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Concept: Dytiscidae

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However complex a visual system is, the size (and growth rate) of all its components-lens, retina and nervous system-must be precisely tuned to each other for the system to be functional. As organisms grow, their eyes must be able to achieve and maintain emmetropia, a state in which photoreceptors receive sharp images of objects that are at infinity. While there has been ample research into how vertebrates coordinate eyes growth, this has never been addressed in arthropods with camera eyes, which tend to grow dramatically and typically in a step-wise manner with each molt (ecdysis). Here, we used histological and optical methods to measure how the larval eyes of Sunburst Diving Beetles (Thermonectus marmoratus, Coleoptera, Dytiscidae) grow, and how well optical and morphological parameters match, during the dramatic growth that occurs between two consecutive larval stages. We find that the eye tubes of the principal eyes of T. marmoratus grow substantially around molt, with the vitreous-like crystalline cone contributing the most to the overall growth. Lenses also reform relatively quickly, undergoing a period of dysfunction and then regaining the ability to project sharp images onto the retina around 8 h post-molt.

Concepts: Insect, Larva, Eye, Photoreceptor cell, Visual system, Beetle, Rod cell, Dytiscidae

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During sexual conflict, males and females are expected to evolve traits and behaviours with a sexually antagonistic function. Recently, sexually antagonistic coevolution was proposed to occur between male and female diving beetles (Dytiscidae). Male diving beetles possess numerous suction cups on their forelegs whereas females commonly have rough structures on their elytra. These rough structures have been suggested to obstruct adhesion from male suction cups during mating attempts. However, some diving beetle species are dimorphic, where one female morph has a rough elytra and the other has a smooth elytra. Here, we used biomechanics to study the adhesive performance of male suction cups on the female morphs in two diving beetle species: Dytiscus lapponicus and Graphoderus zonatus. We compared adhesion on the rough and the smooth female morphs to infer the function of the rough elytral modifications. We found that the adhesive force on the rough structures was much lower than on other surfaces. These findings support the suggestion of sexual conflict in diving beetles and a sexually antagonistic function of the rough female structures. In addition, males differed in their adhesive capacity on different female surfaces, indicating a male trade-off between adhering to smooth and rough female morphs.

Concepts: Human, Male, Female, Sex, Mimicry, Beetle, Adaptation, Dytiscidae

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Variation in the ability to fly or not is a key mechanism for differences in local species occurrences. It is increasingly acknowledged that physiological or behavioral mechanisms rather than morphological differences may drive flight abilities. However, our knowledge on the seasonal variability and stressors creating nonmorphological differences in flight abilities and how it scales to local and regional occurrences is very limited particularly for small, short-lived species such as insects. Here, we examine how flight ability might vary across seasons and between two closely related genera of freshwater beetles with similar geographical ranges, life histories, and dispersal-related morphology. By combining flight experiments of >1,100 specimens with colonization rates in a metacommunity of 54 ponds in northern and eastern Europe, we have analyzed the relationship between flight ability and spatio-environmental distribution of the study genera. We find profound differences in flight ability between the two study genera across seasons. High flight ability for Acilius (97% of the tested individuals flew during the experiments) and low for Graphoderus (14%) corresponded to the different colonization rates of newly created ponds. Within a 5-year period, 81 and 31% of the study ponds were colonized by Acilius and Graphoderus, respectively. While Acilius dispersed throughout the season, flight activity in Graphoderus was restricted to stressed situations immediately after the emergence of adults. Regional colonization ability of Acilius was independent of spatial connectivity and mass effect from propagule sources. In contrast, Graphoderus species were closely related to high connectivity between ponds in the landscape. Our data suggest that different dispersal potential can account for different local occurrences of Acilius and Graphoderus. In general, our findings provide some of the first insights into the understanding of seasonal restrictions in flight patterns of aquatic beetles and their consequences for species distributions.

Concepts: Biology, Species, Difference, Season, Beetle, Genus, Colonialism, Dytiscidae

