa deferentia, ovaria with oviducts), and (b) integumental structures, such as the ductus ejaculatorius and vagina.
In the younger larvæ the vasa deferentia form slender cords along which are situated the seminal glands; these cords are inserted in the integument on the hinder edge of the 9th sternite, where afterwards, during the last moult, the copulatory organs grow out. In the older larvæ the sperm collects in the cavities of these cords. Their walls become expanded, and this section then functions as vesiculæ seminales. The ends of the cords remain contracted and act as ductus ejaculatorii. Common unpaired glandular structures are not present. At the last moult the copulatory organs reach their complete development, and the ducts become open externally.
The oviducts in the larva are at first slender, string-like, and bear the egg-follicles. As soon as the eggs pass out of the follicles and collect in the oviducts, the walls of the latter become stretched, and this portion forms two uterus-like structures. The terminal division of the two passages forms their vaginal portions. But since there is no common vagina, there are no unpaired glands and no receptaculum seminis. The two ducts become open after the last shedding of the skin.
Palmén adds that this paired or double nature of the sexual glands and their external ducts in this group of insects occurs in some Myriopoda (Fig. 3, E, F) and a few Arachnida (Fig. 3, C, D, the outlets being in this class unpaired), numerous Crustacea, and most worms; and as already stated it is very marked in Limulus, where the paired outlets are in both sexes very simple and wide apart (Fig. 3, A). In the worms the paired genital ducts are modified segmental organs. As we have seen, in the young male Lepisma there are two male genital openings. Hence this double nature of the genital passages in the may-flies seems to be very primitive.
In the Dermaptera, also, the genus Labidura was found by Meinert to have two independent ductus ejaculatorii, opening externally in double external slit-like processes (penes). The two ducts arise from a single seminal vesicle, which is either paired (L. advena), or forms a common passage (L. gigantea). In Forficula (Fig. 464, B) only one ejaculatory duct persists, the other is obliterated, and one of the penes is atrophied, the other assuming a position in the middle line of the body. Thus the single ejaculatory duct and seminal vesicle arise from the primitive vasa deferentia, and not from the integument of the body, as is the case in the following examples.
According to the researches of Dufour, Loew, etc., most species of Orthoptera (Œdipoda), Libellula, Perla, Panorpa, Rhaphidia, Myrmeleon, Sialis, and Trichoptera (Hydropsyche) have double vasa deferentia and seminal vesicles, and two ejaculatory ducts. The male genital passages of Rhaphidia have a double opening, Loew describing “the two seminal vesicles as lying near each other and at last uniting in a common passage, with an external opening, which, however, must be very short, since I could only once clearly observe it.” This opening is a deep invagination of the external integument, at the bottom of which the two ducts open independently of each other. In such insects, Palmén states that the single ejaculatory duct morphologically arises by an invagination of the integument.
In another group, forming, as regards the genital apparatus, a step next above the Ephemeridæ, viz. the Perlidæ, the oviducts open near each other at the bottom of a median single “vagina,” situated between the 7th and 8th abdominal segment; it is covered beneath by a valve-like, enlarged sternite of the preceding segment, and Palmén homologizes it with the ovi-valvula of some Ephemeridæ. He regards this bell-shaped vagina as a cup-like, deep, intersegmental fold, which projects into the body-cavity and there receives the two ducts.
This differentiation in the Perlidæ may be regarded as the type for several groups of insects. But in others occur a complication which in some degree modifies the type. Thus the invagination arises out from one segment alone, but several segments during metamorphosis may become so reduced that the ventral portions of all may be invaginated to form the vagina. Thus in the larva of Corethra, according to Leydig, and also Weismann, the two testes are attached by two cords to the integument; the hinder ones are inserted independently, and share in the development of the outlets.
Graber has observed the same relations in the pupa of Chironomus, the efferent genital tubes in both sexes being separate, so that there are two vaginal passages and two penes present. Palmén comments on these relations in the dipterous insects, remarking that during metamorphosis certain parts of the terminal abdominal segments are reduced, while others are hypertrophied; hence the points of insertion of the cords referred to becoming the openings of the vasa are carried within the abdomen; and this part of the integument becomes an unpaired section. In these insects, also, there is an unpaired vesicula seminalis, but its morphological nature (whether formed from the integumental duct or the fused vasa deferentia) can only be settled after special investigation.
In the Lepidoptera, also, it has been shown by Herold, Suckow, Bessels, and recently with full details by Jackson, that the paired larval oviducts are at first solid, but become tubular early in pupal life. A little later, their cavities open into that of the azygos or unpaired oviduct. The paired oviducts open in the female caterpillars on the hind edge of the 7th abdominal segment, afterwards uniting with the unpaired vagina of the 8th segment, which is developed from the hypodermis.
Jackson adds that there are three stages traceable in the evolution of the genital ducts of Lepidoptera: “an ephemeridal stage, which ends towards the close of larval life; an orthopteran stage, indicated during the quiescent period preceding pupation; and a lepidopteran stage, which begins with the commencement of pupal life.”
