The myth of intercalary segment in insect head.

Abstract

Embryonic development of the head of Oxyrhachis tarandus (Membracidae) has been investigated in detail to settle the controversy of head segmentation and to refute the occurrence of an intercalary segment. The head is formed from six distinct elements: the prostominal lobe, the paired cephalic lobes, the antennal segment and the three noncontroversial gnathal segments. The prostomial lobe, which possesses a neuromere and a pair of coelomic cavities, represents the first body segment, called the prostomial segment. The tritocerebral lobes of the brain and the stomatogastric nervous system, consisting of a frontal ganglion, clypeolabral nerves, and the recurrent nerve etc., develop from the neuromere of the prostomial lobe. The tritocerebrum thus belongs to the prostomial segment rather than to an imaginary intercalary segment and mainly represents the ganglionic center of the stomatogastric nervous system in the brain. Frons, clypeus, and labrum develop from the outer wall of the prostomial lobulate plate, whereas the epipharyngeal wall, including the cibarial pump, develops from its inner wall. The presence of three coelomic cavities and of three distinct neural masses in the cephalic lobes during the initial stages of development shows that they have developed by the fusion of three distinct segments during the long phylogenetic history of insects. The portion of the germ band presently considered as the intercalary segment is actually the sternal part of the antennal segment. The neural cells located in this region give rise to the deutocerebrum by shifting forward, around the stomodaeum, and always leaving a commissure behind. The intercalary segment is thus a complete illusion. The antennal segment is postoral in the beginning and bears a pair of coelomic cavities, but later on it shifts forward and its sternal part invaginates into the stomodaeum.