Most of the material was collected during a faunistic survey in Bulgaria using sweep netting (Pramatarova et al. 2025). Additional specimens from Bulgaria, Czechia and Slovakia were examined from the collections of the Moravian Museum, Brno, Czechia. The majority of specimens were dry-mounted, and some were preserved in absolute ethanol. This material is deposited in the Zoological Collection of the Sofia University (BFUS), Sofia, Bulgaria (Table 1). Species identifications were based primarily on Loginova (1963, 1974), Conci and Tamanini (1983), Ossiannilsson (1992), Burckhardt and Lauterer (1989, 1997), Lauterer and Burckhardt (2004) and Rodrigo-Gómez and Burckhardt (2023). Additional slide-mounted specimens were examined from the collections of the Muséum d’histoire naturelle, Genève (MHNG) and the Naturhistorisches Museum, Basel (NHMB).

General morphological terminology follows Bastin et al. (2023), and terminology of surface sculpturing follows Harris (1979). Psyllid taxa are arranged alphabetically following the classification of Burckhardt et al. (2021), while nomenclature and host-plant information follow Ouvrard (2020).

For SEM, specimens lacking wax coverage were mounted on aluminum stubs using double-sided adhesive carbon tape. Approximately two thirds of the specimens were sputter-coated with a conductive gold layer using an Emitech K500 Cool Sputtering System (Emitech Ltd., Ashford, Kent, UK), while the remainder were examined uncoated (Table 1). No substantial differences in image quality were observed between coated and uncoated specimens.

SEM imaging was performed at the Laboratory of Materials Technology and Ellipsometry, Faculty of Physics, Sofia University, using a Tescan Lyra I XMU field emission scanning electron microscope (FE-SEM) (Tescan Orsay Holding, Brno, Czech Republic). The microscope was operated at accelerating voltages of 10–20 kV with secondary electron (SE) or back-scattered electrons (BSE) detector and working distances of 9.8–30 mm.

For each species, the head and thorax of one specimen (sex indicated in Table 1) were examined in dorsal view, as the selected characters are not known to be sexually dimorphic or to exhibit high intraspecific variability. Microsculpture is illustrated by an overview of the head and thorax and by a magnified area (0.1 × 0.1 mm) of the vertex. Whenever possible, this area was selected on the left side of the coronal suture and extended anteriorly towards the median ocellus (Fig. 1).

Phylogenetic relationships were inferred using the molecular dataset of Pramatarova et al. (2024), from which only the subset of 24 species relevant to the present microsculpture study was reanalysed de novo under the maximum likelihood (ML) criterion. The dataset comprised concatenated sequences of two mitochondrial gene fragments, cytochrome c oxidase I (COI) and cytochrome b (cyt b). The alignment was taken directly from Pramatarova et al. (2024). No partitioning scheme was applied, and the matrix was analysed as a single partition. Psyllopsis fraxini (Linnaeus, 1758) (Liviidae: Euphyllurinae) was included as the outgroup to root the molecular phylogeny.

Maximum likelihood analysis was perfomed using IQ-TREE v.1.6.12 with default settings on the IQ-TREE web server (Trifinopoulos et al. 2016). The best-fitting substitution model was selected automatically using the built-in ModelFinder under default settings (Kalyaanamoorthy et al. 2017). Branch support was assessed with 1,000 ultrafast bootstrap replicates (UFBoot; Hoang et al. 2018). The best-scoring ML tree was visualised and exported in Nexus format using iTOL v.5 (Letunic and Bork 2021) and used for subsequent character mapping and ancestral state reconstruction.

