Case design and flow resistance in high-alpine caddisfly larvae (Insecta, Trichoptera).

For evaluating hydraulic stress reduction strategies of caddisfly larvae, our study has three goals. First, creating a database on Reynolds numbers (Re) and drag coefficients valid for Limnephilidae larvae with cylindrical mineral cases. Second, evaluating the effects of submerged weight and biometry in cases with comparable length/width ratios. And third, collecting field data in an alpine environment for gaining insights into the hydraulic niches occupied by thirteen Drusinae species. Biometric data were subsequently combined with published Reynolds numbers and mean flow velocity data measured immediately upstream of Limnephilidae larvae at the moment of dislodgement. This provides drag coefficients for the range of Reynolds numbers obtained in the field. Data reveal that heavy cases strongly benefit from compensating drag by submerged weight, thereby enabling species to utilize high velocity spots, an important benefit for filtering species. Supplementary Information: The online version contains supplementary material available at 10.1007/s10750-022-04981-y.

Comparing head muscles among Drusinae clades (Insecta: Trichoptera) reveals high congruence despite strong contrasts in head shape.

The subfamily Drusinae (Limnephilidae, Trichoptera) comprises a range of species exhibiting differently shaped head capsules in their larval stages. These correspond to evolutionary lineages pursuing different larval feeding ecologies, each of which uses a different hydraulic niche: scraping grazers and omnivorous shredders sharing rounded head capsules and filtering carnivores with indented and corrugated head capsules. In this study, we assess whether changes in head capsule morphology are reflected by changes in internal anatomy of Drusinae heads. To this end, internal and external head morphology was visualized using µCT methods and histological sections in three Drusinae species-Drusus franzi, D. discolor and D. bosnicus-representing the three evolutionary lineages. Our results indicate that Drusinae head musculature is highly conserved across the evolutionary lineages with only minute changes between taxa. Conversely, the tentorium is reduced in D. discolor, the species with the most aberrant head capsule investigated here. Integrating previous research on Drusinae head anatomy, we propose a fundamental Drusinae blueprint comprising 29 cephalic muscles and discuss significance of larval head capsule corrugation in Trichoptera.

External and internal head anatomy of Drusus monticola (Trichoptera, Limnephilidae).

EXTERNAL AND INTERNAL HEAD ANATOMY OF DRUSUS MONTICOLA TRICHOPTERA LIMNEPHILIDAE: Caddisflies have evolved to a staggering diversity, and their larvae inhabit a wide range of different habitats. Also, the larvae differ in their (feeding) ecology, and hydrological niche preference. Consequently, groups differ in their external morphology, a fact that allows to identify many taxa to species-level in the larval stage. However, a comparative treatise on the internal anatomy of larval Trichoptera remains to be presented. Here, we provide a detailed study on the external and internal head anatomy of Drusus monticola, a member of the limnephilid subfamily Drusinae.We found 26 major muscles using µCT-scans, of which the muscles operating the mandibles were the largest. Overall, we could differentiate four main muscle groups: muscles operating the labrum, muscles operating the mandibles, muscles operating the maxillolabium and muscles operating the alimentary canal.The situation as observed in D. monticola is highly similar to that of D. trifidus, the only other Drusinae in which cephalic anatomy is known. We propose that the configuration (muscle origins and number) observed here is characteristic for an evolutionary lineage within Drusinae in which all known members share a scraping grazer feeding ecology. Other Drusinae, including such with modified head capsules, remain to be investigated. ZUSAMMENFASSUNG EXTERNE UND INTERNE ANATOMIE DES KOPFES VON DRUSUS MONTICOLA TRICHOPTERA LIMNEPHILIDAE: Köcherfliegen haben eine beeindru-ckende Diversität, und ihre Larven besiedeln ein breites Spektrum unterschiedlicher Habitate. Zudem unterscheiden sich diese Larven in ihrer (Ernährungs)-Ökologie und der Präferenz bestimmter hydrologischer Nischen. Folglich unterscheiden sich diese Gruppen in ihrer Morphologie, ein Umstand, durch den sie erst bestimmbar werden. Eine umfassende vergleichende Bearbeitung der internen Anatomie von Köcherfliegenlarven steht allerdings noch aus. Hier legen wir eine genaue Studie der Kopfkapselanatomie von Drusus monticola vor, einer Limnephilidae aus der Unterfamilie der Drusinae.Wir konnten mittels µCT-Scans 26 Muskeln feststellen, wobei die Mandibelmuskeln bei weitem die größten sind. Insgesamt konnten wir vier Muskelgruppen differenzie-ren: Muskeln des Labrums, Muskeln der Mandibeln, Muskeln des Maxillolabiums und Muskeln des Verdauungstrakts.Die Organisation, die bei D. monticola vorgefunden wurde, entspricht weitestgehend der, die anhand von D. trifidus beschrieben wurde - der einzigen anderen daraufhin erforschten Drusinae. Wir schließen daraus, dass die beobachtete Konfiguration für die evolutionäre Linie der schabenden Weidegänger innerhalb der Drusinae typisch ist. Bezüglich der Anatomie anderer Drusinae, insbesondere solcher mit abgewan-delten Kopfkapseln, sollten weitere Forschungen angestellt werden.

