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Ant thorax

Ant thoraxOwing to the fusion of the first abdominal segment of the embryo and larva with the hindermost portion of the thorax during pupation, the thorax of the adult ant may be said to consist of four segments, a pro-, meso- and meta-thoracic and a mediary segment, or epinotum. In our description we may follow Emery who has studied the external morphology and reviewed the nomenclature of these four segments in the male, female and worker. The primitive condition of the thoracic region may be readily traced through the ergatoid females and workers of these forms to the much reduced and specialized condition in the workers of more highly developed ants like the Camponotinae.

Emery starts with a primitive form like the male Styeblognathus aethiopicus. In this insect the various elements or sclerites of which the thorax is composed are clearly delimited by sutures. The prothorax is very small and consists dorsally and laterally almost entirely of the unpaired pronotum, with a slender ventral element, the prosternum, to which the coxa of the fore-leg is articulated. Owing to the development of the wings, the mesu­ and metathorax are much larger.

The former is especially well-developed, in correlation with the larger size of the fore wings, and comprises dorsally a large unpaired, convex plate, the mesonotum; ventrally on each side, and articulated below with the coxa of the middle leg, is the mesosternum, which also forms much of the pleural wall of the thorax. The space on each side between the mesonotum and the mesosternum is occupied by a pair of elements, one of which, the mesepisternum, is ventral; the other, the mesepimeron, dorsal.

The fore-wing is articulated just above the mesepimerun and below a small sclerite, which is behind the mesonotum and may be called the mesoparapteron, or praescutellum. The insertion of the forewing is covered by a small chitinous scale, the tegula. Viewed from above the large mesonotum in some male ants presents a Y-shaped groove, known as the Mayrian furrow. Each side of the mesonotum is marked off for some distance from the median portion of the segment by a distinct suture, which may be called the parapsidal suture. The area thus cut off on each side is the parapsis. The sides and the ventral portions of the metathoracic segment are similar to those of the mesothorax, but smaller.

It is possible to distinguish a metasternum, to which the coxa of the hind-leg is articulated, a metepisternum and a metepimeron. Dorsally, however, the metanotum, which, of course, is serially homologous with the mesonotum, is very narrow antero-posteriorly and separated from the mesonotum by a large, unpaired, semi-circular element, the scutellum. Between the scutellum and metanotum, a small piece, the metaparapteron, or post­scutellum, is intercalated on each side. The hind-wing is inserted between this metaparapteron and the metepimeron. The epinotum, which, as we have seen, is morphologically the first abdominal segment, is large and convex and in many ants furnished with a pair of stout spines or teeth. It is closely applied to the metathorax from the posterior edge of the mesonotum above to the ventral edge of the meta­thorax below.

The thorax has on each side three openings, or stigmata, to the respiratory tubes, or tracheae. The first, belonging morphologically to the mesothorax, lies beneath a small flap-like expansion of the pronotum where it abuts on the mesepimeron. The second or meta­thoracic stigmata lies beneath the insertion of the hind-wing and near the posterior end of the mesepimeron. The third stigma, belonging to the first abdominal segment, is distinctly seen on the side of the epinotum.

In the female ant the thorax is constructed on the same plan as that of the male, but is more robust and lacks the Mayrian furrow, which is also absent in the males of many genera. The males and females of most species, however, exhibit a greater simplification, of the pleural region of the thorax, owing to the fusion of the epimera and episterna with each other and often also with the sterna in the meso- and metathorax, and a very intimate fusion of the epinotum with the latter segment.

Turning to the workers, which are wingless, there is noticeable a great reduction in the size of the meso- and metathorax plus the epinotum, so that the three divisions of the thorax are more nearly of uniform size. In certain species, and especially in the ergatoid females and soldiers of a few genera, the various dorsal elements, such as the paraptera, scutellum and metanotum may still be recognized as very small sclerites, but in the workers of the highest and most specialized ants of the genera Formica and Camponotus the thorax appears to consist of three similar segments, owing to the disappearance of the scutellum, paraptera and metanotum as separate sclerites and to the fusion of the various elements in the pleural region of each segment.

The legs of the ant show much less variation in structure than the thorax and are, therefore, of less taxonomic value. Each of these appendages consists of the same fixed number of joints, the coxa, trochanter, femur, tibia, and five tarsal joints.

The first tarsal joint, often called the metatarsus, is much elongated, especially on the middle- and hind-legs, where it functions as a kind of secondary tibia, while the terminal tarsal joint bears a pair of usually simple, but sometimes toothed, or pectinated claws. All of the tibiae may be provided at their distal ends with spurs. These are always large and pectinated on the fore-legs, but may be simple on the middle and hind pairs. The finely and regularly pectinated spur of the fore tibia is of special interest on account of its beautiful structure and its function as a strigil.
It is movable and curved and its concavity is opposite a similar concavity, fringed with bristles, on the base of the metatarsus. The ant draws its antennae and posterior legs between the two opposed, pectinated surfaces and thus wipes off any adhering foreign matter.