Fig. 78—Various forms of worker of Eciton hamatum. Guatemala.
The Dorylides are of great interest, for they exhibit the remarkable phenomenon of a nomadic social life, accompanied by imperfect sight in the wanderers. The sub-family includes two apparently distinct groups: (1) the Ecitonini, peculiar to the New World, and having a close relationship with the Myrmicides; and (2) the Dorylini existing chiefly in the eastern hemisphere, and related closely by its workers to the Ponerides and Amblyoponides. (i.) The Ecitonini consist of the species of the genus Eciton, the wandering ants of America, and of Labidus, which there is now good reason for believing to consist of the males of Eciton. The female is still uncertain. The Eciton are nomad ants having no fixed abode, but wandering from place to place in search of prey, and forming temporary resting-places. The species are rather numerous, and the habits of several have been described by Bates, who, however, was not acquainted with some of the most peculiar features in their biology, these having been since revealed by Belt and W. Müller.
These ants are predaceous in their habits, and some of the species travel in vast hordes; they occasionally enter houses and clear them of much of the vermin with which they may be infested. They have no facetted eyes, some of the forms being quite blind, while others have a pair of peculiar lenses in the position normally occupied by the compound eyes. Usually there are two castes of the workers, and in some species these are very different from one another, the mandibles being in the larger form very elongate, cylindrical and unfit for industrial purposes, while the individuals of the smaller caste have the outer jaws shorter, with their edges apposed and coadapted: in other species individuals with mandibles differentiated from the normal form do not exist. The nomad habits of these ants were described by Bates, but the detection of their temporary resting-places was reserved for Belt, who found that, after their plundering raids, they retired to a place of concealment, and there clustered together in a compact mass like a swarm of bees. Belt says: "They make their temporary habitations in hollow trees and sometimes underneath large fallen trunks that offer suitable hollows. A nest that I came across in the latter situation was open at one side. The ants were clustered together in a dense mass, like a great swarm of bees, hanging from the roof, but reaching to the ground below. Their innumerable long legs looked like brown threads binding together the mass, which must have been at least a cubic yard in bulk, and contained hundreds of thousands of individuals, although many columns were outside, some bringing in the pupae of ants, others the legs and dissected bodies of various Insects. I was surprised to see in this living nest tubular passages leading down to the centre of the mass, kept open, just as if it had been formed of inorganic materials. Down these holes the ants who were bringing in booty passed with their prey. I thrust a long stick down to the centre of the cluster and brought out clinging to it many ants holding larvae and pupae."
Turning now to the Labidus question: many American species of this genus have long been known, though all of them by the male sex only. The discoveries (to be subsequently alluded to) made in the Old World as to the relations between the driver ants and Dorylus raised a suspicion that Labidus might be the male of Eciton, the distinctions in the two cases being very analogous: this conjecture has been almost proved to be correct by the recent observations of Hetschko and W. Müller. The latter, who observed the temporary nests of Eciton hamatum, confirms Belt's statements as to the ants hanging together in clumps, like swarms of bees; he also states that the change from one temporary abode to another takes place at night, though, as is well known, the hunting forays of this ant are carried on in the daytime. The periods of migration appear to be determined by the time at which all the larvae have assumed the pupal state, this at any rate being the time chosen in the case observed by Müller. This naturalist bagged a part of one of the nests by the aid of ether, and found the larger portion to consist of pupae; there were also some larvae and eggs; a specimen of Labidus (L. burchelli) was also found on friendly terms with the Eciton-workers; and myrmecophilous Coleoptera were discovered. The pupae are enclosed in cocoons. Persistent search failed to reveal any female, but the examination was made under great difficulties. Müller also states that the earliest pupated larvae yield soldiers, the latest the smallest forms of workers. From observations made by Forel on a pupa, it seems probable that a wingless form of male may be found to exist. If therefore, as appears practically certain, Labidus is the winged male of Eciton, it is probable also that males of more or less worker-like form exist, as is now known to be the case in some other Formicidae.
We may here notice a peculiar apterous female ant recently described by André under the name of Pseudodicthadia incerta. He thought this might prove to be the female of Eciton-Labidus; but his description and figure are imperfect, and do not greatly support his idea of a connection between Eciton and Pseudodicthadia.
ii. The group Dorylini includes the genus Dorylus, which was founded many years ago for Insects very like Labidus. As in the case of the American Insect named, males only were known; two or three allied genera, consisting exclusively of individuals of the sex mentioned, were subsequently described. In the regions inhabited by these males numerous species of blind ants are known, but only in the worker form, and were, or still are, referred to genera called Typhlopone and Anomma. Nothing that could be considered to be a female pertaining to any of these Insects was discovered until Gerstaecker described under the generic name Dicthadia an extraordinary apterous female ant found in Java, and it was suspected that it might be the long-expected female of the male Dorylus and of the worker Typhlopone or Anomma. This remained for many years without confirmation, but in 1880 Trimen announced the discovery in South Africa of an enormous apterous female ant, allied to Dicthadia; it had been disinterred from a nest of small red ants believed (wrongly) to be Anomma. As Dorylus had been previously found in connection with allied worker ants it has since then been clear that notwithstanding the enormous differences existing between these three forms they may all pertain to one (or to closely allied) species. From this summary the student should understand that he will find in myrmecological literature many references to two or three genera that really belong to one species.
Fig. 79—Dorylus helvolus. Africa. A, male; B, female (Dicthadia); C, worker major (Typhlopone); D, worker minor. (After Emery.)
The workers of the Dorylini at present known are without exception quite blind, and are believed to be all of predaceous habits; it is thought by some that they have no fixed abodes, but, like the Ecitonini, frequently change their residence, and it has been suggested that in doing so they make use of the nests of other ants as temporary abodes; all these points are, however, still unsettled, and as there are several genera it is not unlikely that considerable variety will be found to prevail. The driver ants of Africa, belonging to the genus Anomma, are in some respects similar to Eciton in habits, as they enter human habitations and cause nearly everything else to quit; it is probable that they are also exclusively carnivorous. Savage detected the nests of A. arcens, but the account he has given of them is too vague to permit one to decide whether the assemblages he saw were of a nomad kind. The workers of this species vary greatly in size, and Emery has recently stated that he believes all the supposed species of the genus to be merely varieties of A. burmeisteri. The female of the driver ants is still quite unknown. A Dorylus has been ascertained to be the male of Typhlopone. The male Dorylus (Figs. 79, A, and 80) is of great interest, for the propodeum is in a more primitive form than it is in any other petiolate Hymenopteron known to us, while at the same time the pronotum and mesonotum are very highly developed. The genus Typhlatta Sm. has been recently identified by Wroughton and Forel as the worker-condition of which Aenictus is the winged male. The genus Alaopone will probably be found to have some species of Dorylus as its male.