The tritocerebrum and the clypeolabrum in mandibulate arthropods: segmental interpretations

AbstractBitsch, J. and Bitsch, C. 2010. The tritocerebrum and the clypeolabrum in mandibulate arthropods: segmental interpretations. —Acta Zoologica (Stockholm) 91: 249–266Different interpretations of the segmental composition of the head in mandibulate arthropods are critically reviewed, with particular focus on three closely associated structures: the tritocerebrum, the stomatogastric nervous system and the clypeolabrum. The main conclusions arising from the different discussions are the following. (1) Each tritocerebral ganglion has a dual composition, clearly discernable in some crustacean and hexapod species, including a dorsal portion connected with the second antennae and a ventral portion connected with the stomatogastric nervous system via the frontal ganglion. (2) The suboesophageal commissure linking the tritocerebral lobes of the two sides, can be wholly ascribed to the tritocerebral segment. (3) The stomatogastric nervous system is a morphologically autonomous system that is not fundamentally affected by head metamerization. (4) The clypeolabrum, the epistome–labrum and the hypostome are regarded as homologous formations. The clypeolabrum represents a fundamental structure of the head probably present in the arthropod ground plan. Its close spatial and developmental association with the stomodeum and its derivative, the stomatogastric nervous system, suggests that it is an anterior outgrowth of the forehead arising from a preoral territory (presegmental acron or protocerebral–ocular region?) and secondarily connected with the tritocerebrum, rather than derived from a pair of reduced appendages.