Hydraulic niche utilization by larvae of the three Drusinae clades (Insecta: Trichoptera).

Hydraulic niche descriptors of final instar larvae of nine Drusus species (Trichoptera) were studied in small, spring-fed, first-order headwaters located in the Mühlviertel (Upper Austria), Koralpe (Carinthia, Austria), and in the Austrian and Italian Alps. The species investigated covered all three clades of Drusinae: the shredder clade (Drusus franzi, D. alpinus), the grazer clade (D. biguttatus, D. chauvinianus, D. dudor, D. monticola), and the filtering carnivore clade (D. chrysotus, D. katagelastos, D. muelleri). Flow velocity was measured at front center of 68 larvae, head upstream, on the top of mineral substrate particles at water depths of 10-30 mm, using a tripod-stabilized Micro propeller meter (propeller diameter = 10 mm). Each data series consisted of a sampled measurement lasting 30 s (measuring interval = 1 s). In total, 2040 single velocity measurements were taken. Instantaneous flow velocities and drag at the sites of the 68 larvae varied from 0 to 0.93 m s-1 and 0 to 8346 *10-6 N, respectively. Flow velocities and drag between the three clades were highly significantly different (p < 0.001); mean velocity (± 95% confidence limits) for the three clades were 0.09 ± 0.00 m s-1 for the shredder, 0.25 ± 0.00 m s-1 for the grazer, and 0.31 ± 0.01ms-1 for the filtering carnivore clade; the corresponding data for drag were (85 ± 18)*10-6 N, (422 ± 61)*10-6 N and (1125 ± 83)*10-6 N, respectively. Adhesive friction ranged from (41.07 ± 53.03)*10-6 N in D. franzi to (255.24 ± 216.87)*10-6 N in D. chrysotus. Except in D. franzi and D. dudor adhesive friction was always well below drag force, indicating that submerged weight alone was not sufficient to stabilize the larvae in their hydraulic environment. Reynolds numbers varied between 0 in D. franzi and D. alpinus, and 12,634 in D. katagelastos, with 7% of the total in the laminar (R < 500), 30%in the transitional (R = 500-2000), and 61%in the fully turbulent stage (R > 2000). Froude numbers (Fr) varied from 0 to 2.97. The two Drusus species of the shredder clade and three out of four species of the grazer clade were exposed to subcritical Fr < 1, one species of the grazer clade and two out of three species of the filtering clade to supercritical Froude numbers >1.

Hydraulic stress parameters of a cased caddis larva (Drusus biguttatus) using spatio-temporally filtered velocity measurements.