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High species diversity and endemism within Madagascar is mainly the result of species radiations following colonization from nearby continents or islands. Most of the endemic taxa are thought to be descendants of a single or small number of colonizers that arrived from Africa sometime during the Cenozoic and gave rise to highly diverse groups. This pattern is largely based on vertebrates and a small number of invertebrate groups. Knowledge of the evolutionary history of aquatic beetles on Madagascar is lacking, even though this species-rich group is often a dominant part of invertebrate freshwater communities in both standing and running water. Here we focus on large bodied diving beetles of the tribes Hydaticini and Cybistrini. Our aims with this study were to answer the following questions 1) How many colonization events does the present Malagasy fauna originate from? 2) Did any colonization event lead to a species radiation? 3) Where did the colonizers come from-Africa or Asia-and has there been any out-of-Madagascar event? 4) When did these events occur and were they concentrated to any particular time interval? Our results suggest that neither in Hydaticini nor in Cybistrini was there a single case of two or more endemic species forming a monophyletic group. The biogeographical analysis indicated different colonization histories for the two tribes. Cybistrini required at least eight separate colonization events, including the non-endemic species, all comparatively recent except the only lotic (running water) living Cybister operosus with an inferred colonization at 29 Ma. In Hydaticini the Madagascan endemics were spread out across the tree, often occupying basal positions in different species groups. The biogeographical analyses therefore postulated the very bold hypothesis of a Madagascan origin at a very deep basal node within Hydaticus and multiple out-of-Madagascar dispersal events. This hypothesis needs to be tested with equally intense taxon sampling of mainland Africa as for Madagascar.

Concepts: Biodiversity, Species, Phylogenetics, Beetle, Madagascar, Endemism, Dytiscidae, Madagascar dry deciduous forests

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Exposure to intense predation risk can induce morphological and behavioural phenotypes that prepare prey, often at young ages, for surviving attacks from unknown predators. However, previous studies revealed that this survival advantage depended on the predator species. Here, we used alarm cues from injured conspecifics to simulate a period of high predation risk for embryonic wood frogs, Lithobates sylvaticus. Two weeks post-hatching, we tested whether the embryonic risk exposure influenced survival in encounters with two novel predators: (1) a spider (Dolomedes sp.) that ambushes prey exclusively on the surface of the water, and (2) the adult predacious diving beetle (Dytiscus sp.) which displays underwater sit-and-wait posture and pursuit tactics. Tadpoles exposed to embryonic high-risk survived longer when encountering spiders, whereas background risk had no influence on survival with adult beetles. These findings, coupled with survival studies involving other predator types, indicate that a high-risk environment promotes tadpole survival in future encounters with unknown sit-and-wait predators, but at the cost of increased vulnerability to novel predators capable of active pursuit.

Concepts: Predation, Hunting, Frog, Dytiscidae, Ambush predator

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Coexistence of closely related species has long been a focus of biologists in their efforts to explain mechanisms that drive community assembly. Dytiscidae (predaceous diving beetles) are a group that shows a particularly high affinity for sympatry despite their relatedness. Our objective was to investigate the degree of overlap among Neoporus (Guignot) species (Coleoptera: Dytiscidae) in floodplains of the southeastern United States. We sampled two floodplain habitats (permanent oxbow lakes and temporarily flooded pools) of the Altamaha River (Georgia, USA) for Neoporus species over three years. Six species of Neoporus were collected during our study, and a significant amount of overlap (spatial and temporal) was documented. Analysis suggested that none of the species exhibited a preference toward one habitat type or the other. Temporally, no striking patterns of segregation emerged. No negative correlations between species were documented, but neither were significant positive correlations found. This absence of distinct patterns suggests a lack of segregation among Neoporus species in floodplains of the Altamaha River. While Dytiscidae in general appears to be a particularly sympatric group of organisms, overlap among congeneric species within the family has been documented less frequently. Our study provides new insight into the degree to which dytiscids are capable of coexisting in space and time.

Concepts: Lake, River, Beetle, Speciation, Adephaga, Dytiscidae

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The morphology and distribution of the antennal sensilla of adult diving beetle Cybister japonicus Sharp (Dytiscidae, Coleoptera), have been examined. Five types of sensilla on the antennae were identified by scanning electron microscope (SEM) and transmission electron microscope (TEM). Sensilla placodea and elongated s. placodea are the most abundant types of sensilla, distributing only on the flagellum. Both these types of sensilla carry multiple pore systems with a typical function as chemoreceptors. Three types of s. coeloconica (Type I-III) were also identified, with the characterization of the pit-in-pit style, and carrying pegs externally different from each other. Our data indicated that both type I and type II of s. coleconica contain two bipolar neurons, while the type III of s. coleconica contains three dendrites in the peg. Two sensory dendrites in the former two sensilla are tightly embedded inside the dendrite sheath, with no space left for sensilla lymph. There are no specific morphological differences in the antennal sensilla observed between males and females, except that the males have longer antennae and more sensilla than the females.