As a summary of these results it appears that the genital organs of insects consist of two morphologically different elements: 1. the primitive internal paired structures (testes with the vasa deferentia; ovaries with the ovarian tubes), and 2. integumental structures (Fig. 464). In the most primitive winged insects (Ephemeridæ) the latter structures are only represented by the two external sexual openings, the entire reproductive system being paired. The paired parts become in the more highly differentiated forms united into single parts, while, a, a common integumental division, grows in, forming the ductus ejaculatorius, or the vagina; or, b, the inner passages anastomose together, i.e. the openings fuse together; or, c, both of these cases occur at once; or, finally, we have d, where the superfluous paired parts by reduction become single.
Fig. 464.—Evolution of the unpaired from the paired sexual organs of insects: A-E, male organs. The parts arising by invagination of the integument indicated by thick black lines. A, an Ephemerid. B, Forficula auricularia. C, nymph of Orthoptera in general. D, Œdipoda. E, Cetonia aurata. F, female organs of Æschna.—After Palmén, from Lang.
The male ducts open behind the 9th, the female passages of Ephemerids behind the 7th abdominal segment, those of other insects behind the 8th, except in the Stylopidæ (Strepsiptera), in which they open much in front.
Figure 464 graphically shows their relation. In the Odonata (F) the chitinous lining or integumental invagination extends inwards where the two oviducts begin, in the Coleoptera (E) the vagina, bursa copulatrix, and receptaculum seminis being lined by a thick chitinous layer. While in Perla the two seminal ducts pass directly into the copulatory organ, in the Coleoptera they open into the unpaired ductus ejaculatorius at a distance from the copulatory organ.
The morphological results obtained by Palmén, and for the Lepidoptera by Jackson, were apparently confirmed from an embryological point of view by Nusbaum, from observations on the development of the sexual passages in two genera of Pediculidæ, and are as follows:—
1. The prevalent impression that the larval ducts unite with each other and give origin to the whole system of sexual ducts is incorrect; they form only the vasa deferentia or the oviducts.
2. All other parts of the efferent apparatus (uterus, vagina, receptaculum seminis, ductus ejaculatorius, penis, and appended glands) develop from the hypodermis.
3. The connective tissue and the musculature of the efferent apparatus are derived from mesoblast cells present in the body-cavity.
4. The efferent ducts originate as paired rudiments. All unpaired (azygos) parts (uterus, penis, receptaculum seminis, unpaired glands, etc.) are at first paired. The unpaired efferent apparatus of insects must therefore be regarded as morphologically a secondary and more complicated form.[[76]]
5. The male and female efferent ducts are strictly homologous.
6. The cavities of the oviducts, uterus, vagina in the female, of the vasa deferentia, appended organs, and ductus ejaculatorius of the male arise independently, and come into connection secondarily.
The presence of two genital openings, viz. a bursa copulatrix or copulatory pouch, and of the primitive oviducal orifice behind the 9th segment, is peculiar to Lepidoptera, and the inquiry naturally arises whether they represent the outlets of two pairs of segmental organs. The question has been fully set at rest, however, by Jackson, who shows that the copulatory pouch is a secondary invagination of the ectoderm, being derived from the hypodermis, while the second aperture is a special adaptation. It is, however, the partial homologue of the vaginal orifice in other orders of insects. It opens behind the sternite of the 8th abdominal segment, the typical position of the vaginal aperture as shown by Lacaze-Duthiers. The lateral position of the bursa and its separation from the azygos oviduct are probably late features in the phylogenetic history of the Lepidoptera, subsequent even to the closure of the furrow.
“The existence of a second or posterior aperture is probably to be attributed to the advantage gained by a terminal position for the aperture through which the ova are laid. The remarkable way in which this aperture shifts backwards seems to point very distinctly to this explanation, especially as the Lepidoptera are entirely devoid of the outgrowths which form the ovipositor in some orders; e.g. most Orthoptera.”
The original condition of things appears to have been retained in a moth, Nematois metallicus, which, according to Cholodkowsky, possesses but a single external aperture, the bursa opening into the dorsal wall of the unpaired oviduct.
a. The male organs of reproduction
Bearing in mind that the testes with their efferent ducts are, like the ovaries and egg-tubes, primitive structures, there are various secondary or adaptive structures which are either due (1) to modifications of the male efferent ducts, or of the ovarian tubes, or (2) to various accessory organs, mostly glandular, resulting from the invagination of the ectoderm.
Fig. 465.—A, diagram of male sexual organs of Carabus. B, of Blaps. C, of Hydrophilus. The heavy black lines represent the ectodermal organs; t, testis; a. g., accessory glands.—After Escherich.
The male organs are, then, the following:—
1. Two testes (Figs. 465–469, t, H, ho).
2. The two seminal ducts (vasa deferentia, v, sl, SL), whose lower or outer (distal) division becomes enlarged and acts as a seminal vesicle (vesicula seminalis; Figs. 467–469, bl, SB).
3. The common ejaculatory duct (ductus ejaculatorius), with the penis (Figs. 467–469, ag, uSG).
4. Accessory glands at the base of the vasa deferentia (glandulæ mucosæ, Figs. 465–469, a. g., dr, D), whose secretion mixes with the semen or serves for the formation of the seminal packets (sematophores).
