Pachysima aethiops, Emery, Emery

Wheeler, W. M., 1922, The ants collected by the American Museum Congo Expedition., Bulletin of the American Museum of Natural History 45, pp. 39-269 : 114-119

Pachyaima aethiops (Smith)

Text Figures 25, 26, and 27

Avakubi, [[worker]], [[male]]; Stanleyville, [[queen]]; Ambelokudi, [[worker]]; Isangi, [[worker]]; Panga, [[queen]]; Medje,[[worker]] [[queen]], [[male]]; Bafwabaka, [[queen]] (Lang and Chapin).

This shining, jet-black ant, the worker of which measures 9 to 10 mm., the male and female 13 to 14 mm., is the largest, most widely distributed, and therefore best known to taxonomists of all the Ethiopian species formerly included in the genus Sima. It is represented in the collection by numerous adults and larva; and pupae in all stages. The specimens from Medje and Ambelokudi were living in the twigs of Barteria fistiulosa (Plates XXVIII and XXIX). "When disturbed the workers came out in great numbers. The natives, who call them 'gumaguma,' fear them on account of their sting."

Referring to specimens taken by Tessmann in Spanish Guinea and the Cameroon, Stitz says that "this ant is often found on the trunks of Epitaberna myrmaecia K. Schum., the thickened twigs of which it inhabits. It is called 'engunkun' by the natives and its sting is greatly feared as it is supposed to cause fever."

Father Kohl (1909, Natur u. Offenbarung, LV, p. 97, et seq.) gives a much more extensive account of the habits of P. aethiops and especially of the plant Barteria fistulosa which it inhabits. According to his observations in the Congo, it is restricted to this plant and an allied species, B. Dewevrei De Wildeman and Th. Durand. It inhabits the peculiarly swollen, lateral branches and keeps large coccids in their cavities. The openings to the cavities are not made at definite points predetermined by a peculiar histological structure, as in the case of the Neotropical Cecropiae associated with species of Azteca . After the marriage flight, the aethiops queen gnaws its way into an already hollow twig and while she is establishing her colony the orifice, as in Cecropia, closes by growth of the plant tissue, so that it has to be reopened from within by the workers of the young colony. As several queens enter different internodes of the same plant, their various colonies probably eventually unite to form a single huge colony possessing all the cavities in common, as in the case of Cecropiae tenanted by Azteca . Concerning the behavior of aethiops, Kohl writes as follow:

The Simae arc extremely pugnacious and always ready for a fight as they are equipped with excellent weapons, their stings and mandibles. If a Barteria tree is roughly handled or even shaken, innumerable hosts of the ants rush out of all the openings and woe to him who approaches them too closely! I have had many sore experiences with their pointed stings while studying or amputating the branches.

The pain spreads instantly over the whole affected limb and continues for a long time and on the following morning returns with full intensity during one's ablutions. One day my black servant told me that it was customary in his part of the country to punish unfaithful wives by tying them to plants inhabited by the Sima.

On examining the series of aethiops larvae, I was struck with their extraordinary appearance. A further study of them and of the larvae of the only other known species of Pachysima ( P. latifrons ) throws considerable light on the raison d'etre of the peculiar ethological relations of larval ants to their nurses, as I have shown in a recent paper.1

Four distinct stages, probably separated by moults or ecdyses, may be recognized in the aethiops larva. The first stage larva, just after hatching, is represented in Fig. 26a-b as it appears in ventral and lateral view. The body is curved, convex dorsally and concave ventrally, and terminates behind in a cylindrical projection, with the anus shifted to the ventral surface near its base. The creature is strongly hypocephalic like the larvae of Tetraponera , Viticicola , and Pseudomyrma, i.e., with the head on the ventral side. The head is surrounded by a cluster of prominent, tubercle-like appendages. On the prothorax, which is large and forms a hood over the head, there are three pairs of these appendages, an anterior truncate pair, a median pointed pair and a large posterior pair, which are swollen and rounded and embrace the sides of the head. These correspond to the single prothoracic pair figured in the larva of Viticicola tessmanni . The mesothoracic segment has a pair of smaller appendages nearer the midventral line. Between them arises a very peculiar organ, with a swollen, pear-shaped base prolonged into a slender, apparently erectile, tentacle-like process which extends up in front of the head and terminates in a small ampulla. The first abdominal segment bears a pair of large swollen appendages, which lie at the lateral bases of the mesothoracic pair and are united with a large and very prominent midventral tubercle. This tubercle and its lateral appendages are represented in the larva of V. tessmanni but the others, with the exception of the third thoracic pair, are absent. Sections and stained, cleared preparations of the whole larva show that the various tubercles contain portions of the fat-body, at least in the basal portions of their cavities, and next to the hypodermis a dense, granular substance, evidently a coagulated liquid produced by the adipocytes or trophocytes. The liquid also fills the impaired tentacle, except its pear-shaped base, which contains fat-cells. Around the bases of the tubercles are muscles so arranged that their contraction increases the pressure of the fat and granular liquid on the appendages and in all probability causes the liquid to exude through the hypodermis and delicate chitinous cuticle onto the surface. The whole arrangement of the tubercles, in fact, constitutes a system of exudate organs or "exudatoria," as I shall call them, adapted to produce a substance that can be licked up by the ants when they are feeding and caring for the larvae. In this stage the mandibles are small, soft, and unchitinized, so that the ants must feed the larva by regurgitation on liquid food. The labium of the larva has a peculiar pair of swollen appendages, shown just beneath the mandibles in the figure. The body is naked, except for a few sparse, pointed bristles on the dorsal surface and the median pair of prothoracic appendages. As nothing like this larval stage is known among ants or indeed among the Hymenoptera, I propose to call it the "trophidium."