To investigate evolutionary patterns of vertex microsculpture, character states of terminal taxa were mapped onto the molecular phylogeny and ancestral states were reconstructed using ML under the Mk1 (equal-rates) model in Mesquite v.4.1 (Maddison and Maddison 2025). Reconstructions were performed using the original branch lengths of the molecular tree, and character states were treated as unordered. Node likelihoods were visualised as proportional likelihoods of each state (displayed as pie charts at nodes). Character fit to the molecular phylogeny was assessed using the consistency and retention indices (CI and RI), two classical parsimony‑based measures of homoplasy (Kluge and Farris 1969; Farris 1989). Phylogenetic signal in the vertex microsculptural types was further quantified using the δ statistic, which measures the correspondence between a categorical trait and a phylogeny based on Shannon entropy across ancestral states (Borges et al. 2019; Ribeiro et al. 2023). Ancestral character states for this purpose were estimated with PastML using the MPPA algorithm under the F81 model (Ishikawa et al. 2019), and δ values were computed from the resulting marginal probabilities. The observed δ was compared with a null distribution generated by randomly permuting character states across the tips (Nsim = 1000). All δ-related analyses were performed in Python 3.11.9 using the official implementation provided by Ribeiro et al. (2023).

Six additional Aphalaridae species examined for microsculpture but absent from the molecular dataset of Pramatarova et al. (2024) were attached to the terminals to which they were inferred to be most closely related, based on overall morphological similarity and previous phylogenetic analyses (Burckhardt and Lauterer 1997; Burckhardt and Queiroz 2013).

Six types of cuticular microscupltures were distinguished in Aphalaridae (Table 2). These correspond to four broader categories of Harris (1979): type 1 (smooth–imbricate) represents an ’unsculptured surface’, type 2 (tuberculate) belongs to ’protuberant nonparallel sculpturing’, types 3 (rugose) and 4 (areolate–rugose) correspond to ‘irregular and nonparallel sculpturing’, and types 5 (imbricate) and 6 (alveolate) represent ’regular and nonparallel sculptured patterns’.

The examined taxa show greater variation in cuticular microsculpture on the vertex than on the frontal and ventral parts of the head (frons, genae, clypeus, scape and pedicel), which consistently exhibit imbricate (scale-like) sculpturing in all species (Fig. 1). The following account therefore focuses on the vertex.

Within Aphalarinae, all eight examined species of Aphalara possess imbricate microsculpture on the vertex. The overlapping scale-like structures are generally similar in shape but vary slightly among different species in edge thickness (from rounded to angular) and the presence or absence of small basal tubercles (Fig. 2A–H).

In Colposcenia, rugose microsculpture occurs in C. bidentata, C. osmanica and C. traciana, with C. osmanica showing the deepest furrows and a complete absence of scale-like structures along the outer vertex margin (Fig. 2J–L). In contrast, C. aliena is distinctive in having a predominantly imbricate vertex with dense scale-like elements (Fig. 2I).

Species of Craspedolepta exhibit several types of vertex microsculpture. In most species the surface is imbricate, but with notable variation: (i) poorly defined “scale” bases, accompanied by granules and macroscopic setae in C. araneosa and C. conspersa (Fig. 3A, D); (ii) closely arranged, flat, weakly protruding “scales” in C. artemisiae and C. latior (Fig. 3B, F); and (iii) irregularly rounded to angular “scales” in C. bulgarica (Fig. 3C) and C. innoxia, the latter also bearing prominent basal tubercles similar to those in some Aphalara species (e.g. A. affinis) (Fig. 3E). In contrast, C. nervosa and C. pontica have an areolate–rugose pattern divided into smaller irregular fields, reminiscent of a vertebrate cerebral cortex (Fig. 3I, K), whereas C. malachitica shows a distinctly alveolate pattern with regular, deep, angular cavities (Fig. 3G). Finally, C. omissa, C. nebulosa and C. subpunctata have an almost smooth vertex, with only sparse scale-like elements, here termed the smooth–imbricate type (Fig. 3H, J, L).

Both Eumetoecus kochiae and Rhodochlanis bicolor exhibit imbricate vertex microsculpture. In the former, some “scales” have irregularly serrated margins (Fig. 4A), whereas in the latter they are rounded (Fig. 4B).