In his paper on the internal male organs of beetles, Escherich states that those of the Carabidæ illustrate the simplest, most primitive condition (Fig. 465). A simple blind tube on each side produces spermatozoa, stores the elements, and secretes mucus. Each of these tubes opens into a somewhat larger duct, and the two unite in a common ejaculatory canal. The terminal portion in these beetles is lined with chitin, and is therefore ectodermal, and not the result of the union of the mesodermic vasa deferentia. The region corresponding to the testes, vasa deferentia, and seminal vesicles are mesodermic. Blaps (Fig. 465, B) is intermediate between the Carabidæ and Hydrophilus (Fig. 465, C). The accessory glands (a. g.) are developed, and the seminal vesicles are situated in the middle, and not at the lower end of the vasa deferentia, as in Hydrophilus.
The testes.—Each testis is composed of follicles or corresponding parts, which according to the group of insects in which they occur are united in different ways; or each testis consists of a single hank or skein-like blind tube which is enveloped by a membrane, as in the Carabidæ, Dyticidæ, or Lucanidæ.
Fig. 466.—t, testis; v, vas deferens; g, seminal vesicle of Acheta campestris.—After Carus, from Gegenbaur.
Fig. 467.—Male sexual apparatus of a bark-beetle: sl, vas deferens; ho, testis; bl, seminal vesicle; dr, accessory gland; ag, ductus ejaculatorius.—After Graber.
The number of testicular tubes is small in most Hemiptera, but very great in the Cicadidæ, Orthoptera, Coleoptera, and many Hymenoptera. Although the testes are usually separated from each other, they are closely united in certain Orthoptera (Gryllotalpa, Ephippigera), Coleoptera (Galerucella), in many Lepidoptera, and in a number of Hymenoptera (Scolia, Pompilus, Crabro, and others).
The two testes of most Lepidoptera are so closely grown together or coalesced into a single body that one might regard them as a single testis. But in the different families there occur all grades, from the unpaired testes of most Lepidoptera to Hepialus with separate testes. Cholodkowsky therefore distinguishes four types:—
1. The embryonal or primitive type, with two testes, whose seminal follicles are entirely separate. (Brandt.) These testes are contained, as in all other Lepidoptera, in a well-developed thick chitinous membrane or scrotum, analogous to that of the higher vertebrates, which envelops each separate seminal follicle (Hepialus humuli).
2. The larval type, with two testes, whose four follicles are enclosed by a common scrotal membrane (Bombyx mori, Gastropacha quercifolia, Ichthyura anachoreta and anastomosis, Saturnia pyri, Aglia tau).
3. The pupal type (since it first occurs in the pupa state), with a single testis, which possesses an external median lace-like covering. (Adela, Lycæna.)
4. The imaginal type, with a single testis enveloped by a lace-like scrotum, within which the follicles are wound around the longitudinal axis of the testis. (Most Lepidoptera.)
In Nematois there are twenty seminal follicles, the number of ovarian tubes being the same. (Cholodkowsky.)
In many insects the testes are not composed of tubes (follicles), but of button-like bodies, each of which has its own duct.
The color of the testes is usually white, but they may be orange (Decticus), yellowish green (Locusta viridissima), or deep yellow (Chrysopa).
The testes of Asilid flies are enveloped by a common dark-red membrane rich in tracheæ, like that in Lepidoptera which clothes the separate testicular follicles. The two testes of Calliphora are enveloped by an orange-yellow capsule, outside of which is a special membrane formed by the fat-body. (Cholodkowsky.)
In the honey-bee the testis has two envelopes, the outer of which is formed by the fat-body, the inner coat of connective tissue. The entire testis corresponds to a portion only of that of Bombyx mori.
Fig. 468.—Male organs of a weevil, Hylobius abietis: H, testis; SL, vas deferens; D, slime gland; SB, seminal vesicle; uSG, ejaculatory duct.
Fig. 469.—Male organs of Tomicus. Lettering same as in Fig. 468.—This and Fig. 468 from Judeich and Nitsche.
The seminal ducts.—The vasa deferentia are fine tubes, which vary much in length; being short in many beetles and locusts, very short in many Diptera (Syrphidæ, etc.), very long in Cicada and many beetles; according to Burmeister, being in Dyticus about five times, in Necrophorus and Blaps eight to ten times, in Cicada 14 times, in Cetonia aurata 30 times, as long as the body. They either resemble a skein of silk, or form a tangled mass.
The distal or lower end of the vasa is in many insects dilated into a sac or seminal vesicle, which serves for the reception and storage of the seminal fluid after it passes through the vasa deferentia. In the honey-bee the vas deferens is given off from the reservoir, forms loops in and outside of the testis, and passes to the seminal vesicle. The canal into which the vesicle narrows does not open into the ductus ejaculatorius, but into the glandulæ mucosæ; its epithelial cells are much vacuolated, and have, therefore, a spongy appearance. (Koschewnikoff.)
Fig. 470.—A, spermatozoön of a beetle (Copris), partly macerated to show structure of flagellum, which consists of a supporting fibre (s.f) and a fin-like envelope (f); n, nucleus; a, a, apical body divided into two parts. B, anterior part of that of Calathus, with barbed head and finmembrane.—After Ballowitz, from Wilson.
