Chapter 2: Comparative morphology of the peripheral olfactory system of disease vector arthropods

Abstract

Haematophagous arthropods rely on their peripheral olfactory systems to locate hosts, refuges, oviposition sites and to communicate intraspecifically. Olfactory appendages comprise the antennae and certain mouthparts in insects, and the first pair of legs in other arthropods. Olfactory sensory neurons (OSNs) are housed within hair-like cuticular structures called sensilla. There are two morphologically distinct types of olfactory sensilla: those with single walls and those with double walls. Both types are found throughout insect orders and in acarines. Additional subtypes of olfactory sensilla have also been described, while morphologically similar sensilla have been identified based on physiological function. To fully characterise a sensillum type, scanning electron microscopy, transmission electron microscopy, plus physiological and molecular assays aimed at elucidating the chemical ‘tuning’ of OSNs are required. Once the olfactory sensillum types and subtypes have been defined, precise taxonomic comparisons are possible, leading to a better understanding of the contributions of components of olfactory systems and inspiring new hypotheses about their evolution. In this chapter, we compare the peripheral olfactory systems of haematophagous arthropods, which act as vectors of disease-causing pathogens. Our survey includes descriptions of the various sensillum types and subtypes across taxa. When ultrastructural analyses of sensilla and physiological responses are available, unequivocal functional comparisons can be made. Unfortunately, comprehensive studies of peripheral olfactory systems are uncommon. Despite the lack of robust comparative information, we and others propose that haematophagous species locating hosts at long distances display more highly developed peripheral olfactory systems than species that live very near to, or on, their hosts. Moreover, the loss of the blood-feeding habit seems to correlate with reduced numbers of olfactory sensilla. We suggest that collaborative efforts are necessary, wherein specialised equipment and skills are distributed across research groups, to improve our understanding of arthropod peripheral olfactory systems more rapidly.