Larval Mouthparts and Feeding Mechanism of the Hessian Fly (Diptera: Cecidomyiidae)

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Abstract
The morphology of the larval mouthparts of the Hessian fly, Mayetiola destructor (Say), was studied using electron microscopy to explain the feeding mechanism. Major structures, such as the clypeus-labrum, paired mandibles and maxillae, and labium, were identified. The clypeus-labrum appears to be a composite structure bearing two, prominent, dorsomedial pits. Mandibles of the first instar are bladelike, tapering distally to a single, sharp-pointed tooth, and appear to be adapted for piercing plant tissue. Each tooth is deeply grooved and contains a minute hole at its base that connects to a canallike passage extending longitudinally through the mandible. Ultrastructure of the mandible suggests that the canallike passage may function in conveying salivary secretions. The mandible of the second instar terminates in a foreshortened tooth, broadly rounded and flared at the apex. The maxilla comprises three distinct plates: a large median outer plate, bearing a crescentshaped palp; a small dorsal inner plate bearing a minute, one-segmented papilla; and a small ventral plate. The labium is well defined, tapers dorsally to a central sclerite, and widens ventrally into a thick, deltoid structure bearing four minute papillae. A functional model of the first instar's feeding mechanism is proposed based on the hypothesis that salivary fluids are injected into the plant via highly specialized mandibles and that these substances react with plant cell components and cause physiological alterations in the plant, which allow the larva to suck liquids through the functional mouth.