Head

About half the head is a convex plate called the head capsule (image at right), carrying the antennae. The other half is made up of the mouthparts, which are joined to the capsule near the top of the head ('joint' in image).
 
From the side, what you can see of the mouthparts is the mandible, which is in three parts: cardo (plural cardines), stipes (plural stipetes) and gnathal lobe. The cardines+stipetes are the fat 'cheeks' on the head of a polydesmidan.
 
As illustrated at right, the head capsule is on top of the mouthparts, but as you can see from the image above, the capsule is normally anterior to the mouthparts.

Even though the head capsule is in one piece, parts of it are named. The top is the vertex. A shallow median groove called the vertigial sulcus runs from the back of the vertex to a point just above the antennae. The sulcus is not well-defined in some Polydesmida, and may be shorter than the one shown here.
 
Below the vertex is the frons ('front'), where the antennae attach. At the lower, front edge of the capsule is the labrum ('upper lip', see below) and between the frons and the labrum is the clypeus.

Each antenna emerges from a well-defined socket (left antenna removed in specimen at right). The spacing between the sockets (s) varies from species to species, and is usually measured with reference to socket diameter (d). In the species shown (Somethus tasmani, Paradoxosomatidae, Tasmania), the spacing is about 1X a socket diameter, but the spacing can be greater than 2X in many Polydesmida.
 
The postantennal groove lies lateral to the socket and its depth and width vary between species. Antennomere 2 rests in the groove when the antennae are folded back.

A 'top' view of the head (image at right). Some Polydesmida specialists use the word occiput for the part of the head where I have placed the label 'vertex'.

A 'bottom' view of the head (image at right), showing the flat, rectangular plate which closes off the mouth cavity. This plate is called the gnathochilarium. Its fused-together parts are the mentum (m), the paired gnathochilarial stipetes (gs) and the paired lingual lobes (l).
 
There is very little variation in the gnathochilarium across the Polydesmida. Differences between families and genera have mainly been reported in the other mouthpart, the mandible, where the cardo and stipes can be more or less swollen and may have 'keels'.

At right is the open mouth of a cleared specimen of Lissodesmus perporosus (Dalodesmidae, Tasmania). The mouth opening (o) has the labrum above, with its three small teeth (t), and the gnathochilarium below, with two palps (p) at the tip of each gnathochilarial stipes. On either side of the opening are the complex tips of the gnathal lobes of the mandible, with tooth plates (tp) and comb rows (cr).
 
There is even more structure to the gnathal lobe hidden inside the mouth. Most of a polydesmidan's chewing is done sideways by the mandibles, rather than up and down as in vertebrates.

In this specimen the mouth is closed. Without the labels, see if you can recognise the antennal socket; the postantennal groove; the cardo, stipes and gnathal lobes of the mandible; and the palps on the gnathochilarium. Note also that there are many setae on the frons, but that the clypeus is almost bare.
 
Shown: Noteremus infimus (family uncertain), Tasmania.