By studying hydraulic stress parameters of larvae of the cased caddisfly Drusus biguttatus (Pictet, 1834) in a tributary of the Schwarze Sulm (Carinthia, Austria), we aimed on (1) detecting the flow properties of the spatio-temporally filtered velocity measurements taken, and (2) on defining the hydraulic niche of this caddisfly larva. For this, we took 31 measurement series lasting 30 to 300 s, yielding 2176 single velocity measurements. The probability density functions of the 31 data series were Gaussian or sub-Gaussian, and the mean recurrent interval between velocity maxima within a data series was only 15.00 s. As a consequence, the Trichoptera larvae studied have to face strong flow accelerations in short intervals which is a much higher stress than conventional mean velocity measurements would suggest. The hydraulic niche of Drusus biguttatus is defined by instantaneous flow velocities ranging from 0.04 to 0.69 m s-1, by drag forces from 13 × 10-6 to 3737 × 10-6 N, by Froude numbers from 0.13 to 1.20, and mostly by Reynolds numbers > 2000. Under such conditions, only 5.1% of the drag force is compensated by submerged weight, whereas the remainder has to be counterbalanced by the active efforts of the larvae to remain attached to the substrate.

Lagrangian chaos in steady three-dimensional lid-driven cavity flow.

Steady three-dimensional flows in lid-driven cavities are investigated numerically using a high-order spectral-element solver for the incompressible Navier-Stokes equations. The focus is placed on critical points in the flow field, critical limit cycles, their heteroclinic connections, and on the existence, shape, and dependence on the Reynolds number of Kolmogorov-Arnold-Moser (KAM) tori. In finite-length cuboidal cavities at small Reynolds numbers, a thin layer of chaotic streamlines covers all walls. As the Reynolds number is increased, the chaotic layer widens and the complementary KAM tori shrink, eventually undergoing resonances, until they vanish. Accurate data for the location of closed streamlines and of KAM tori are provided, both of which reach very close to the moving lid. For steady periodic Taylor-Görtler vortices in spanwise infinitely extended cavities with a square cross section, chaotic streamlines occupy a large part of the flow domain immediately after the onset of Taylor-Görtler vortices. As the Reynolds number increases, the remaining KAM tori vanish from the Taylor-Görtler vortices, while KAM tori grow in the central region further away from the solid walls.

A new Drusinae species from the western Alps with comments on the subfamily and an updated key to filtering carnivore larvae of Drusinae species (Insecta: Trichoptera: Limnephilidae).

A new Drusinae species, Drusus katagelastos sp. nov., of the Drusus chapmani Species Complex, is described based on a male and associated larvae. Adult-larval association was achieved through DNA barcoding. The male of the new species differ from that of its congeners in the formation of the intermediate appendages and parameres. Information on the morphology of the larva is given, and important diagnostic features are discussed. In the context of filtering carnivore Drusinae, the larva of the new species can be separated from other filtering carnivore species by the dense cover of long translucent bristles within the frontal cavity surrounded by a circular corona of long bristles. Drusus katagelastos sp. nov. is known from only northwestern Italy (Piemonte).

Project overview: Intricate bodies in the boundary layer - bridging fluid mechanics, morphology and ecology in larval Drusinae (Insecta: Trichoptera).

This paper summarizes the layout, the three work packages and the intended outcome of the project 'Intricate bodies in the boundary layer P 31258-B29', funded by the Austrian Science Fund (FWF ; project start: October 2018).

Topology of hydrothermal waves in liquid bridges and dissipative structures of transported particles.