The ejaculatory duct during coition conducts the sperm into the copulatory pouch of the female. In consequence of the stretching of the integumental membrane the end of the duct can be erected and again withdrawn. For this purpose the end of the duct is thickened and is said to be provided with powerful muscles. The evaginable terminal portion is covered by a strong chitinous membrane forming the penis or intromittent organ (Fig: 462, h), which is externally enveloped by a pair of chitinous lobes, which in many beetles are converted into a capsule. The ductus ejaculatorius of the honey-bee is inserted by two chitinous branches into the point of union of the two glandulæ mucosæ; it and the entire copulatory apparatus are devoid of muscles, though it is, however, well developed beneath the mucous glands. (Koschewnikoff.)
The accessory glands of the vasa deferentia are tubes whose secretions either directly mix with the semen, or in many cases form seminal packets (spermatophores). In Coleoptera, Lepidoptera, and Diptera there is usually one pair. In many insects there are several pairs, as in Hydrophilidæ and Elateridæ; they are branched in Hemiptera, and in Orthoptera bushy. The single glandular tubules are very long, and form a skein-like mass. In Orthoptera, in the larger number of accessory glands, two forms may be distinguished, which differ from each other in their contents (Siebold). In the cockroach (Fig. 461) these glands form the “mushroom” shaped gland of Huxley, which was at first regarded as the testis.
The spermatozoa.—These very minute bodies, the sexual homologues of the eggs, abound in the seminal fluid, and are formed in the follicles of the testes from a germinal layer or epithelium, as are the eggs. They are hair- or thread-like, usually consisting of a head, a body or middle-piece, and a long, thread-like tail (flagellum), which vibrates rapidly, causing the spermatozoön to move actively forwards (Fig. 470).
In beetles, according to Ballowitz, there are two main types of spermatozoa, connected, however, by intermediate forms. There is a double-tailed type, already described by Bütschli and v. la Valette St. George, and there are others which are single-tailed. Bütschli showed that in the double spermatozoön one tail-filament is straight and stiff, the other being undulating and contractile. Ballowitz describes this type in Calathus (Fig. 470, B), Chrysomela, and Hylobius, etc., and shows that the straight or supporting portion of the tail is elastic, but somewhat stiff, resistant to reagents, and without any fibrillar structure, while the contractile fringe consists of an extremely complicated system of fibrils (Fig. 470). The single-tailed type of spermatozoön, as seen, e.g., in Melolontha and Hydrophilus, has no supporting fibres. The tail is twisted in a spiral, corresponds to the contractile fringe of the double type, and exhibits a complicated fibrillar structure. The fringed type works its way ahead like the screw of a steamer.
Fig. 471.—C, anterior end of spermatid of a moth (Pygæra). D, young spermatozoön of the same; af, axial filament; c, centrosome; m, middle-piece or mitosoma; n, nucleus; p, paranucleus; e, envelope of the tail.—After Platner, from Wilson.
Each spermatozoön is a modified but complete cell, and the nucleus contains the chromatin, a deeply staining substance of the nuclear network and of the chromosomes and the supposed bearer of heredity.
Formation of the spermatozoön.—It arises from a primordial germcell called spermatogonium. This cell contains a large, pale nucleus and a dark body, the accessory nucleus of Bütschli. The spermatogonia subdivide, but at a certain period pause in their subdivisions, and undergo considerable growth. “Each spermatogonium is thus converted into a spermatocyte, which, by two rapidly succeeding divisions gives rise to four spermatozoa, as follows: The primary spermatocyte first divides to form two daughter-cells, known as spermatocytes of the second order, or sperm mother-cells. Each of these divides again—as a rule without pausing, and without the reconstruction of the daughter-nuclei—to form two spermatids or sperm-cells. Each of the four spermatids is then directly transformed into a single spermatozoön; its nucleus becoming very small and compact, its cytoplasm giving rise to the tail and to certain other structures.... As the spermatid develops into the spermatozoön, it assumes an elongated form, the nucleus lying at one end, while the cytoplasm is drawn out to form the flagellum at the opposite end.” (Wilson’s The Cell, from La Valette St. George.)
Henking finds that the primordial sperm-cells correspond to the primordial ova, both forms of cells in the insect he studied containing the characteristic number of twenty-four chromosomes.
The spermatogenesis of Laphria, according to Cholodkowsky, is very peculiar, and strongly resembles that described by Verson in Bombyx mori. In the blind end of the testicular tubes lies a colossal cell visible to the naked eye, the spermatogone, from which the entire contents of the testes originate. In Bombyx this spermatogone appears in the larva state. Such colossal spermatogones also occur in Lepidoptera of different families (Hyponomeuta, Vanessa, and in the pupa of Chareas graminis), in Trichoptera, and in Hemiptera (Syromastes); and Cholodkowsky inquires whether they may not be typical of insects. Toyama has observed these colossal cells not only in the testes but also in the ovaries of the silkworm. He regards them as supporting cells.
The spermatozoa are inclined to remain in bundles, and in this state are expelled during copulation. These bundles are either root-like, bushy, string-like, sinuous, or worm-like.
