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4. Head
The head is composed of cranium, clypeus, antenna, eye (compound eye), ocellus and mouth parts. Fig. 1.3 shows the name of each part of the head. The features of the head are very important in identifying species and genera of ants.
Cranium: The posterior area of the cranium is called the occiput. The margin is often elevated (occipital collar). The antenna insertion point is sometimes covered with a protrusion called the frontal carina. In some genera, antennal grooves develop from the base of the antenna in the posterior direction. The triangular area surrounded by the clypeus and the frontal carina is called the frontal area. The median line, posterior to this area, often shows a groove called the frontal groove.
Clypeus: The clypeus is firmly connected to the cranium. Its border (epistomal suture) is sometimes unclear. The degree of clypeus development varies among genera. It is extremely small in the genus Odontomachus. The shape of the anterior margin of the clypeus is sometimes important when distinguishing among different genera.
Eye and ocellus: The eye (compound eye) is composed of several facets. The degree of eye development is variable. Some ants have an eye composed of only one facet. There are some ants which have no eye. The location and degree of development of eyes are taxonomically important. Workers often have no ocellus.
Antenna: The antenna of the worker and the female is usually composed of 12 segments, while that of the male is often composed of 13 segments. The number of segments is smaller in some genera. The segments of the antenna are numbered, beginning from the segment closest to the base. The first segment, which is called the scape, is relatively long and forms a knee-like joint with the other parts. The second and subsequent segments are collectively called the funiculus. The several segments at the tip of the antenna are often thicker and longer, sometimes forming a club. The number of antennal segments, the relative length of the scape, the presence or absence of club, and the number of segments constituting the club are important for distinction of genera.
Mouth parts: Mouth parts include the labrum, mandible, maxilla and labium. The features of mouth parts often used for classification include the shape of the mandible, the number of teeth, and the palp formula (PF, the number of constituent segments) of the maxillary palpus and labial palpus). The side of the mandible where the right and left portions occlude is called the masticatory margin, which is often equipped with teeth. The side of each tooth closer to the clypeus is called the base. The maxillary palpus and the labial palpus are easy to observe when the ventral plane is viewed from the head level. The PFs of maxillary and labial palpus are often expressed in combination. For example, PF = 6,4 means that the PF is 6 for the maxillary palpus and 4 for the labial palpus.
(Comments by Kazuo Ogata)