Ultrastructural and Functional Organization of Maxillary Palps in Ladybird Species (Coleoptera: Coccinellidae) With Different Feeding Preferences.

Insects antennae serve as their primary sensory organs, playing a crucial role both in intra- and interspecific communication, a trait shared across the entire class Insecta. Representatives of Coccinellidae (ladybird) are known for being important natural enemies of economically relevant pests, and occasionally for being pests themselves. In this study, we investigated the ultrastructural organization of maxillary palps in several ladybird species exhibiting different feeding preferences, including entomophagous species (Harmonia axyridis, Scymnus interruptus, Delphastus catalinae, Cryptolaemus montrouzieri, Ceratomegilla undecimnotata, Propylea quatuordecimpunctata) and phytophagous species (Subcoccinella vigintiquatuorpunctata). We analyzed the ultrastructure of the maxillary palps using scanning and transmission electron microscopy. In all species, the maxillary palps were segmented, with the main sensory area located at the apical part of the enlarged distal segment. Our investigation revealed the presence of five distinct sensilla types on the maxillary palps, including uniporous, multiporous, campaniform, trichoidea, and digitiform sensilla. Interestingly, the sensilla shared a similar morphology across the investigated species. Uniporous and multiporous sensilla were the most abundant and were exclusively located in the apical sensory area. Campaniform sensilla were located on the apical sensory area, but positioned along the marginal zone, surrounding the area occupied by uniporous and multiporous sensilla. Digitiform sensilla were positioned in a restricted area of the dorsal side of the last palpomere. Our study hypothesizes that in Coccinellidae, maxillary palps play the role of primary sensory organ, along with the antennae. The comprehensive data presented here lay a robust foundation for further investigations into the role of maxillary palps in the chemical ecology of ladybirds.