The second stage larva is shown in Fig. 27a, The various exudatoria are small in proportion to the remainder of the body but are still much like those of the trophidium. The body is more elliptical, the mandibles are more pointed and distinctly falcate but, even in this stage, they are unchitinized and therefore nonfunctional. The coarse hairs are visible on the dorsal surface but a more uniform investment of small hairs has made its appearance. They are blunt or even clavate, especially on the prothoracic segment. In this and the trophidium stage, I am unable to find any salivary glands in cleared preparations, though rudiments of these organs may, perhaps, be present.

The third stage larva (Fig. 27b) is larger and very regularly elliptical. The exudatoria can all be recognized, except the impaired tentacle. It is, however, present in some of the younger individuals but in a greatly reduced and vestigial condition at the bottom of the deep depression which now forms a definite pocket just back of the mouth and under the midventral swelling of the first abdominal segment. In many larvae I found in this pocket a small rounded, dark-colored pellet which puzzled me at first. In sections it was at once seen to consist of triturated and compacted bodies and parts of small insects. It is, in fact, a food-pellet placed by the worker ants in the pocket just behind the larva's mouth and proves to be merely the pellet which is originally formed in the infrabuccal pocket of the adult ants. In this stage, therefore, the larva is fed on solid food and the strongly chitinized, acute, and bidentate mandibles corroborate this statement. Slender salivary glands may also be detected in this stage indicating that the substance of the food-pellet is subjected to extra-intestinal digestion. The longer hairs on the dorsal integument have almost completely disappeared. The first pair of appendages on the prothorax have disappeared and the second pair is smaller or obsolescent.

In the fourth or adult stage (Fig. 27c) the larva is more elongate and cylindrical and much more hypocephalic, the prothorax forming a great protuberance in front of the head. The exudatoria are still recognizable, with the exception of the first and second prothoracic pairs, which have disappeared completely. The labial appendages are reduced. A food-pellet was found in the postcephalic pocket in several of the larvae of this stage but is not represented in the figure. The coarse hairs have disappeared from the integument, which is now uniformly covered with very short, delicate hairs and the structure of the posterior end of the body is very different from that of the preceding stages.

The conclusions which I draw from the study of these larvae and from those of P. latifrons and Paedalgus infimus (vide infra) are that the young larvae are fed by regurgitation, the older larvae with pellets of crushed insects, and that, especially during their younger stages, the larvae are so assiduously fed and cared for because they furnish liquid exudates, small in quantity, to be sure, but of such a quality as to excite the appetite of their nurses and induce regurgitation. I believe that the salivary glands, as soon as they develop, take on the function of supplying exudates and at the same time aid in the extra-intestinal digestion of the food placed in the postcephalic pocket. That the salivary glands may be important as exudate organs throughout life is indicated by certain genera of Myrmicinae (e. g., Paedalgus ), the larvae of which have no exudatoria but greatly developed salivary glands, though the latter are never used for spinning cocoons in the prepupal stage. Thus in ants very much the same "oecotrophobiotic" relations exist between the adults and young as Roubaud1 has so beautifully described for the wasps of the genera Belonogaster, Ropalidia (= Icaria), and Polistes. To these relations, established by a mutual exchange of food-substances and which I have called "trophallactic," the social life of ants in all probability owes its origin, development, and maintenance. Moreover, the exudates of larval ants are strictly comparable with those of various castes of termites among themselves, of the queens of parasitic ants and even of workers (e. g., Crematogaster inflata of the East Indies), with the excrement of coccids and aphids, the secretions of lycaenid larvae and the nectar of the extrafloral nectaries of plants. Thus trophallaxis, myrmecophily, termitophily, trophobiosis, and the relations of ants with certain plants (myrmecophytes) are all seen to be merely so many particular manifestations of the same fundamental instinct of ants to foster and defend and, if possible, to feed and transport any small living object which can furnish droplets of agreeable secretion or exudates.

The only account of the aethiops larva in the literature is by Emery.2 He describes the adult larva very briefly and figures its anterior end with some of the exudatoria but erroneously interprets the large prothoracic pair as "ebauches de pattes," or rudiments of the anterior pair of imaginal legs.

In the same paper Emery created the subgenus Pachysima for the accommodation of aethiops and latifrons , because those species have the frontal carinae of the worker and female much more widely separated than the numerous other species of Tetraponera (= Sima). I have raised Pachysima to generic rank, because the larvae of the two species are so very different from those of Tetraponera .