Fig. 67—Pheidologeton laboriosus, large and small workers. East India.
i. The Myrmicini proper are defined by Forel as having the antennae inserted near the middle, a little behind the front, of the head, which has carinae on the inner sides, but none on the outer sides, of the insertions of the antennae; the clypeus extends between the antennae.
Fig. 68—Formicoxenus nitidulus, male. (After Adlerz.)
Certain genera of small European ants of the group Myrmicini display some most anomalous phenomena. This is especially the case in Formicoxenus, Anergates and Tomognathus. The facts known have, however, been most of them only recently discovered, and some obscurity still exists as to many of even the more important points in these extraordinary life-histories.
Fig. 69.—Anergates atratulus. Europe. A, male, with part of hind leg broken off; B, female, with wings: C, female, after casting the wings and becoming a queen.
It has long been known that the little Formicoxenus nitidulus lives as a guest in the nests of Formica rufa, the wood-ant; and another similar ant, Stenamma westwoodi, which shares the same life, was declared by Nylander and Smith to be its male; it was however shown some years ago by André that this is a mistake, and that S. westwoodi is really the male of another ant that had till then been called Asemorhoptrum lippulum. This correction left the workers and females of Formicoxenus nitidulus destitute of a male, but Adlerz has recently discovered that the male of this species is wingless and similar to the worker, the female being a winged Insect as usual. It is very curious that the characters by which the male is distinguished from the worker should vary in this species; but according to Adlerz this is the case, individuals intermediate in several points between the males and workers having been discovered. This phenomenon of quite wingless males in species where the female is winged is most exceptional, and is extremely rare in Insects; but it occurs, as we shall see, in one or two other Myrmicides. Charles Darwin made the very reasonable suggestion that winged males may be developed occasionally as an exceptional phenomenon, and it is very probable that this may be the case, though it has not yet been demonstrated. Formicoxenus nitidulus occurs in England in the nests of Formica rufa and of F. congerens, but we are not aware that the male has ever been found in this country. The genus Anergates is allied to Formicoxenus, and occurs in Central Europe, but has not been found in Britain; the female, as in Formicoxenus, is winged and the male wingless, but there is no worker-caste; the male is a rather helpless creature, and incapable of leaving the nest. The species lives in company with Tetramorium caespitum a little ant very like Myrmica, and not uncommon in South-East England. The female Anergates is at first an active little creature with wings, but after these are lost the body of the Insect becomes extremely distended as shown in Fig. 69, C; the creature is in this state entirely helpless, and as there are no workers, the Anergates is completely dependent, for the existence of itself and its larvae, on the friendly offices of the Tetramorium that lives with it. The mode of the association of these two Insects is at present both unparalleled and inexplicable, for only workers of the Tetramorium are found in company with the ♂ and ♀ Anergates; the community, in fact, consisting of males and females of one species and workers of another. The nests of Anergates are so rare that only a few naturalists have been able to observe them (Schenk, von Hagens, and Forel may be specially mentioned), but in the spots where they occur, nests of the Tetramorium, containing all the forms of that species, are numerous, and it therefore seems probable that a young fertile female of the Anergates may leave a nest in which it was born, enter a nest of the Tetramorium, and, destroying the queen thereof, substitute herself in the place of the victim; but if this be really the case, the larvae and pupae of the Tetramorium must also be destroyed, for no young of the Tetramorium are ever found in these strange associations. It is very difficult to believe that the Tetramorium workers should be willing to accept as their queen a creature that commenced her acquaintance with them by destroying their own queen or queens and a number of their young sisters; especially as the Tetramorium is a more powerful ant than the Anergates, and could readily dispose of the murderous intruder if it were disposed to do so. It is known, however, that colonies of Tetramorium completely destitute of queens sometimes occur, and Wasmann has suggested that the female Anergates may seek out one of these, and installing herself therein as a substitute, may be accepted by the orphaned colony. This plausible hypothesis has still to be verified.
The genus Cardiocondyla also exhibits the phenomenon of apterous, worker-like males, while in one species, C. emeryi, a winged male is also known to exist.
Tomognathus sublaevis is a little Myrmicid ant, found rarely in Denmark and Sweden, where its habits have recently been studied by Adlerz. A band of the Tomognathus attack the nest of another little Myrmicid, Leptothorax acervorum, and succeed by their own pertinacity and the fears of the Leptothorax in obtaining possession of it; the legitimate owners disappear, leaving the Tomognathus in possession of their larvae and pupae; these complete their development only to find themselves the slaves of Tomognathus. The subsequent relations of the two ants are friendly, the slaves even preventing their masters from wandering from the nest when they wish to do so. If an established mixed community of this nature is in want of additional servitors, the Tomognathus secure a supply by raids after the fashion of the Amazon-ant, bringing back to their abode larvae and pupae of Leptothorax to be developed as slaves. It was formerly supposed that the Tomognathus continued its species by perpetual parthenogenesis of the workers, for neither males nor females could be found. Adlerz[[68]] has, however, now discovered the sexual individuals. The male is an ordinary winged ant, and is so like that sex of the Leptothorax, that Adlerz had failed to distinguish the two before he reared them. The females are apterous, and in fact like the workers. It would perhaps be more correct to say that the workers of this species vary greatly but never become winged; some of them have ocelli and a structure of the thorax more or less similar to that of winged females, though none have been found with wings. Certain of these females possess a receptaculum seminis, and Adlerz treats this as the true distinction between female and worker. In accordance with this view the female of Tomognathus may be described as a worker-like individual possessing a receptaculum seminis, and having more or less of the external structures of winged females, though never being actually winged. It is probable that other workers reproduce parthenogenetically. The males of this species will not unite with females from the same nest, thus differing from many other ants, in which union between individuals of the same nest is the rule. Finally, to complete this curious history, we should remark that the larvae of the Tomognathus are so similar to those of the Leptothorax that Adlerz is quite unable to distinguish the two.