Auerbach has observed the spermatozoa of Dyticus marginalis in their passage through the convoluted seminal vesicles. All those arising from one testicular tube are united in a bundle. Each has a very complex structure, bilateral but unsymmetrical. The right side of the head is concave, the left convex; the whole head is longitudinally curved to right or left; and on the posterior half of the right side there is a projecting ridge bearing a hook-shaped cyanophilous “anchor,” at the free end of which an erythrophilous spherule appears. The most remarkable fact is that the spermatozoa unite in pairs in a perfectly definite way, opposed and crossed in a manner somewhat suggestive of a pair of scissors, with the right sides of the heads in contact. During this conjugation, or “dejugation” as Auerbach calls it, the anchors change their shape, and the little spherules are lost. Hundreds of these double spermatozoa are found together in little balls. The conjugation is a temporary one, but it may permit a molecular exchange of substance, perhaps with the result of mixing the hereditary qualities and limiting variability. (Journ. Roy. Micr. Soc., 1893, p. 622.)
In many insects which lack a true penis, the bundle of spermatozoa are united in the ejaculatory duct, forming packets which are enveloped by the secretion of the accessory glands which stiffens into a hard case. These packets are called spermatophores. They are either introduced into the vagina of the female or simply remain outside. Graber has repeatedly observed that the male crickets, in the absence of the female, let their spermatophores fall to the earth; whether it is afterwards made available is not known, because hitherto no case is reported that females seeking impregnation search, as in the case of the Isopod crustacean, Porcellio, for the spermatophores.
In the Gryllidæ and Locustidæ the spermatophore lies in a cup-like cavity under the penis. This is called the “spermatophore cup” (Chadima, 1871), into which the ejaculatory duct of the testis opens.
According to the views of Schneider, the spermatophores, with their capsule, usually consist solely of seminal filaments, which stick closely to each other, and only exceptionally have a capsule formed by a glandular secretion. In Locusta, however, and perhaps also in Gryllus, the sperm is enveloped by the secretion of the accessory glands of the seminal ducts; the spermatophores pass, still fluid, out of the sexual opening of the male into that of the female, but become chilled on the outer surface, so that the sperm, without coming in contact with the air, passes into the receptaculum seminis.
The mode of grouping of the spermatozoa of the Locustidæ as they occur in the spermatheca of the female is remarkable. Their heads lie so close to each other that they form a long shaft, while the numerous threads are arranged so as to look like the two vanes of a feather, the entire mass being like a very long heron’s feather. (Siebold.)
In the honey-bee the spermatophore is likewise enveloped by the secretion of the accessory glands, and thereby becomes a sort of seminal cartridge. This is a peculiar oval body which is carried during the marriage-flight into the air within the upper part of the penis, the so-called penis-bulb. (Leuckart.)
b. The female organs of reproduction
The different parts of the female reproductive organs are the following:
1. The two ovaries.
2. The two oviducts.
Fig. 472.—Female organs of generation of a saw-fly (Athalia centifoliæ): a, b, c, the 18 ovarial tubes originating from each of the two oviducts (d), and containing the immature eggs; e, common oviduct; f, spermatheca; g, poison-sac; h, poison-glands; 10, last ganglion.—After Newport.
3. The common egg-passage in nearly all insects (its distal or hindermost part forming the uterus or vagina).
4. The receptaculum seminis, or spermatheca.
5. The bursa copulatrix, or copulatory pouch.
6. The accessory glands (cement, sebific, or colleterial glands, or “oil reservoirs,” glandulæ sebaceæ, coleterium).
The ovaries and the ovarian tubes.—As in the testes, so each ovary consists of a variable number of ovarian tubes, by some called ovarioles, united by a thread at the distal end, and at the lower or hinder end opening into the oviduct. Each ovarian or egg tube is divided into three sections: (1) the terminal thread; (2) the terminal chamber, and (3) the actual ovarian tube, or chambered main division, this forming the longest part of the egg-tube.
The slender terminal thread serves to attach or suspend each egg-tube near the dorsal vessel (not directly to the heart, as formerly supposed), becoming lost in the fat-body.
Fig. 473.—Ovarian tube of P. orientalis: A, section near the end; tf, base of terminal filament. B, section lower down; ec, egg-cells in egg-chamber.—After Brandt.
The terminal chamber contains undifferentiated cell elements, supposed to be the remains of the ovarian rudiments. From these arise (either in the embryo or larva) first, the follicle epithelium of the ovarian tubes; and, second, the material for the formation of the new eggs, and nutritive cells. “In the terminal chamber these cell-elements remain undifferentiated, excepting when required for the removal of the follicle epithelium, eggs, and nutritive cells in the adult insect.” (Lang.) This portion of the ovariole is called the germarium. In Blatta it is filled with protoplasm in which numerous small nuclei are imbedded. (Wheeler.) The chambered main division of the egg-tube contains the ripening eggs, one in each compartment, the tube appearing like a string of beads.
The egg-tubes are of two types: (1) those without, and (2) those with nutritive cells, the first kind being the simplest, and occurring in the Synaptera (except Campodea) and in Orthoptera. As an example may be cited that of the cockroach (Fig. 473), where in each tube there is a simple continuous row of eggs from the terminal chamber to the oviduct. The tube being constricted between these consecutive eggs, gives it a beaded appearance.