Concepts: Electron, Electron microscope, Neuron, Transmission electron microscopy, Beetle, Scanning electron microscope, Dytiscidae, Cybister

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The Hydroporus memnonius species group includes both widespread and range restricted diving beetle taxa in the western Palaearctic, some of which have been divided into a number of geographical subspecies. Of these, Hydroporus necopinatus is distributed in the far west of Europe, from central Spain to southern Britain, and has been split into three subspecies, occurring in Iberia (necopinatus sst.), France (robertorum) and England (roni) respectively-the last of these being a rare example of an insect taxon apparently endemic to northern Europe. Here we explore inter-relationships between populations and subspecies of H. necopinatus and related members of the Hydroporus melanarius subgroup, using mitochondrial COI sequence data. We reveal widespread discordance between mitochondrial DNA sequence variation and morphology in areas where H. necopinatus and H. melanarius come into contact, consistent with historical introgressive hybridization between these taxa. In light of this discordance, the lack of clear genetic divergence between H. necopinatus subspecies, and the fact that both robertorum and roni are morphologically intermediate between H. necopinatus sstr. and H. melanarius, we suggest that these taxa may be of hybridogenic origin, rather than representing discrete evolutionary lineages.

Concepts: DNA, Genetics, Evolution, Species, Fungus, Europe, Beetle, Dytiscidae

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In 2006, the forewing trabeculae of Cybister tripunctatus Olivier (i.e., Cybister) beetles were reported to be hollow, and a biomimetic structural model (i.e., Song’s model) was reported to exhibit better compressive mechanical properties than a solid-core trabecula-honeycomb model (i.e., Chen’s model). To test these assertions, the current study first observed the trabecular microstructure of the Cybister beetle and confirmed that the trabeculae are solid. Second, the finite element method (FEM) was used to perform a contrast analysis of the compressive mechanical properties of Song’s and Chen’s biomimetic models. The results indicated that Chen’s model exhibited better compressive mechanical properties. These findings, which are completely opposite of Song’s findings, were obtained because the comparison models designed for use in Song’s study were not comparable to that of Chen’s model in terms of the core volumes. This study will benefit the development of beetle forewing biomimetic research.

Concepts: Engineering, Finite element method, Model, Beetle, Partial differential equation, Insect wing, Trabecula, Dytiscidae

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In most lineages, most species have restricted geographic ranges, with only few reaching widespread distributions. How these widespread species reached their current ranges is an intriguing biogeographic and evolutionary question, especially in groups known to be poor dispersers. We reconstructed the biogeographic and temporal origin of the widespread species in a lineage with particularly poor dispersal capabilities, the diving beetle genus Deronectes (Dytiscidae). Most of the ca. 60 described species of Deronectes have narrow ranges in the Mediterranean area, with only four species with widespread European distributions. We sequenced four mitochondrial and two nuclear genes of 297 specimens of 109 different populations covering the entire distribution of the four lineages of Deronectes, including widespread species. Using Bayesian probabilities with an a priori evolutionary rate, we performed (1) a global phylogeny/phylogeography to estimate the relationships of the main lineages within each group and root them, and (2) demographic analyses of the best population coalescent model for each species group, including a reconstruction of the geographical history estimated from the distribution of the sampled localities. We also selected 56 specimens to test for the presence of Wolbachia, a maternally transmitted parasite that can alter the patterns of mtDNA variability. All species of the four studied groups originated in the southern Mediterranean peninsulas and were estimated to be of Pleistocene origin. In three of the four widespread species, the central and northern European populations were nested within those in the northern areas of the Anatolian, Balkan and Iberian peninsulas respectively, suggesting a range expansion at the edge of the southern refugia. In the Mediterranean peninsulas the widespread European species were replaced by vicariant taxa of recent origin. The fourth species (D. moestus) was proven to be a composite of unrecognised lineages with more restricted distributions around the Western and Central Mediterranean. The analysis of Wolbachia showed a high prevalence of infection among Deronectes, especially in the D. aubei group, where all sequenced populations were infected with the only exception of the Cantabrian Mountains, the westernmost area of distribution of the lineage. In this group there was a phylogenetic incongruence between the mitochondrial and the nuclear sequence, although no clear pattern links this discordance to the Wolbachia infection. Our results suggest that, in different glacial cycles, populations that happened to be at the edge of the newly deglaciated areas took advantage of the optimal ecological conditions to expand their ranges to central and northern Europe. Once this favourable ecological window ended populations become isolated, resulting in the presence of closely related but distinct species in the Mediterranean peninsulas.

Concepts: Evolution, Biology, Species, Ecology, Mediterranean Sea, Europe, Northern Europe, Dytiscidae