Sensory structures on mouthpart palps in Trichoptera: ground plan and basal evolution trends

Comparative study of sensory structures on maxillary and labial palps in 71 species from 14 families by scanning electron microscopy and light microscopy revealed significant diversity of sensory structures. Seven principal types of sensory structures were found: pointed trichoid, blunt chaetoid, campaniform, thin basiconic, thick basiconic, petaloid, and pseudoplacoid sensilla. Pointed trichoid and blunt chaetoid sensilla occur on every palp segment. First and, especially, second segments of maxillary palps have bunches of very large blunt chaetoid sensilla on medial surfaces. Campaniform sensilla were found only on basal segments. Pseudoplacoid sensilla are common on the terminal segments of both labial and maxillary palps except for Ptilocolepidae and Hydroptilidae. The petaloid sensilla forming the sensory fields are found in groups surrounded by the soft cuticle, generally in depressions, on the apical segments either on maxillary and labial palps in Hydrobiosidae and Rhyacophilidae, only on labial palps in other studied Integripalpia, or on apical labial palp segment and third and fourth maxillary palp segments in Annulipalpia. The pointed tips of both maxillary and labial palps in lower families have apical sensory complexes looking like small conical outgrowths without microtrichia, each with one large thick basiconic sensilla on its tip and several shorter thick basiconic sensilla on lateral surfaces. We consider these seven types of sensilla along with the apical sensory complex and the assemblage of the petaloid sensilla as a part of Trichoptera ground plan. This primitive diversity changes in evolution so the apical sensory complex, the fields of petaloid sensilla, the groups of very long blunt trichoid sensilla of basal segments, and the pseudoplacoid sensilla disappear in some advanced instances, more often on the maxillary palps. Interspecific variations of sensilla might be important for species discrimination, while the distribution of certain sensory structures is important for higher taxonomy.