In the cockroach (Periplaneta orientalis) each egg-tube has a beaded appearance. Its wall consists of a transparent elastic membrane, lined by epithelium, with an external peritoneal layer of connective tissue. The terminal filament (tf) is filled with a clear protoplasm, with a few nuclei. In the terminal chamber (tc) are large nucleated cells, with separate nuclei, both entangled in a network of protoplasm. In the third, or egg-chamber (ec), are about twenty ripening eggs, arranged in a single row. “Between and around the eggs the nuclei gradually arrange themselves into one-layered follicles, which are attached, not to the wall of the tubes, but to the eggs, and travel downwards with them. As the eggs descend, the yolk which they contain increases rapidly, and the germinal vesicle and spot (nucleus and nucleolus), which were at first plain, disappear. A vitelline membrane is secreted by the inner surface and a chitinous chorion by the outer surface of the egg-follicle.
“The lowest egg in an ovarian tube is nearly or altogether of the full size; it is of elongate-oval figure, and slightly curved, the convexity being turned towards the uterus. It is filled with a clear albuminous fluid, which mainly consists of yolk. The chorion now forms a transparent yellowish capsule, which, under the microscope, appears to be divided up into very many polygonal areas, defined by rows of fine dots. These areas probably correspond to as many follicular cells.” (Brandt, from Miall and Denny.)
In the second type, i.e. those egg-tubes with nutritive cells, there are two kinds. In the first the egg-chambers and yolk- or nutritive chambers alternate, each of the latter containing one or more nutritive cells, which serve for the nourishment of the ripening egg contained in the neighboring chamber. “The egg- and yolk-chambers may be distinctly separated externally by constrictions (Hymenoptera and many Coleoptera), or one nutritive and one egg-chamber may lie in each section of the ovarian tube, which is externally visible as a swelling (Lepidoptera, Diptera).”
In the second kind with nutritive cells, the actual tube consists (Fig. 474, C) of ovarian chambers only; the nutritive cells here remain massed together in the large terminal chamber. The single egg in the tube is united with the terminal chamber by connective strands (d. s.), which convey the nutritive material to the eggs. (Lang.)
Egg-cells, nutritive cells, and the cells of the follicle-epithelium (epithelium of the chambers of the ovarian tubes) are, says Lang, according to their origin, similar elements, like the egg and yolk-cells of the flat worms (Platodes); division of labor leads to their later differentiation. Only a few of the numerous egg-germs develop into eggs, the rest serving as envelopes and as food for these few.
Korschelt considers that all the chief elements of the egg-tubes, viz. egg, nutritive, and epithelial cells, arise by a direct transformation of the elements of the terminal chamber, and that the last may be traced to the indifferent elements of the terminal thread, the elements in question originating from the nuclear elements by a breaking down of the syncytium (or masses of protoplasm with nuclei scattered through it) composing it (Fig. 475).
The latest work is that of Wielowiejski (Zoologische Anzeiger, ix, 1886, p. 132), whose observations are based on a study of the ovarian tubes and the growing eggs of the Hemiptera (Pyrrhocoris), the Coleoptera (Telephorus, Saperda, Cetonia and Melolontha, Carabidæ, and Hydradephaga), etc.
Wielowiejski divides the ovaries of insects into three groups:—
1. Comprising such ovaries in the ends of whose egg-tubes (terminal filament) the embryonal cells in the early stages are accumulated, and are transformed into egg-, yolk-, and epithelial cells respectively. (Ovaries of Orthoptera, geodephagous and hydradephagous Coleoptera, Lepidoptera, Diptera, and Hymenoptera).
2. Comprising ovaries whose ends above the egg-cells and egg-germs (Eianlagen) possess throughout life a more or less voluminous solid accumulation of cells (terminal chamber), but which stand in no close relation with the first. (Ovaries of Coleoptera, with the exception of the Geodephaga and Hydradephaga, and Aphidæ in part.)
Fig. 474.—Various types of ovarian tubes, diagrammatic: A, ovarian tube without nutritive cells. B, egg-tube with alternating nutritive and egg-compartments. C, ovarian tubes in which the terminal chamber (ek) is developed into a nutritive chamber, with which the developing eggs remain connected by means of threads (ds); ef, terminal filaments; efa, egg compartments or chambers; fe, follicle epithelium; df, yolk-chambers.—After Lang (C from Claus).
Fig. 475.—Upper portion of the ovary in Forficula, showing eggs and nurse-cells; below, a portion of the nearly ripe egg (e) showing deutoplasm-spheres and germinal vesicle (gv). Above it lies the nurse-cell (n), with its enormous branching nucleus. Two successively younger stages of egg and nurse-cell are shown above.—After Korschelt, from Wilson.
Fig. 476.—A, ovarian egg of a butterfly (Vanessa), surrounded by its follicle; above are the nurse-cells (n. c.), with branching nuclei; g.v, germinal vesicle. B, egg of Dyticus, living; the egg (o.v.) lies between two groups of nutritive cells; the germinal vesicle sends amœboid processes into the dark mass of food-granules.—After Korschelt, from Wilson.
