Scanning electron microscopy and light microscopy were used to elucidate the morphology of labium and mandibular and maxillary stylets of the aphids Stomaphis quercus (L.) and S. graffii Cholodkovsky. The mechanism of labium shortening associated with feeding process was described as well. Stomaphis quercus and S. graffii have cone-shaped labium of 13 and 10 mm in length, respectively, that strongly extend behind the abdomen. The stylets bundle comprises a pair of mandibular and maxillary stylets which are on average as long as labium. Serial cross sections of labium revealed that the first segment is inverted inside and the second is pulled into it; both segments are shifted into abdomen. This study provides new information on S. quercus and S. graffii mouthparts that may help to understand their feeding behavior.
Keywords: Feeding; Labium; Mandibles; Maxillae; Morphology.