Arthropod Head Problem - Head Components

Head Components

A typical insect head possesses a pair of antennae; eyes; mandibles, labrum, maxillae and labium (the latter four forming the cluster of "mouth parts", no. 32. in the diagram). Lying above the oesophagus is the brain or supraesophageal ganglion, divided into three pairs of ganglia: the protocerebrum, deutocerebrum and tritocerebrum from front to back (collectively no. 5 in the diagram). Nerves from the protocerebrum lead to the large compound eyes; from the deuterocerebrum to the antennae; and from the tritocerebrum to the labrum and stomatogastric nervous system. Circum-oesophageal connectives lead from the tritocerebrum around the gut to connect the brain to the ventral ganglionated nerve cord: nerves from the first three pairs of ganglia lead to the mandibles, maxillae and labium, respectively. The position of the mouth and the circum-oesophageal connectives allows a distinction to be made between pre- and post-oral structures; although it should be borne in mind that because structures can move around during development, a pre-oral position of a structure in the adult does not necessarily prove that its developmental origin is from there. The myriapod head is very similar to that of the insects.

The crustacean head is broadly similar to that of the insects, but possesses, in addition, a second pair of antennae that are innervated from the tritocerebrum. In place of the labium, crustaceans possess a second pair of maxillae.

Chelicerate head structures differ considerably from those of mandibulates (i.e. insects, crustaceans and myriapods); they possess eyes and a single pair of grasping appendages innervated from the brain, plus a labrum-like structure. Behind the mouth lies another pair of mouthparts, the pedipalps, and behind them lie the series of walking limbs. In chelicerates, the leg-bearing segments are fused with the anterior segments to form a prosoma, so that in living arthropods a distinct head only exists in mandibulates.