# Morphological Modification of the Mouthparts of Aphids (Hemiptera: Sternorryncha: Aphididae)

## TL;DR

This paper examines the mouthpart structures of nine aphid species, revealing significant differences that suggest adaptations to their host plants.

## Contribution

The study provides detailed comparative morphological data on aphid mouthparts, highlighting unique features in Trama sp. and Lachnus roboris.

## Key findings

- Trama sp. has a conical labrum and unique sensilla types not found in other species.
- Trama sp. and Lachnus roboris have five-segmented labia, unlike most species with four segments.
- Cuticular processes and granular protrusions are exclusive to Trama sp. and Lachnus roboris.

## Abstract

This study compares the mouthpart structures of nine aphid species, with a focus on the labrum and labium. Most species have a triangular labrum and a four-segmented labium, while Trama sp. and Lachnus roboris have five segments. Trama sp. stands out with a conical labrum and unique types of sensilla not found in other species. Sensilla trichodea and basiconica are distributed differently across species, with Uroleucon sp., L. roboris, and Trama sp. showing the highest numbers and most complex patterns. Special surface features, such as cuticular processes and granular protrusions, appear only in Trama sp. and L. roboris. These differences suggest that aphid mouthparts are adapted to different host-plant structures, reflecting their feeding specialisations and evolutionary diversification within the Aphididae.

Comparative morphological analysis of the labrum and labium among nine aphid species—Uroleucon sp., Glyphina betulae, Myzus cerasi, Panaphis juglandis, Chaitophorus sp., Lachnus roboris, Forda sp., Paracletus cimiciformis, and Trama sp., belonging to six subfamilies—reveals marked interspecific variation in structure, segmentation, and sensilla equipment. The labrum is generally triangular and plate-like, ranging from 88.8 μm (M. cerasi) to 358.1 μm (L. roboris). However, Trama sp. exhibits a distinctive conical labrum (311.1 μm) bearing three pairs of sensilla trichodea (St2)—unique among the examined taxa. Most species possess a four-segmented labium, while Trama sp. and L. roboris exhibit five segments. The second segment is the longest and most elaborate, bearing dense arrays of sensilla or spiniform tubercles in several species. Sensilla trichodea (St1–St5) are widespread across taxa, showing the highest densities in Chaitophorus sp St1, Trama and L. roboris St2, Uroleucon sp St3, and P. cimiciformis St4, whereas Trama sp. uniquely combines sensilla St2, St3, St5, and sensilla basiconica (Sb2). Sensilla basiconica (Sb1) are consistently positioned at the base of the labrum and the fourth labial segment, except in Trama sp., which presents sensilla St3. Distinct cuticular modifications—including apical cuticular processes and granular protrusions—occur only in Trama sp. and L. roboris, suggesting lineage-specific adaptations. These morphological patterns indicate that aphid mouthpart diversity reflects functional specialisation linked to host-plant structural variation.