Ant colonies > Internal structure of ants > Reproductive organs

Reproductive organs

The Reproductive Organs

In the female, or queen ant, the repro­ductive organs comprise the two ovaries, each of which consists of a number of tubes, or ovarioles, in which the elliptical eggs are formed in a single series, very small at the distal or anterior and gradually increasing in size towards the proximal or posterior end of the tube. Each egg is surrounded by a follicular epithelium, which secretes frotn its inner surface the thin, transparent chorion enveloping the ripe egg, and is accompanied by a cluster of nurse cells. The ovarioles, which are bound together in a fascicle by richly ramifying tracheze, are attached at their tapering anterior ends to the pericardium in the antero-dorsal region of the gaster (Fig. 15, ot).

The number of ovarioles in each ovary varies considerably in the queens of different ants. Miss Bickford (1895) gives the following numbers for several European species: Formica rufa 45, F. rufibarbis 18-20, Lasius niger 30-40, L. flavus 24, L. brunneus 9-11, Camponotus 39-40, Myrmica ruginodis 8, M. levinodis 12, M. scabrinodis 8-9, M. sulcinodis 9-1 1, Anergates atratulus 12, Plagiolepis pygmae 4-5; and Miss Holliday (1903) finds the following numbers in several American ants: Pachycondyla harpax 5-7, Odontomachus clarus 5, Eciton schmitti about 250, Leptothorax emersoni 2, Cremasto­gaster minutissima 2, Colobopsis etiolata 6-7, Camponotus decipiens 12, C. festinatus 15-18, C. sansabeanus 6-17, Pogonomyrmex mole­faciens 25-30.

The ovarioles of each ovary unite at their posterior ends to form a short oviduct, and the two oviducts in turn unite to foriii the uterus, which bears on its dorsal surface a small stthshherical pocket, the seminal receptacle, which is filled with sperm by the male duringthe nuptial flight. Tile sperm is kept alive in th.e receptacle for years, apparently by a nutritive fluid secreted into the cavity of the organ near its orifice by a pair of appendicular glands. The eggs are fertil­ized as they pass through the uterus by sperm which is permitted to escape in small quantities from the orifice of the receptacle. The uterus opens behind into the short vagina, which bears on its dorsal surface a rather thin-walled sac, the copulatory pouch (Fig. 15, bc ) . The vagina (vg) opens to the exterior by means of a transverse slit (zw) just in front of the sting or its vestige on the sternal articular membrane of the seventh abdominal (fourth gastric) segment.

In the worker the ovaries are also present, but, as a rule, with a greatly reduced and often highly variable number of ovarioles. Adlerz (1887) gives the following numbers for each ovary in the work­ers which he examined: Formica sanguinea, 3-6, Camponotus herculeanus I-5, Polyergus rufescens 3, Lasius flavus I, Tapinoma erraticum I, Har­pagoxenus sublevis, 3-6, and Miss Bickford gives the following data: F. pratensis 2-6, F, rufa 4-10, L. fuliginosus 1-2, Myrmica levinodis, ruginodis, scabrinodis, Aphaenogaster subterranea and Cremastogaster scrutellaris I. The numbers observed by Miss Holliday are: Leptogenys elongata 2-3, Pachycondyla harpax 2-9, Odontomachus c1arus 2-8, Lepthothorax emersoni 2-4, Colobopsis etiolata I, Camponotus decipiens 1-4, C. festinatus 1-11, C. sansabeanus I, Pogonomyrmex molefaciens 1-7, Lespez (1863), Adlerz and Miss Bickford failed to find any tubules in the worker Tetramoroium cespitum, and Miss Holiday had no better success with the worker of Ecition schmitti. It is very doubtful, however, that the ovaries have completely disappeared in the workers of any of the Formicidae. A well-developed seminal receptacle was found in the workers of quite a bumer of species by Miss Holiday, but copulation of workers with males has not been observed.

In the male ant each testis (Fig. 19, ts) consists of a number of com­pact lobules (according to Adlerz 17 in Camponotus ligniperdus, 21 in Formica sanguinae, ,3 in Leptothorax acervorum and Anergates atratulus; according to Janet 4 in Myrmica), occupying a position int he gaster like that of the ovaries in the female. The lobules, which are crowded with cysts containing mature sperm, or testicular cells in various stages of spermatogenesis (sp), unite in each testis to form a

