An ant's head is its most distinctive feature. Located on the ants head is most of the organs responsible for sensory perception, as well as the ants primary means of interacting with its environment. Due to the highly precise nature of these tasks and the highly varying natural histories of different species, the head of one ant can look exceedingly different from that of another. Accordingly, the ant head is quite often most telling for a taxonomist.
The ant head is composed of three major parts, the eyes, the antennae, and the mouth parts, all of which are contained in or attached to the cranium.
The ants that have eyes generally have two varieties. The first is the compound eyes, which is situated laterally, and are often visible to our naked eye. The compound eye is composed of many ommatidia, which are in turn composed of several layers. These layers include, from the external to internal, Cornea which serves as the lens, the Coneagen Layer, which is a single layer of cells responsible for excreting the cornea, the retina which is responsible for converting the light stimulus to nervous impulse, and lastly the central nervous connections. The number of ommatidia that an ant may possess in a single lateral eye can be as great as 1,200 in the male of Formica pratensis, to 1 in the eye of workers of Ponera punctatissima, to zero in the worker of the African driver ant. The second type of eye is the ocelli, or simple eye. Most ants have three such eyes on the vertex of the cranium, placed in a triangle. The ocelli are of a different construction from the compound eyes, a fact that is thought by some to indicate that ocelli are ancient structures, evolved long before compound eyes. Ants that have no compound eyes generally do not have ocelli either.
The antennae are by far the ant's most important sensory input devices. They are multi-jointed, with the first segment comprising the scape, while the remaining segments comprise the funiculas. The first segment, the scape, is elongated in females. The multiple joints of the funiculas can be uniform in size, however the terminal joint is often elongated and club shaped. Under magnification one can see that sensory apparatus are located on the last two segments. These apparatus are responsible for both tactile and olfactory perception. It is for tactile perception that the last two segments of the antennae are covered in hairs. Also visible on the antennae are the four olfactory receptors: the Clubs of Fovel, the Clubs Lining the Elliptical Pits, the Champagne Cork Organs of Fovel, and the Flask Shaped Organs of Lubbock and Fovel. The Clubs of Fovel are slight pits which contain hairs separated by a very thin chitinous layer from their accompanying nerve cell. This nerve cell is very sensitive, and can be triggered by as little as a single pheromone molecule. The Clubs of Fovel also secret a substance that is in function analogous to the saliva that coats our tongues. Without the mucus analogue the ant could not 'smell.' The other three olfactory receptors are similar to the Clubs of Fovel, with structural modifications like the internalization of the hair and the deepening of the pit.
The ants mouth can serve a tremendous number of purposes. Some of the extraordinary adaptations of the mouth include hunting and fighting, weaving, transport (many ants can transport water by holding single droplets in there mandibles using static pressure), and, in one extremely interesting case, for propulsion by a snapping of the mandibles at a very rapid pace against a solid object. The mouth is composed of the labrum, the mandibles, the maxillae, the labium, and the hypopharynx. The labrum functions as the ants upper lip, and is considered to be vestigial. The mandibles are the part most of us are familiar with, the working part of the mouth used for grasping. The mandibles of an ant, like all insects, open horizontally, however an ant's mandibles can open and close independently of the rest of the mouth parts, an adaptation rare in the insects. The mandibles take on many diverse shapes, and are the most specialized structure of the ant's morphology. The fastest moving biological structure known to man is the mandibles of the Odontomacus, which spring shut at an amazing 8.5 meters a second! The mandibles of an ant are similar to our hands; it is their means of interaction with the external world. The maxillae are a pair of acessory jaws. The are modified limbs, and appear as such. The labium are similar to our lower lip. The hypopharynx is the ant's tongue, and acts very much as ours does.