Fig. 477.—A, lower portion of one of the two ovaries of Sphinx ligustri, the four egg-tubes uniting to form the slightly developed calyx (ov). The egg-tubes above contain ripe eggs still surrounded by the follicle; e. c, the empty egg-chamber. Beyond the empty egg-chambers (e. c) are three egg-chambers with ripe eggs and the connecting cord. The whole tube is surrounded by the peritoneal membrane and musculature.—After Korschelt.
3. Comprising ovaries whose ends above the egg-germs contain a well-developed mass of cells functioning as a yolk-forming organ, between whose special elements grow root-like offshoots of nearly ripe egg-cells. (Hemiptera.)
Fig. 478.—Ovary of a beetle, drawn somewhat diagrammatically: o, egg-tube; s, stalk of the same; c, egg-calyx; ov, oviduct.—After Korschelt.
When the egg is ripe the food-chamber disappears because its contents have served for the formation of the egg below it. In Lepidoptera especially, the egg-tubes resemble strings of pearls because most of the numerous eggs ripen simultaneously and are likewise deposited at the same period, which is naturally not the case in those insects whose eggs gradually ripen (Fig. 477). In other cases the egg- or food-compartments are transformed into each other, but only one egg- and one food-compartment can be situated in the same dilatation of the ovarian tube. Finally, there are insects in whose egg-tubes the egg-compartments are arranged in a single row, while the capacious terminal chamber contains a large mass of food-cells.
Egg-cells, nutritive cells, as well as the cells of the follicle epithelium (epithelium of the chambers of the ovarian tubes), originate as similar or homologous elements, division of labor leading to their later differentiation. Only a few of the numerous egg-germs develop into eggs, the rest serving as envelopes and also as food for these few. (Lang.)
In many insects the egg-tubes open into an egg-calyx (Fig. 478, c), in which the ripe eggs collect before passing into the oviduct (ov).
As the result of his investigations on the origin of the cellular elements of the ovaries of insects Korschelt concludes:—
1. The different cell-elements of the egg-tubes, eggs, nutritive cells, and epithelium arise from identical undifferentiated elements situated in the contents of the earliest germ of the egg-tubes.
2. The first formation of the cellular elements present, and the differentiation of the individual compartments of the egg-tube, occur during embryonic and larval life.
3. The undifferentiated elements of the terminal chamber correspond to the embryonic condition, while in post-embryonic time, and even during imaginal life, a new formation of the different kinds of cells takes place.
4. The mode of origin of the different kinds of cells from the undifferentiated elements varies greatly in different insects.
5. From their histological nature, and from the mode of origin of their elements, the most complex egg-tubes and those provided with nutritive compartments are phylogenetically derived from those without such nutritive compartments.
6. The nutritive cells in certain cases originate in the same way and at the same time as the germ-cells, and are therefore to be regarded as germ-cells which have abandoned the function of egg-making, and exchanged it for the production of nutritive material.
7. In the egg-tubes with numerous nutritive compartments the nutritive cells can originate at the same place as the egg-cells, and they afterwards still lie intermingled with these in the beginning or upper part of the egg-tubes.
8. While the capability of egg-making of the germ-cells originally situated in the extremity of the terminal chamber gradually becomes transferred to those at the base of the terminal chamber, and the first transform into nutritive cells, egg-tubes with nutritive compartments at the base may be found.
9. The nutritive cells of certain forms arise independently of the germ-cells and therefore could not have previously originated from them.
10. The epithelium has in all forms nearly the same mode of formation; it everywhere shows a close similarity to the undifferentiated elements of the terminal chamber, out of which it directly develops. As to the fact of formation of epithelium through the germ-vesicles (Keimblaschen), nutritive-cell nuclei, or the so-called “oöblasts,” I could not feel certain.
11. Neither the eggs of Hemiptera or of other insects arise through the agency of “oöblasts,” but like the epithelial and nutritive cells arise by a gradual differentiation from the indifferent elements of the ovarian tubes.
12. The different elements of the egg-tubes, also the eggs, have the morphological value of cells.
Origin of incipient eggs in the germ of the testes.—Heymons has detected in the germ of the testes of the male larvæ of Phyllodromia germanica 7 mm. in length, young or incipient eggs, similar to those seen in the ovarian tubes of the female larva of the same size. In another male larva of the same size also occurred short cylindrical tubes each with a terminal thread, which had the appearance of rudimentary egg-tubes. Hence he thinks that every part of the genital germs (Anlagen) in the male, which are not concerned in the formation of testicular follicles, represents the germ of a female genital gland. As is well known, no insects are hermaphroditic, but this case of the practical origin of eggs and egg-tubes in the lowest division of the male efferent passage, which is homologous with the egg-producing division of the female ovarian tubes, points back to hermaphroditic ancestors. And Heymons suggests that the frequent occurrence of hermaphroditism in insects probably confirms this view.
Fig. 479.—Abdomen of queen bee, under side, × 8: P, petiole; o, o, ovaries; hs, position filled by honey-sac; ds, place through which the digestive canal passes; od, oviduct; co.d, common oviduct; E, egg passing oviduct; s, spermatheca; i, intestine: pb, poison-bag; p.g, poison-gland; st, sting; p, palpi. B, vestigial ovaries of ordinary worker; sp, vestigial spermatheca. C, partially developed ovaries of fertile worker; sp, vestigial spermatheca.—After Cheshire.