Fig. 20. Poison apparatus of Forrrrica, A, Lateral and slightly dorsal aspect of apparatus in Formica rufibarbis. (Fore].) a, Duct of poison vesicle, or reservoir; b, cushion formed by the long convoluted portion of the duct from the two free glandular tubes (c) ; d, tracheae; e, ring-muscles of vesicle wall; f, nerves; g, intima of vesicle; h, accessory gland ; i, its orifice ; in, gland-cells of its walls ; n, muscles; o, sting­sheath; p, vestigial sting-groove ; r, somewhat dislocated, vestigial right sting-stylet ; s, piece of the cloacal membrane which has been almost entirely removed. B, Trans­verse section through poison apparatus of Formica rufa. (Beyer.) w, Dorsal wall of vesicle; .r, sections of cunvoluted duct forming the cushion-shaped mass; y, opening of duct into the vesicle, the lining of which is represented by the more heavily shaded layer (c:).

long deferent duct (vd). Each duct is enlarged near its posterior end to form a thick-walled seminal vesicle ( vs ) . The two deferent ducts unite to form a slender ejaculatory duct (de) which opens on the ninth abdominal (sixth gastric) segment at the base of the paired penis (p).

The poison apparatus belongs morphologically to the sting, and is, therefore, absent in all male ants. It appears under two different forms, which Forel (1878) distinguishes as the pulvinate and the bourreleted. The former is confined to the ants of the subfamily Camponotinae (Formica, Lasius, Camponotus, etc.), a group in which the parts of the sting have all but completely disappeared, the latter occurs in all the other subfamilies, which have the sting either highly developed or very small. In Formica the poison apparatus consists of a large, elongated, thin-walled but muscular sac or vesicle and a glandular portion (Fig. 20). The former opens by means of a rather large orifice between the scarcely recognizable sclerites of the highly vestigial sting. To the inside of the dorsal wall of this vesicle is applied an elongate, elliptical, flattened cushion made up of a delicate, much convoluted and somewhat branched glandular tubule, which is fully Zo cm. in length when uncoiled.

One end of this tubule opens into the vesicle at the middle of the ventral surface of the cushion, the other leaves the posterior end of the vesicle in the mid-dorsal line and bifurcates to form a pair of glandular tubules which terminate blindly and lie freely in the body cavity. The walls of these tubules consist of polygonal cells, each of which has a minute duct starting within its cytoplasm and opening into the axial duct, or lumen of the tubules.

The second, or bourreleted type of poison apparatus is of a much simpler structure. It, too, consists of a vesicular and a glandular por­tion, with the former opening into the groove of the sting. The vesicle is smaller, however, and more pyriform or globular, and its duct to the exterior is more slender than in the pulvinate type. The glands are a pair of tubules which unite and enter, and in Myrmica (Fig. 21, 13 ) and many other genera, form an unpaired and somewhat convoluted tubule within the vesicular cavity. This tubule is enlarged or button­shaped at the free end where its opening is situated. In Bothriomyr­mex (Fig. 21, A ) Forel found the unpaired tubule reduced to a small sub-globular structure with the opening on its summit. As the bour­releted gland is usually associated with a well-developed sting, except in the Dolichoderinae, and, moreover, closely resembles the poison gland of the wasp and bee, it must be regarded as the more primitive of the two types. The pulvinate gland secretes a more copious amount of liquid, which is stored in the vesicle whence it can be either ejected by some ants (Formica rufa and its allies) in a fine spray to a distance of 20-50 cm., or injected into wounds inflicted with the mandibles. Beyer (1890), who has made a comparative study of the development of the poison apparatus in the honey-bee, wasp, Myrmica and Formica, finds that it is smallest in the forms with the largest sting (bee) and largest in forms with only a functionless vestige of this organ. The enlargement and extraordinary convolution of the gland in the Campo­notinae is therefore correlated with a degeneration of the sting as an organ of defence and the development of a novel method of using the poison in conflicts with hostile ants and other animals.
A paper by Melander and Brues (1906) has been published on the chemical constitution of the poison of ants in general. These authors find appreciable traces of formic acid, as a

FIG 21. Poison apparatus of a Dolichoderine and a Myrmicine ant. (Forel.) A, Bothriomyrmex meridionalis; B, Myrmica levinodis, a, Sting; b, sting-groove; c, sting-sheath; d, accessory gland; e, duct of poison vesicle; f, poison vesicle; g, bourrelet-like termination of poison glands; h, poison glands; i, unpaired, convoluted portion of poison gland; 1:, film of secretion(?) surrounding bourrelet.

rule, only in the Camponotinae, that is, in the forms with the pulvinate glands. In this group, as would be expected, the species of Formica head the list with more than twice as much acid relatively to their size as the species of Camponotus. In the Doryline ants (various species of Eciton ) the secretion has a very strong and nauseating, fecal odor like that of the lace-wings (Chrysopa). Melander and Brues believe this to be due to leucine, and they state that "these ants are totally blind, and migratory in their habits, so that they must depend almost entirely upon a sense of smell to follow one another about. Thus it can easily be seen how such a strong odor might be developed through the action of natural selection, from the small trace of leucine that is usually present in insect feces." As I have found a secretion precisely like that of Eciton in certain carnivorous Pheidole (P12. ecitonodora and antillensis), I infer that its chemical constitution may, perhaps, depend on the diet of the insects.

