Intricate bodies in the boundary layer

The morphologies of most caddisfly species in streams seem to be not well adapted to the high hydrodynamic forces acting on them. This ecological paradox also fully applies to the Drusinae, a caddisfly subfamily inhabiting cold, turbulent running water in high-gradient, hard-substrate channels. Fitting their feeding types, head capsules of Drusinae larvae are morphologically very different from other stream biota, because many species show deep frontal concavities, often combined with internal modifications of the skeleto-muscular system of the head capsule and with pronota fitted with steep dorsal humps or high, sharp ridges. On the other hand, Drusinae larvae lack elaborate attachment devices and rely only at their standard single tarsal claws in order to get hold at the substrate. In order to resolve this paradoxon, the first work package of this proposal aims at providing data sets on the actual duration and magnitude of hydraulic stress acting at Drusinae larvae belonging to different morphological types in the field and over a range of activity scenarios. For this, we will measure hydraulic stress acting at the larval locations using acoustic Doppler velocimetry; this information will be combined with video documentations of body postures and flow exposition using a waterproof endoscope camera both at the sediment surface and within substrate interstices. In the second work package, we will test our hypothesis that the evolution of external head capsule shapes of the seven morphotypes defined in Drusinae larvae has triggered shifts in the origins of the main muscle bundles and in innervation patterns. Comparative anatomical studies of the seven morphotypes using micro-computed tomography will enable us to identify anatomic key innovations promoted by feeding type evolution and exploitation of hydrological niches in Drusinae. For interpreting the evolutionary significance of morphological (micro-) structures, we will generate morphotype-specific data sets on flow fields around larvae at different current velocities and body postures using numerical flow simulations. Data sets obtained by micro-computed tomography will allow for modelling the effects of roughness elements on morphotype-specific hydrodynamic drag and lift forces over a wide range of Reynolds numbers and for interpreting hydrodynamic profiles obtained in the field. The outcomes of this interdisciplinary research of work package 3 will contribute to a better understanding of the ecological significance of body shapes and morphological structures in an aquatic environment and will provide new insights in the fields of trait evolution and niche utilization of aquatic insects.

Intricate bodies in the boundary layer The morphologies of most caddisfly species in streams seem to be not well adapted to the high hydrodynamic forces acting on them. This ecological paradox also fully applies to the Drusinae, a caddisfly subfamily inhabiting cold, turbulent running water in high-gradient, hard-substrate channels. Fitting their feeding types, head capsules of Drusinae larvae are morphologically very different from other stream biota, because many species show deep frontal concavities, often combined with internal modifications of the skeleto-muscular system of the head capsule and with pronota fitted with steep dorsal humps or high, sharp ridges. On the other hand, Drusinae larvae lack elaborate attachment devices and rely only on their standard single tarsal claws in order to get hold at the substrate. In order to resolve this paradox, the first work package of the project aimed at providing data sets on the actual duration and magnitude of hydraulic stress acting on Drusinae larvae belonging to different morphological types in the field. We succeeded in obtaining data sets covering 13 species from all three clades of Drusinae: shredders, grazers and filtering carnivores. Flow velocities and drag between the three clades were highly significantly different, with mean velocities being least in shredders and highest in filtering collectors. A similar trend was also observed for other descriptors of hydraulic stress. In the second work package we tested our hypothesis that the evolution of external head capsule shapes of the seven morphotypes defined in Drusinae larvae has triggered shifts in internal head morphologies of the three clades. To this end, internal and external head morphology was visualized using micro-computed tomography and histological sections. Our results indicate that Drusinae head musculature is highly conserved across the evolutionary lineages with only minute changes between taxa. Conversely, parts of the internal head skeleton (tentorium) has found to be reduced in Drusus discolor, the species with the most aberrant head capsule investigated here. In addition, the volume of chewing muscles was found to be significantly higher in shredders than in grazers and filtering collectors. For interpreting the evolutionary significance of morphological structures, we generated morphotype-specific data sets by micro-computed tomography as a basis for modelling the effects of body shapes on morphotype-specific hydrodynamic drag and lift forces. The data showed that in all three evolutionary lineages lift is significantly smaller than total drag. The highest streamwise normal stresses were observed upstream of the head of filter feeders where a horseshoe-shaped vortex redirects flow directly to the filtering bristles of the extended legs, thereby optimizing the catching success.