CHAPTER II.

THE EXTERNAL STRUCTURE OF ANTS.

Διὸ δεῖ μὴ δυσχεραίνειν παιδικῶς τὴν περὶ τῶν ἀτιμοτέρων ζώων ἐπίσκεψιν. Ἐν πᾶσι γὰρ τοῖς φυσικοῖς ἔνεστί τι θαυμαστόν· καὶ καθάπερ Ηράκλειτος λέγεται πρὸς τοὺς ξένους εἰπεῖν τους βουλομένους αὐτῷ ἐντυχεῖν, οἱ ἐπειδὴ προσιόντες εἶδον αὐτὸν θερόμενον πρὸς τῷ ἴπνῳ, ἔστησαν· ἐκέλευσε γὰρ ἀυτους εἰσιέναι θαῤῥούντας· εἶναι γὰρ καὶ ἐνταῦθα θεούς· οὕτω και πρὸς τὴν ζήτησιν περὶ ἑκάστου τῶν ζώων προσιέναι δεῖ μὴ δυσωπούμενον, ὡς ἐν ἅπασιν ὄντος φυσικοῦ καὶ καλοῦ.

"Wherefore we ought not childishly to neglect the study even of the most despised animals, for in all natural objects there lies something marvellous. And as it is related of Heraclitus that certain strangers who came to visit him, when they found him warming himself at the kitchen fire, stopped short-he bade them enter without fear, for there also were the gods: so we ought to enter without false shame on the examination of all living beings, for in all of them resides something of nature and beauty."-Aristotle, "De Partibus Animalium,” 1, 5.

The ants form a natural family (Formicida), or, according to some authorities, a superfamily (Formicina or Formicoidea), comprising five subfamilies (Ponerinæ, Dorylinæ, Myrmicinæ, Dolichoderinæ and Camponotinæ ), embracing about 5,000 described species, subspecies and varieties, and are placed at the head of the order Hymenoptera, a vast assemblage of insects including also the bees, wasps, ichneumon flies, velvet-ants, saw-flies and many smaller groups. From all the other members of the order the ants may be readily distinguished by a series of characters, perhaps the most striking of which is the differentiation of the abdomen into two strongly marked regions, a slender one- or two-jointed, highly mobile pedicel, and a larger, more compact terminal portion, the gaster. Another distinguishing character is furnished by the antennæ which are elbowed and have the first joint greatly elongated in the female. The species are all social, and with the exception of a few parasitic forms, are always at least trimorphic, i. e., the female is not only sharply differentiated from the male, but itself appears under two very distinct phases, a fertile, queen, or female phase proper, and a usually sterile worker phase. The former is nearly always winged like the male, but loses the wings after fecundation, the latter, except in rare abnormalities, never bears these organs. In a few species the females, and in many the workers may again show differentiation into two sub-phases (Fig. 1). Owing to this remarkable morphological instability or tendency of the female to assume different

aspects, the Formicidæ have also been called Heterogyna. All of the species have retained in their development the four salient stages known as the egg, larval, pupal and imaginal instars, which are peculiar to all holometabolic insects. These stages and their relations to the polymorphism of ants will be considered in subsequent chapters. In this and the two following chapters I shall endeavor to give a rapid survey

of the external and internal anatomy, as these have become known to us through the careful researches of a number of investigators, notably Adlerz, Berlese, Dewitz, Emery, Forel, Janet, Lubbock, Meinert and Nassonow.

The Segmentation of the Body.-There can be little doubt that the ants are phylogenetically related, through the lower families of Hymenoptera with the oldest and most primitive of all the existing insects, the Blattoidea, or cock-roaches. But while the Blattoid body. as seen, for example, in the common cock-roaches, is generalized, that of the ants in its sharp demarcation of the head, thorax and abdomen is highly specialized. These accentuated subdivisions enable anyone to recognize an ant at a glance. typical of insects, and may be the insectum were originally applied. acters make it seem a far call from the ant to its remote Blattoid ancestors, it must be borne in mind that the individual ant still passes

In this respect the ants are the most ones to which the terms Evтoμov and While these and many other char

in its embryonic development through a stage in which the body consists of twenty like, or homonomous segments. Six of these belong morphologically to the head, three to the thorax and the remaining eleven to the abdomen. The first and third segments bear no appendages, the second bears the antennæ, the three thoracic segments bear the three pairs of legs, and the second and third of these segments in the males and females develop, at a much later stage, the two pairs of wings. The first abdominal, which has long been known as the mediary segment, becomes fused with the hind portion of the third thoracic segment during pupal development, as Janet and Emery have demonstrated, and becomes the epinotum of the latter author. The pedicel consists of the second abdominal segment, or of this and the third segment, while the remaining seven or eight form the gaster.