The bursa copulatrix.—The copulatory pouch in most insects is a special cup-shaped appendage of the vagina adapted for the reception of the male organ during sexual union. Its mode of formation in the cockroach is thus described by Haase:—
“By the retreat of the female sexual aperture, situated in the 8th ventral plate, a considerable space, the genital pouch, is produced; this is formed chiefly by the extended connective membrane between the elongated 7th and 8th ventral plates. This serves for the development of the egg-cocoon, which is retained by the internal appendages of the posterior gonapophyses.”
The fertilization of the female takes place once for all a long time previous to oviposition; the semen in the receptaculum seminis passes out as the eggs slip down the egg-passage, and a spermatozoön gains entrance into the interior of the egg through the micropyle. In Œcanthus, according to Ayers, fecundation probably takes place while the egg is passing into the vagina, “since it is hardly possible that the male element could gain access to the follicles before the chorion is secreted.”
In the Lepidoptera, as has been stated, the copulatory pouch opens separately from the opening of the oviduct (vagina), but a slender canal connects the pouch with the vagina (Fig. 310, bc). The outlet (“vagina” of Burgess) of the copulatory pouch opens between the 7th and 8th segments, that of the oviduct (vagina) on the 9th segment being “situated immediately below the anus and hardly separated from it, between the lappets of the 9th segment.” (Burgess.) The opening of the copulatory pouch is, as we have seen, the genuine or primitive sexual opening.
Fig. 480.—Spermatheca of the honey-bee, queen, × 40: a, space filled by a clear fluid; b, mass of spermatozoa; c, duct; d, d, active spermatozoa.— After Cheshire.
Fig. 481.—Female sexual organs of Scolytus: ER, egg-tubes; pEL, paired oviducts; ST, spermatheca; BT, copulatory pouch; KD, cement-glands; Sch, vagina.—After Lindeman, from Judeich and Nitsche.
The spermatheca.—This is a sac or pouch for the reception and storage or preservation of the semen. While in most of the higher insects it opens into the dorsal wall of the vagina (Fig. 472, f), in the cockroach, locusts, and grasshoppers it opens into the bursa; but in other European Orthoptera, as in most insects, it lies upon the dorsal wall of the vagina. (Berlese.) In the cockroach, it is a short tube dilated at the end and wound into a spiral of about one turn. “From the tube a cœcal process is given off, which may correspond with the accessory gland attached to the duct of the spermatheca in many insects (e.g. Coleoptera, Hymenoptera, and some Lepidoptera). The spermatheca is filled during copulation, and is always found to contain spermatozoa in the fertile female. The spermatozoa are no doubt passed into the genital pouch from time to time, and there fertilize the eggs descending from the ovarian tubes.” In Meloë the spermatheca is exceedingly large. (Miall and Denny, pp. 170, 171.)
The colleterial glands.—We have already briefly referred to these glands. Those of the cockroaches form a number of long blind tubes opening into the vagina. They furnish the material for the egg-capsule or oötheca, viz. chitin and large crystals of oxalate of lime.
In Phyllodromia germanica “these glands are glistening white till the time of oviposition approaches, when they assume a yellow tint, and the octahedral crystals are seen imbedded in a viscid substance which fills their lumina. This viscid substance is soluble in potassium hydrate, and is consequently not chitin. When excreted to form the oötheca, it slowly hardens, deepens in color, and becomes insoluble in potassium hydrate. Light has nothing to do with this change, which is possibly produced by the oxygen in the air. It is the same change which is undergone by the cuticula of the insect itself immediately after ecdysis.” (Wheeler.)
The vagina or uterus.—This is simply the end of the common oviduct, which, when dilated, is called the vagina, and, in the pupiparous forms, the uterus.
In the cockroach the vagina opens by a median vertical slit situated in the 8th sternite, into the genital pouch or bursa, upon the dorsal wall of which the orifice of the spermatheca is situated. In the sheep-tick the oviduct is enlarged to form the so-called uterus, which furnishes a milk-like secretion for the nourishment of the larva during its intra-uterine life.
In insects in general, the external opening of the vagina is simple, the chitinous structures (valves) at the opening being adapted to receive the male intromittent organ.
When the eggs are to be deposited deep below the surface of the earth, or in wood, or in wood-boring larvæ, or in the body of caterpillars, etc., they are inserted by the ovipositor (see p. 167).
Signs of copulation in insects.—Leydig has collected, partly from his own observations and partly from those of others, a number of cases in which female insects bear traces of having had sexual union, in the form of tags or plates attached to the body, and apparently formed from material secreted by the male. Such probably is the “pouch” on the abdomen of Parnassius apollo, and a somewhat similar structure in Fulgora laternaria, and such is the plate which is found on the hinder part of the abdomen of Dyticus latissimus and D. marginalis. Leydig compares these structures with the white plate in Astacus fluviatilis, and with the little white lid on the spider Argenna, and finds analogues among vertebrates. (Arbeit. Zool. Zoot. Inst. Wurzburg, x, 1891, pp. 37–55, 2 Figs.)