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

Comparative morphological analysis of the labrum and labium among nine aphid species-<i>Uroleucon</i> sp., <i>Glyphina betulae</i>, <i>Myzus cerasi</i>, <i>Panaphis juglandis</i>, <i>Chaitophorus</i> sp., <i>Lachnus roboris</i>, <i>Forda</i> sp., <i>Paracletus cimiciformis</i>, and <i>Trama</i> 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 (<i>M. cerasi</i>) to 358.1 μm (<i>L. roboris</i>). However, <i>Trama</i> 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 <i>Trama</i> sp. and <i>L. roboris</i> 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 <i>Chaitophorus sp</i> St1, <i>Trama</i> and <i>L. roboris</i> St2, <i>Uroleucon</i> sp St3, and <i>P. cimiciformis</i> St4, whereas <i>Trama</i> 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 <i>Trama</i> sp., which presents sensilla St3. Distinct cuticular modifications-including apical cuticular processes and granular protrusions-occur only in <i>Trama</i> sp. and <i>L. roboris</i>, suggesting lineage-specific adaptations. These morphological patterns indicate that aphid mouthpart diversity reflects functional specialisation linked to host-plant structural variation.