Within Rhinocolinae, Agonoscena pistaciae, A. targionii, Megagonoscena gallicola and M. viridis all possess a homogeneous imbricate microsculpture on the head (Fig. 4C–F). The “scales” in M. viridis are nearly flat, whereas in A. pistaciae and M. gallicola they bear small basal granules. In contrast, Rhinocola aceris displays a distinctive tuberculate microsculpture composed of distinct rounded, convex elements (Fig. 4G).

In most species, the microsculpture observed on the vertex extends also onto the dorsal surface of the thorax (Figs 2, 3, 4, 5, 6, 7), with several exceptions.

In Aphalara, the pronotum (Fig. 2A–H) typically bears an imbricate microsculpture composed of transverse scale-like elements, whereas on the mesoscutum these structures become more rounded and smoother, often appearing globular or irregularly polygonal (Fig. 5A–C, E, F, H). In A. exilis and A. nigrimaculosa, the “scales” remain consistently rounded across both head (Fig. 2D, G) and thorax (Fig. 5D, G).

In Colposcenia aliena, although the vertex is imbricate, the pronotum exhibits a distinctly rugose pattern (Fig. 2I–L), as in all other members of the genus (Fig. 5A). In C. osmanica, the rugose pattern extends across the entire thoracic dorsum, whereas in other species, the thoracic dorsum except for the pronotum shows a more or less imbricate microsculpture (Figs 2K, 5K).

In Craspedolepta araneosa, the pronotum (Fig. 3A) is distinctly imbricate with transverse scale-like elements interspersed with small granules. These granules are numerous on mesopraescutum and mesoscutum, forming transverse rows on the mesopraescutum and densely covering the mesoscutum, where the imbricate pattern remains visible only near the posterior and lateral margins (Fig. 6A). A similar condition occurs in C. conspersa, although here the granules form polygonal clusters on mesoscutum (Fig. 6D). In other species of the genus, the thoracic dorsum is predominantly imbricate (Fig. 6B, C, E–L). In C. nebulosa and C. subpunctata, the scuplture becomes smoother near the suture between mesopraescutum and mesoscutum (Fig. 6H, L).

Ancestral state reconstruction mapped onto the molecular phylogeny indicates that imbricate vertex microsculpture is the most likely ancestral condition for Aphalaridae, with high proportional likelihood at basal nodes (Fig. 8). All other sculptural types (rugose, areolate–rugose, smooth–imbricate, alveolate, and tuberculate) appear to have evolved independently from this ancestral state.

Imbricate microsculpture remains dominant across most internal nodes, including Rhinocolinae, Aphalarinae, and the Aphalara and Craspedolepta clades. Within Colposcenia, however, rugose microsculpture is reconstructed as ancestral, with a reversal to the imbricate type in C. aliena. In Craspedolepta, smooth–imbricate microsculpture evolved independently in C. omissa and in the C. nebulosa + C. subpunctata clade; the areolate–rugose type represents a synapomorphy of the C. nervosa + C. pontica clade, whereas the alveolate type is an autapomorphy of C. malachitica. Tuberculate microsculpture is an autapomorphy of Rhinocola aceris.

The vertex microsculpture character thus showed moderate homoplasy and phylogenetic information (7 steps, minimum = 5; CI = 0.714; RI = 0.6), partly due to two autapomorphic states. Despite this, the δ statistic indicates significant phylogenetic structure (δ = 3.93, p = 0.003), with rare derived states distributed non-randomly across the tree.

Observations of additional species not included in the molecular dataset (due to lack of sequence data) support these patterns. Aphalara borealis, A. exilis, Craspedolepta araneosa, C. artemisiae, C. latior, Eumetoecus kochiae and Megagonoscena viridis displayed imbricate vertex microsculpture (Fig. 8). Slide-mounted material from MHNG and NHMB further indicates that most additional genera of Aphalarinae (Brachystetha, Crastina, Gyropsylla and Limataphalara) and Rhinocolinae (Cerationotum and Crucianus) also possess predominantly imbricate microsculpture, although reductions or modifications occur in some taxa (e.g. Caillardia, Leurolophus, Tainarys), where rugose or alveolate patterns may be locally developed or partially reduced.