Gas exchange and dive behaviour in the diving beetle Platynectes decempunctatus (Coleoptera: Dytiscidae)

Many aquatic insects use bubbles on the body surface to store and supply O₂ for their dives. There are two types of bubbles: air stores, which store O₂ gained from air at the surface, and gas gills that allow passive extraction of O₂ from water. Many insects using air stores and gas gills return to the surface to replenish their bubbles and, therefore, their requirement for O₂ influences dive behaviour. In this study, we investigate gas exchange and dive behaviour in the diving beetle Platynectes decempunctatus that uses a sub-elytral air store and a small compressible gas gill. We measure the PO₂ within the air store during tethered dives, as well as the amount of O₂ exchanged during surfacing events. Buoyancy experiments monitor the volume of gas in the gas gill and how it changes during dives. We also directly link O₂-consumption rate at three temperatures (10, 15 and 20 °C) with dive duration, surfacing frequency and movement activity. These data are incorporated in a gas exchange model, which shows that the small gas gill of P. decempunctatus contributes less than 10% of the total O₂ used during the dive, while up to 10% is supplied by cutaneous uptake.

Hydrophobic-hydrophilic crown-like structure enables aquatic insects to reside effectively beneath the water surface

Various insects utilise hydrophobic biological surfaces to live on the surface of water, while other organisms possess hydrophilic properties that enable them to live within a water column. Dixidae larvae reside, without being submerged, just below the water surface. However, little is known about how these larvae live in such an ecological niche. Herein, we use larvae of Dixa longistyla (Diptera: Dixidae) as experimental specimens and reveal their characteristics. A complex crown-like structure on the abdomen consists of hydrophobic and hydrophilic elements. The combination of these contrasting features enables the larvae to maintain their position as well as to move unidirectionally. Their hydrophobic region leverages water surface tension to function as an adhesive disc. By using the resistance of water, the hydrophilic region serves as a rudder during locomotion.

Suzuki, Takaku, Hariyama and colleagues report on a crown-like structure found on the heads of midge larvae. This structure, analysed using scanning electron microscopy and experimental methods, enables subsurface adhesion and aids in control of locomotion in this region of the water column.

Predator density modifies mosquito regulation in increasingly complex environments

BACKGROUND

Predation plays a pivotal role in the composition and functioning of ecosystems. Both habitat complexity and predator density are important contexts which may determine the strength of trophic and non-trophic interactions. In aquatic systems, the efficacy of natural enemies in regulating vector pest species could be modified by such context dependencies. Here, we use a functional response (FR) approach to experimentally quantify conspecific multiple predator effects across a habitat complexity gradient of two notonectids, Anisops sardea and Enithares chinai, towards larvae of the vector mosquito Culex pipiens pipiens.

RESULTS

E. chinai exhibited significantly greater consumption rates than A. sardea across habitat complexities, both as individuals and conspecific pairs. Each predator type displayed Type II FRs across experimental treatments, with synergistic multiple predator effects (i.e. prey risk enhancement) displayed in the absence of habitat complexity. Effects of increasing habitat complexity modified multiple predator effects differentially between species given behavioral differences, with habitat complexity causing significant antagonism (i.e. prey risk reduction) with multiple A. sardea compared to E. chinai.

CONCLUSION

Habitat complexity effects on multiple predator interactions can manifest differently at the species level, suggesting emergent effects which complicate predictions of natural enemy impact in heterogenous environments. Considerations of density, diversity and habitat effects on efficacies of natural enemies should thus be considered by pest management practitioners to better explain biocontrol efficacies in increasingly diverse environments. © 2020 Society of Chemical Industry.

Surprising differences in the respiratory protein of insects: A spectroscopic study of haemoglobin from the European honeybee and the malaria mosquito

Only recently it was discovered that haemoglobin (Hb) belongs to the standard gene repertoire of insects, although their tracheal system is used for respiration. A classical oxygen-carrying function of Hb is only obvious for hexapods living in hypoxic environments. In other insect species, including the common fruit fly Drosophila melanogaster, the physiological role of Hb is yet unclear. Here, we study recombinant haemoglobin from the European honeybee Apis mellifera (Ame) and the malaria mosquito Anopheles gambiae (Aga). Spectroscopic evidence shows that both proteins can be classified as hexacoordinate Hbs with a strong affinity for the distal histidine. AgaHb1 is proposed to play a role in oxygen transport or sensing based on its multimeric state, slow autoxidation, and small but significant amount of five-coordinated haem in the deoxy ferrous form. AmeHb appears to behave more like vertebrate neuroglobin with a complex function given its diversified distribution in the genome.

