THE ANTERIOR TENTORIAL ARMS IN INSECTS AND THEIR SIGNIFICANCE IN INTERPRETING THE MORPHOLOGY OF THE CRANIUM OF THE CICADAS

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Abstract
The anterior tentorial pits in the orthopteran type of head lie in the ventral ends of the frontogenal sutures at their junction with the frontoclypeal and subgenal sutures, immediately dorsal to the anterior mandibular condyle and, with few exceptions, they always maintain this position. The base of the tentorial arms may expand vertically on the frontogenal ridge, horizontally on the frontoclypeal or subgenal ridge, or on both, but it always retains its connection with the end of the frontogenal ridge when this is present.When the genae descend ventrally, along the sides of the clypeus, the pits lie in the laterofacial sutures at the junction of their frontogenal and clypeogenal components. A transverse line drawn between the ventral ends of the pits always marks the boundary between the frons and clypeus.Laterofacial sutures border the dorsal region of the large median facial plate in the cicadas and the tentorial pits lie near the middle of the sutures. The region of the plate dorsal to the pits is therefore the frons, that ventral to the pits, the postclypeus.It is suggested that the cranium of the cicada is derived from one resembling that of the psocids by dorsoventral compression of the frontoparietal region and the postgenae. The mandibles have migrated inwards and the definitive form of the cranium is completed by the incorporation in it of the lateral walls of the hypopharynx and the basal plates of the maxillae.

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