_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 {162}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.