Prey preferences of notonectids towards larval mosquitoes across prey ontogeny and search area

BACKGROUND

Predatory biological control agents can be effective natural means of managing pests, vectors and invasive species. However, the strength of predator-prey interactions can be regulated through context-dependencies that often remain unquantified. In particular, refuge effects can influence the efficacy of biological agents towards target species, and such effects are often driven by prey size and search area differences. In this study, we quantify the prey preferences of two predaceous notonectids, Anisops breddini and Anisops sardeus, towards four different aquatic larval instar stages of the medically important mosquito Culex quinquefasciatus across variations in surface area and water depth.

RESULTS

Consumption rates differed significantly among the four larval sizes but not between the notonectids. Search area variations also elicited differences in consumption rates. Both predators tended to prefer second-instar mosquito prey among surface area and water depth variations, while generally avoiding the largest (fourth instar) and smallest (first instar) prey instar stages. For both predators, differential selectivity traits were emergent across surface area variations and water depth, with refuge effects for small prey generally greatest at intermediate-large depths with high surface areas. We thus demonstrate that predatory impacts of notonectids towards mosquito larvae differ significantly according to prey size, and likely peak at intermediate size classes.

CONCLUSION

Different mosquito size classes often coexist and compete, selectivity has important implications for adult mosquito proliferations. Further, in ephemeral aquatic habitats where surface areas and water depths are highly variable spatiotemporally, the efficacy of notonectids in controlling mosquito prey may differ substantially. © 2019 Society of Chemical Industry.

Water depth-dependent notonectid predatory impacts across larval mosquito ontogeny

BACKGROUND

Context-dependencies can modulate the strength of predatory interactions and often remain unquantified. In particular, differences in water depth within aquatic systems could influence predator efficiencies towards prey which utilise three-dimensional space through the water column. Differences in prey size could drive prey size-refuge effects, influencing the efficacy of natural enemies towards vector species. We thus quantify the predatory impact of two notonectid predators, Anisops breddini and Anisops sardeus, towards four different larval instars of Culex quinquefasciatus prey across a water depth gradient, using functional responses (FRs).

RESULTS

Consumption rates differed significantly between the predators, and interspecific differences in responses to variations in water depth were emergent. Both notonectids were able to handle C. quinquefasciatus prey across all instar stages, yet predation rates were generally higher towards early as opposed to late instar prey. Anisops sardeus was most voracious, and differential predation rates of this species were most pronounced in shallow waters. Type II FRs were displayed by notonectids in the majority of treatments; however, Type III FRs were emergent in specific treatment groups, with potential implications for prey population stability. Both capture rates and handling times were often greater at greater depths, and thus maximum feeding rates reduced as depth increased. Our results further demonstrate substantial predatory impacts of notonectid predators towards mosquito, and quantify biotic and abiotic context-dependencies which modulate their impact.

CONCLUSION

Given notonectids are capable of aerial dispersal between ephemeral aquatic habitats of varied volumes, their promotion in aquatic systems could help reduce proliferations of medically important mosquitoes. © 2019 Society of Chemical Industry.

Gas exchange and dive characteristics of the free-swimming backswimmer Anisops deanei

Many aquatic insects utilise air bubbles on the surface of their bodies to supply O2 while they dive. The bubbles can simply store O2, as in the case of an ‘air store’, or they can act as a physical ‘gas gill’, extracting O2 from the water. Backswimmers of the genus Anisops augment their air store with O2 from haemoglobin cells located in the abdomen. The O2 release from the haemoglobin helps stabilise bubble volume, enabling backswimmers to remain near neutrally buoyant for a period of the dive. It is generally assumed that the backswimmer air store does not act as a gas gill and that gas exchange with the water is negligible. This study combines measurements of dive characteristics under different exotic gases (N2, He, SF6, CO) with mathematical modelling, to show that the air store of the backswimmer Anisops deanei does exchange gases with the water. Our results indicate that approximately 20% of O2 consumed during a dive is obtained directly from the water. Oxygen from the water complements that released from the haemoglobin, extending the period of near-neutral buoyancy and increasing dive duration.

