Morphological and biomechanical adaptations of larval mandibles in Trichoptera (Insecta)

This study examines the larval mandibles of five caddisfly species,
documenting their morphological and biomechanical adaptations to different
feeding strategies. Three predatory species (Molanna angustata,
Plectrocnemia conspersa, Rhyacophila fasciata) and two algae-grazing
species (Silo nigricornis, Tinodes pallidulus) were investigated using
scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy
(EDX), confocal laser scanning microscopy (CLSM), and nanoindentation. SEM
analysis revealed distinct mandible structures: grazing species possessed
various collecting setae and sharp mandible cutting edges, while predatory
species exhibited more pointed incisors. By EDX, low concentrations of Ca,
Cl, Cu, Fe, K, Mg, Mn, P, S, Si, and Zn were detected with no relationship
to mechanical properties, suggesting limited elemental reinforcement in
larval Trichoptera mandibles. CLSM imaging, however, revealed regional
material heterogeneities related to the mechanical properties, indicating
that the mechanical parameters depend on the degree of cuticle tanning. We
detected harder and stiffer mandible cuticle in predatory species, likely
enhancing the mechanical resistance of the material during prey capture.
Nanoindentation analysis identified two functional mandible types in
grazers, adapted for scraping and collecting, with differential regional
cuticle hardness and stiffness. Predatory species exhibited three mandible
types, likely specialized for grabbing and crushing, piercing and cutting,
or grabbing, piercing, and cutting. Decreased mechanical properties of
condyles in predators suggested enhanced flexibility for prey handling,
whereas grazers, showing higher Young’s moduli and hardness values in
condyles, likely required higher pressure for scraping. In some species
(Silo nigricornis and Rhyacophila fasciata), the heterogeneity between
lateral and medial mandible cuticle indicated that self-sharpening
mechanisms could be present. These findings provide insight into the
functional morphology and material adaptations of mandibles of
trichopteran larvae in relation to their feeding ecology.