File:BSA3 01.jpg

FIGURES 1–3. Head capsule anatomy (diagrams based on Megalomyrmex longinoi). Abbreviations: at, anterior tentorial pit; ce, compound eye; fc, frontal carina; fr, frons; gn, gena; hm, hypostomal margin; hs, hypostoma; lo, lateral ocellus; ma, malar area; mo, median ocellus; occ, occipital carina; ocp, occiput; of, occipital foramen; po, postocciput; pgb, postgenal bridge; pgs, postgenal suture; pt, posterior tentorial pit; tl, torular lobe; tm, temple; vx, vertex. 1. Full-face view. 2. Profile of head capsule oriented in full-face view. 3. Oblique ventral view.

Head. To increase descriptive precision in this and future studies, and to accurately reflect the homology of head capsule elements, we carefully apply the following cranial terminology (figs. 1–3). The frons is the area posterior to the anterior tentorial pits, and which is delimited laterally by the medial margin of the compound eyes, and posteriorly by the anterior margin of the median ocellus (an educated guess for the median ocellus location will have to be made for workers). The vertex is the cranial region between the anterior margin of the median ocellus and the occipital carina, and which is delimited laterally by a line drawn from the medial margin of the compound eye to the occipital carina. The gena is the lateral area of the head capsule ventral to the medial margin of the compound eye and is ventrally delimited by an imaginary line from the apex of the occipital carina to the hypostomal margin. The gena is further divided into the anterior malar area—between the anterior eye margin and the mandibular insertion—and the temple, which is the area posterad the compound eye. The occiput is the area surrounding the raised postocciput and is delimited by the occipital carina. Although the region of the occiput ventrad of the postocciput is called the postgena, the occiput is not anteroventrally closed by the occipital carina in many ants and the use of postgena is thus ambiguous. These head regions are distinct in queens and males, but the posterior frons and anterior vertexal margins are ambiguous in workers which lack ocelli, although in this case without reference to reproductives the vertex can be assumed to begin at slightly before midlength between the posterior compound eye margins and posterior margin of the head.

Confusion exists in the literature about the ventral sclerotization of the Formicidae head capsule. Gauld & Bolton (1988) state that the ventral surface is formed by the fusion of the genae forming a genal bridge, while others suggest the structure is the subgenal bridge (Gotwald 1969; Ronquist et al. 1999; Vilhelmsen 2009, 2011; Serna & Mackay 2010). Until the homology of this region among ants and other Apocrita is explicitly clarified, we use postgenal bridge in deference to broader studies of hymenopteran cranial morphology (Rasnitsyn 1988; Ronquist et al. 1999; Mikó et al. 2007; Vilhelmsen 2009, 2011). It should be noted that Serna & Mackay (2010) erroneously labeled the postgenal suture (the ventromedian longitudinal cranial suture) as the “genal bridge”, and their interpretation of tripartite bridge composition contradicts the more general interpretations for the Formicidae (Gauld & Bolton 1988; Rasnitsyn 1988; Ronquist et al. 1999).

Full-face view is achieved when the head capsule is oriented such that the anterior clypeal margin and posterior cranial margin are in the same plane of focus, regardless of whether the posterior margin is composed of the occipital carina or vertex. We define the head dorsum as the portion of the head capsule visible in full-face view, which is an arbitrary combination of the frons, vertex, and gena. For the antenna, we use the following terms: scape for the first antennomere; funiculus for the antennomeres distad the scape (i.e. antennomeres 2–11 or 2–13); and pedicel for the second antennomere.

Boudinot, B.E. Sumnicht, T.P. and Adams, R.M.M. 2013. Central American ants of the genus Megalomyrmex Forel (Hymenoptera: Formicidae): six new species and keys to workers and males. Zootaxa 3732:1-82.