High-resolution three-dimensional numerical simulations are carried out for hydrothermal waves in a thermocapillary liquid bridge with Prandtl number Pr=4 and length-to-radius aspect ratio Γ=0.66. The flow topology is analyzed using Poincaré sections in a frame of reference co-rotating with the phase velocity of the wave. We find regions of regular and chaotic motion. The regular regions are shown to be of key importance for dissipative structures of transported particles. Suspended particles which are passively advected in the bulk, but experience dissipation in a thin layer below the free surface, can rapidly form dissipative structures, also called particle accumulation structures. The shape and the formation time of the particulate structures are determined by the location of the invariant tori of the flow field with respect to the sub-surface layer in which the dissipation of the particle motion acts. The results from a hard-wall particle-free-surface interaction model are in good agreement with experimental observations.

Airflow elicits a spider's jump towards airborne prey. II. Flow characteristics guiding behaviour.

When hungry, the wandering spider Cupiennius salei is frequently seen to catch flying insect prey. The success of its remarkable prey-capture jump from its sitting plant into the air obviously depends on proper timing and sensory guidance. In this study, it is shown that particular features of the airflow generated by the insect suffice to guide the spider. Vision and the reception of substrate vibrations and airborne sound are not needed. The behavioural reactions of blinded spiders were examined by exposing them to natural and synthetic flows imitating the fly-generated flow or particular features of it. Thus, the different roles of the three phases previously identified in the fly-generated flow and described in the companion paper could be demonstrated. When exposing the spider to phase I flow only (exponentially increasing flow velocity with very little fluctuation and typical of the fly's approach), an orienting behaviour could be observed but a prey-capture jump never be elicited. Remarkably, the spider reacted to the onset of phase II (highly fluctuating flow) of a synthetically generated flow field with a jump as frequently as it did when exposed to natural fly-generated flows. In all cases using either natural or artificial flows, the spider's jump was triggered before its flow sensors were hit by phase III flow (steadily decreasing airflow velocity). Phase III may tell the spider that the prey has passed by already in case of no prey-capture reaction. Our study underlines the relevance of airflow in spider behaviour. It also reflects the sophisticated workings of their flow sensors (trichobothria) previously studied in detail. Presumably, the information contained in prey-generated airflows plays a similar role in many other arthropods.

Particle-accumulation structures in periodic free-surface flows: inertia versus surface collisions.

The role of particle inertia and particle-free-surface collisions in periodic free-surface flows is evaluated in the framework of an analytical flow model for a thermocapillary liquid bridge. Inertia and particle-free-surface collisions lead to particle accumulation, but on different time scales, and can lead to different accumulation patterns. A comparison with experimental results provides strong evidence that the experimentally observed accumulation patterns are due to particle-free-surface collisions.

Airflow elicits a spider's jump towards airborne prey. I. Airflow around a flying blowfly.

The hunting spider Cupiennius salei uses airflow generated by flying insects for the guidance of its prey-capture jump. We investigated the velocity field of the airflow generated by a freely flying blowfly close to the flow sensors on the spider's legs. It shows three characteristic phases (I-III). (I) When approaching, the blowfly induces an airflow signal near the spider with only little fluctuation (0.013 ± 0.006 m s(-1)) and a strength that increases nearly exponentially with time (maximum: 0.164 ± 0.051 m s(-1) s.d.). The spider detects this flow while the fly is still 38.4 ± 5.6 mm away. The fluctuation of the airflow above the sensors increases linearly up to 0.037 m s(-1) with the fly's altitude. Differences in the time of arrival and intensity of the fly signal at different legs probably inform the spider about the direction to the prey. (II) Phase II abruptly follows phase I with a much higher degree of fluctuation (fluctuation amplitudes: 0.114 ± 0.050 m s(-1)). It starts when the fly is directly above the sensor and corresponds to the time-dependent flow in the wake below and behind the fly. Its onset indicates to the spider that its prey is now within reach and triggers its jump. The spider derives information on the fly's position from the airflow characteristics, enabling it to properly time its jump. The horizontal velocity of the approaching fly is reflected by the time of arrival differences (ranging from 0.038 to 0.108 s) of the flow at different legs and the exponential velocity growth rate (16-79 s(-1)) during phase I. (III) The air flow velocity decays again after the fly has passed the spider.