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.

The aim of the study is a review of the sexual generation of European species of the aphid genus Stomaphis Walker. Oviparous females and males of S. (Parastomaphis) longirostris and S. (Stomaphis) radicicola are described and figured in detail. Poorly known sexuales of S. (S.) bratislavensis, S. (S.) quercus, S. (S.) wojciechowskii, S. (P.) graffii, and S. (P.) juglandis are redescribed and figured in detail. Biometric data for the oviparous females and males are reported and keys for the sexual generation of the studied species are provided. On the basis of the morphological characters of the sexuales, supported by biological data, the taxonomic status of the subgenus Parastomaphis Pašek, 1953 stat. rev. is discussed. Aberrations of male morphology are presented, with reference to the general phenomenon of male morphology deterioration in the genus Stomaphis. The discussion also refers to the mating behavior and general evolutionary factors influencing the development of male dwarfism in Stomaphis.

The two species of the genus Stomaphis feeding on oak and birch, respectively, although morphologically similar, are considered to be separate species. However, the birch-feeding S. betulae Mamontova is considered to be a synonym of the oak and birch feeding S. quercus (L.) by some authors. The purpose of this study was to determine whether the birch feeding and oak feeding populations attributed to S. quercus belong to the same species. The mitochondrial cytochrome-c oxidase I (COXI) and II (COXII) were used to determine whether these two populations differ. There are no significant differences in these markers from oak and birch feeding individuals, indicating that these populations are conspecific. However, morphologically and ecologically distinct populations of Stomaphis were discovered feeding on oak. The molecular analysis confirmed that these populations are distinct, which resulted in the description of the new oak-feeding species, Stomaphis wojciechowskii Depa, sp. n. This new species previously remained unrecognized due to its very cryptic mode of life.