An overview of campaniform sensilla arrangement and location on hexapod legs.

Campaniform sensilla (CS) are mechano-sensors in the cuticle of insects which are able to detect cuticle deformation. On insect legs, CS play a role in the modulation of locomotion behavior because they provide feedback on cuticle bending. The presence and location of these sensors on the legs across insects is virtually unknown except for a few classical model species such as Drosophila melanogaster (Diptera), Periplaneta americana (Blattodea), Carausius morosus (Phasmatodea) and a few others. We studied representatives of all insect orders, except Strepsiptera, regarding the morphology, location, and arrangement of CS on their legs to infer whether (i) CS location and arrangement is phylogenetically structured, (ii) there is any variation in CS morphology in relation to leg function, and (iii) CS morphology and arrangement is fundamentally different between holo- and hemimetabolous insects. We found enormous variation in all three aspects studied here - CS morphology, location, and arrangement - with no tangible phylogenetic structure or any distinction between holo- and hemimetabolous insects. Moreover, species from orders which are commonly considered to walk only for very short distances such as Odonata or Raphidioptera showed high quantities of CS on their legs compared to orders which are commonly considered to have "good" walking abilities such as Coleoptera or Diptera. We could also find putative CS on the coxa in Odonata, Dermaptera, Plecoptera, Orthoptera, Mantophasmatodea, Grylloblattodea, Embioptera, Blattodea, Hymenoptera, Raphidioptera, Neuroptera, Coleoptera, and Mecoptera which contradicts a common paradigm that CS occur only from the trochanter towards more distal leg segments. With its high variability, it appears that CS location and arrangement evolves according to the particular ecological and neurophysiological niche of each species with no phylogenetic constraints and a rather high adaptivity instead. Our study provides a baseline for future studies on CS function in insects with different ecological niches.