In all ants, both in those with the pulvinate and those with the bour­releted glands, there is present a so-called accessory, or Dufour's gland (Fig. 20, h, Fig. 21, d), which opens into the duct of the poison vesicle very near its termination. This gland is ventral to the poison apparatus and though of variable form ( pyriform, cylindrical or bilobed ) is rather uniform in structure throughout the family Formicidae. It is a small, elongated sac, with rather thin walls composed of polygonal gland cells enveloped by delicate muscles and tracheae. Several authors have regarded its rather thick, yellowish secretion as a lubricant for the parts of the sting, but Janet has shown that no such lubricant is necessary. Moreover, the gland is often best devel­oped in virtually stingless ants like the Camponotinae. Others have surmised that the secretion is added as a necessary ingredient to the poison. Janet finds that it is alkaline and conjectures that its chief use is to neu­tralize any of the highly acid poison which may happen to adhere to the ant's own body or remain on the parts of the sting or on the anal circlet after the gland has been dis­charged. He also finds that all the other integu­mentary glands, except those of the poison appa­ratus, have an alkaline re­action, and believes that this is important in pre­venting the nest chambers from becoming acid, for the secretions of the poison glands, if allowed to accumulate in a closed cavity, soon

Fig. 22. Repugnatorial glands and vesicles of worker Bothriomyrmex meridionalis. (Forel.) A, Whole structure seen from above; a, vesicles; o, common orifice of same; g, clusters of unicel­lular glands; d, duct; i, intima ; in, muscles in wall of vesicle. B, Single gland cell (c) contain­ing the convoluted termination of the ductlet (e) in its cytoplasm; d, main duct; t, trachea.

become fatal to the ants. This is easily demonstrated when Campo­notinae are confined in a vial and irritated till they discharge their secretions. but as there seems to be little or no acid in the poison of any species except those belonging to this subfamily, Janet's con­jecture, at least so far as the accessory gland is concerned, is far from being applicable to all ants.

The repugnatorial, or anal glands (Figs. 22 and 23), were dis­covered by Forel. They are present only in the female and worker Dolichoderinae and coexist with well-developed poison glands of the bourreleted type. They consist of grape-like clusters of large, spher­ical gland-cells, the fine intracytoplasmic ducts of which unite to form a pair of much larger ducts that open into the posterior portions of two large, thin-walled sacs, dorsal to the poison gland and closely applied to each other in the medium sagittal plane of the ant's body. These sacs have muscular walls and serve as reservoirs for the gland­ular secretion. They have a con-non opening just dorsal to the anus. Their secretion is quite unlike that of the poison glands described above, being more sticky and having in nearly all Dolichoderinae a very characteristic odor, which Forel calls the " Tapinoma odor" because it is very noticeable in the common species of this genus in Europe and North America (T. erraticum and sessile). Others aptly describe the odor as that of " rotten cocoanots."

FIG. 23. Sagittal section through tip of gaster of worker Bothriomyrmex meridionalis (Forel.)­ a. Orifice of repugnatorial vesicles; b. anus; c. orifice of poison vesicle; d, orifice of accesory gland; e. vaginal orifice; f. terminal ganglion of ventral cord; g. repugnatorial of right side.

Melander and Frues have studied the secretion in Iridomyrmex analis (Forelius foetidus) and find that "when distilled with steam the odor passes over and remains dissolved in the aqeous distillate. Thus freed it retains the very evident odor of rencid cocoanuts. By saponification with potassium hydroxide solution it loses all odor, 'but on adding dilute sulphuric acid to excess an odor closely re­sembling that of fresh cocoanuts is developed. From this it is quite evident that the odorous principle is an ether of some sort." During conflicts with other ants the Dolichoderinae smear the secretion of their repugnatorial glands on the bodies of their enemies, and from the behavior of the latter it is evident that the liquid is fatal, or, at any rate, very irritating, and that it constitutes a most efficient protection even for the most diminutive and soft-bodied species of Iridomyrmex, Tapinoma, Azteca, etc.