Respiratory function of the plastron in the aquatic bug, Aphelocheirus aestivalis (Hemiptera, Aphelocheiridae)

The river bug Aphelocheirus aestivalis is a 40 mg aquatic insect that, as an adult, relies totally on an incompressible physical gill to exchange respiratory gases with the water. The gill (called a ‘plastron’) consists of a stationary layer of air held in place on the body surface by millions of tiny hairs that support a permanent air-water interface, so that the insect never has to renew the gas at the water's surface. The volume of air in the plastron is extremely small (0.14 mm3), under slightly negative pressure, and connected to the gas-filled tracheal system through spiracles on the cuticle. Here, we measure Po2 of the water and within the plastron gas with O2-sensing fibre optics to understand the effectiveness and limitations of the gas exchanger. The difference in Po2 is highest in stagnant water and decreases with increasing convection over the surface. Respiration of bugs in water-filled vials varies between 33 and 296 pmol O2 s−1, depending on swimming activity. The effective thickness of the boundary layer around the plastron is calculated from respiration rate, Po2 difference and plastron surface area according to the Fick diffusion equation and verified by direct measurements with the fibre-optic probes. In stagnant water, the boundary layer is approximately 500 µm thick, which nevertheless can satisfy the demands of resting bugs, even if the Po2 of the free water decreases to half of air-saturation. Active bugs require thinner boundary layers (ca. 100 µm) that are achieved by living in moving water or by swimming.

Physical gills in diving insects and spiders: theory and experiment

Insects and spiders rely on gas-filled airways for respiration in air. However, some diving species take a tiny air-store bubble from the surface that acts as a primary O2 source and also as a physical gill to obtain dissolved O2 from the water. After a long history of modelling, recent work with O2-sensitive optodes has tested the models and extended our understanding of physical gill function. Models predict that compressible gas gills can extend dives up to more than eightfold, but this is never reached, because the animals surface long before the bubble is exhausted. Incompressible gas gills are theoretically permanent. However, neither compressible nor incompressible gas gills can support even resting metabolic rate unless the animal is very small, has a low metabolic rate or ventilates the bubble's surface, because the volume of gas required to produce an adequate surface area is too large to permit diving. Diving-bell spiders appear to be the only large aquatic arthropods that can have gas gill surface areas large enough to supply resting metabolic demands in stagnant, oxygenated water, because they suspend a large bubble in a submerged web.

Characterization of the hemoglobin of the backswimmer Anisops deanei (Hemiptera)

While O(2)-binding hemoglobin-like proteins are present in many insects, prominent amounts of hemoglobin have only been found in a few species. Backswimmers of the genera Anisops and Buenoa (Notonectidae) have high concentrations of hemoglobin in the large tracheal cells of the abdomen. Oxygen from the hemoglobin is delivered to a gas bubble and controls the buoyant density, which enables the bugs to maintain their position without swimming and to remain stationary in the mid-water zone where they hunt for prey. We have obtained the cDNA sequences of three Anisops deanei hemoglobin chains by RT-PCR and RACE techniques. The deduced amino acid sequences show an unusual insertion of a single amino acid in the conserved helix E, but this does not affect protein stability or ligand binding kinetics. Recombinant A. deanei hemoglobin has an oxygen affinity of P(50) = 2.4 kPa (18 torr) and reveals the presence of a dimeric fraction or two different conformations. The absorption spectra demonstrate that the Anisops hemoglobin is a typical pentacoordinate globin. Phylogenetic analyses show that the backswimmer hemoglobins evolved within Heteroptera and most likely originated from an intracellular hemoglobin with divergent function.

Oxygen binding properties of backswimmer (Notonectidae, Anisops) haemoglobin, determined in vivo

Aquatic backswimmers (Anisops spp.) collect oxygen from the atmosphere in order to breathe underwater, carrying it within a bubble of air on the ventral surface of their body and bound within haemoglobin-filled cells inside their abdomen. These oxygen stores are interconnected via the abdominal spiracles and the tracheal system. Fibre optic oxygen probes were used to measure PO(2) changes within the air bubbles of submerged backswimmers (Anisops deanei) and these measurements were transformed into in vivo haemoglobin-oxygen equilibrium curves (OECs) using a biotonometric approach. The haemoglobin displayed a triphasic, highly sigmoid OEC with a P(50) of 3.90 kPa. Comparisons made with a previous in vitro analysis of Anisops haemoglobin demonstrate that while the apparent cooperativity and oxygen affinity are considerably higher in vivo, both measurements share unusual Hb-O(2) binding characteristics. The affinity and cooperativity of the backswimmers' haemoglobin appears adaptive as it lengthens dives and promotes neutral buoyancy. While there are limitations associated with biotonometry, the in vivo OEC accurately represents the loading and unloading of biologically available oxygen within the backswimmers' haemoglobin cells. Potential errors associated with determining the OEC are small, as evaluated with sensitivity analyses in numerical models.