The Integument.-The chitinous investment, or integument varies greatly in thickness in the different species of ants, being very hard and brittle in many of the more primitive groups (Ponerinæ, Myrmicinæ, Dolichoderus, Polyrhachis) and thinner and more pliable in the more recently developed forms (most Dolichoderinæ and Camponotinæ). The microscopic character of the integument is of considerable importance to the taxonomist, especially in the more delicate discrimination of geographical subspecies and varieties and may be considered under the captions of sculpture, pilosity, pubescence and color. These all present a bewildering variety of modifications. In some ants the surface of the body is very glabrous and shining, in others opaque, punctate, foveolate, rugulose, rugose, tuberculate, striate or reticulate, and these sculptural characters may be combined in the most diverse patterns.

The term pilosity applies to the longer, reclinate, erect or suberect hairs, the term pubescence to the minute, appressed tomentum, which may cover the whole or portions of the body and appendages. Both the hairs and pubescence vary greatly in length and density or abundance, and the former may be tapering and pointed, straight, flexuous, or hooked, obtuse or clavate, or dilated and flattened to form scales.

No doubt all of these differentiations in sculpture, pilosity and pubescence are correlated with the delicate tactile sense of the ants. Certainly one who has examined many species of ants will have no difficulty in understanding why these blind or nearly blind insects seem to display such keen delight in palpating with their antennæ and burnishing with their tongues the exquisitely chased or chiselled armor of their fellows. In some ants the hairs may be specialized for particular functions on certain portions of the body. I find this to be the case, for example, in several genera of desert ants (Fig. 2), which have the

hairs on the lower surface of the head greatly elongated and directed forward (Pogonomyrmex, Ocymyrmex, Cratomyrmex, Messor, Goniomma, Oxyopomyrmex, Holcomyrmex), or arranged in a tuft on the lower lip (Myrmecocystus, Melophorus). These hairs, which I have called gular and mental ammochætæ (1907), are employed by the ants in removing the dust and sand from the strigils or combs on the fore

legs (vide infra, p. 24). In deserts these insects easily become covered with the dry soil or sand and have to remove it from their bodies and limbs by means of the strigils. These organs are then thrust along the ammochætæ in much the same way as we clean a comb by means of threads. The clypeus and mandibles of many ants are also fringed with unusually long hairs (clypeal and mandibular ammochætæ)

which are employed in re

moving the dust, etc., from the surfaces of the fore-legs.

The colors of ants are, as a rule, testaceous, yellow, brown, red, or black, but a few genera (Rhytidoponera, Calomyrmex, Macromischa, Iridomyrmex) and a few North American species of Pheidole (metallescens and splendidula) have metallic colors. The non-metallic tints are often highly variable, even within the limits of single species. Color patterns are rarely developed and are usually found only on the upper surface of the gaster, a region which often differs in color from the head and thorax. The appendages, as in other insects, are apt to be paler than the trunk. The coloration of the hairs and pubescence, like that of the surface, may be extremely variable in the same species. To the integument belong also a number of glands, but these will be described in connection with the glands of the internal organs.

The Head. After this very general review of the segmentation and. integument we may take up the different parts of the body in somewhat. greater detail. The head varies enormously in shape. It may be circular, elliptical, rectangular or triangular, and all its parts may show an extraordinary diversity of adaptive characters (Fig. 3). It consists of the cranium proper, which is very much constricted behind at its

articulation with the thorax, the eyes, the clypeus, or epistoma, a plate of very variable outline and immovably articulated with and set into the anterior portion of the cranium, the antennæ, and the mouth-parts,

FIG. 3. Heads of various ants. (Original.) A, Mystrium rogeri, worker; B, Myrmecia gulosa, worker; C, Eciton hamatum, soldier; D, Harpegnathus cruentatus, female; E, Daceton armigerum, worker; F, Leptomyrmex erythrocephalus, worker; G, Cheliomyrmex nortoni, soldier; H, Pheidole lamia, soldier; I, Thaumatomyrmex mutilatus, worker; K, Odontomachus hæmatodes, worker; L. Cryptocerus clypeatus, soldier; M, Cryptocerus varians, soldier; N, Opisthopsis respiciens, worker; O, Leptogenys maxillosus, worker; P, Azteca sericea, soldier; Q, Acromyrmex octospinosus, worker; R, Dolichoderus attelaboides, worker; S, Colobopsis impressa, soldier; T, Camponotus cognatus, soldier; U, Camponotus mirabilis, female.