THE ORIGIN AND DEVELOPMENT OF THE HEMIPTERAN CRANIUM

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Abstract
A possible explanation of how the ancestral hemipteran head could have evolved from a basic pterygote plan is presented. It is proposed that the lateral cranial region descended ventrally as a parietal lobe, giving rise to the lamina maxillaris and maxillary plate, while the hypopharynx expanded laterally to form the loral plate. The maxillary and mandibular appendage anlagen produced only the stylets, and did not contribute to the head capsule. These stylets were first concealed in the atrial cavity between the parietal lobe and the hypopharynx; the cavity later closed laterally to form separate stylet sacs. The food and salivary pumps arose through the lateral closure of the cibarium and salivarium respectively. These changes brought about a closure of the external clefts between various parts of the cranium. In some modern Hemiptera the lines of fusion have been retained as sulci; in others they have disappeared. Although the homopteran cranium is very similar to the hypothetical ancestral head, the heteropteran cranium shows further modifications, such as the formation of a posterior cranial wall, the loss of the tentorium, and the obliteration of many external sulci. Both modern hemipteran orders show much variation in cranial structure.