Air sacs are a key adaptive trait of the insect respiratory system.

Air sacs are a well-known aspect of insect tracheal systems, but have received little research attention. In this Commentary, we suggest that the study of the distribution and function of air sacs in tracheate arthropods can provide insights of broad significance. We provide preliminary phylogenetic evidence that the developmental pathways for creation of air sacs are broadly conserved throughout the arthropods, and that possession of air sacs is strongly associated with a few traits, including the capacity for powerful flight, large body or appendage size and buoyancy control. We also discuss how tracheal compression can serve as an additional mechanism for achieving advection in tracheal systems. Together, these patterns suggest that the possession of air sacs has both benefits and costs that remain poorly understood. New technologies for visualization and functional analysis of tracheal systems provide exciting approaches for investigations that will be of broad significance for understanding invertebrate evolution.

High carbohydrate consumption increases lipid storage and promotes migratory flight in locusts.

Migration allows animals to track favorable environments and avoid harmful conditions. However, migration is energetically costly, so migrating animals must prepare themselves by increasing their energy stores. Despite the importance of locust migratory swarms, we still understand little about the physiology of locust migration. During long-distance flight, locusts rely on lipid oxidation, despite the fact that lipids are relatively rare in their leaf-based diets. Therefore, locusts and other insect herbivores synthesize and store lipid from ingested carbohydrates, which are also important for initial flight. These data suggest that diets high in carbohydrate should increase lipid stores and the capacity for migratory flight in locusts. As predicted, locust lipid stores and flight performance increased with an increase in the relative carbohydrate content in their food. However, locust flight termination was not associated with complete lipid depletion. We propose potential testable mechanisms that might explain how macronutrient consumption can affect flight endurance.

Field bands of marching locust juveniles show carbohydrate, not protein, limitation.

Locusts are grasshoppers that migrate en masse and devastate food security, yet little is known about the nutritional needs of marching bands in nature. While it has been hypothesized that protein limitation promotes locust marching behavior, migration is fueled by dietary carbohydrates. We studied South American Locust (Schistocerca cancellata) bands at eight sites across Argentina, Bolivia, and Paraguay. Bands ate most frequently from dishes containing carbohydrate artificial diets and minimally from balanced, protein, or control (vitamins and salts) dishes-indicating carbohydrate hunger. This hunger for carbohydrates is likely explained by the observation that local vegetation was generally protein-biased relative to locusts' preferred protein to carbohydrate ratio. This study highlights the importance of studying the nutritional ecology of animals in their environment and suggests that carbohydrate limitation may be a common pattern for migrating insect herbivores.

The dynamics and the timeline of speciation in the gall-forming aphid Geoica spp. within and among Pistacia host tree species.

Trees of the genus Pistacia serve as obligate hosts for gall-forming aphids (Hemiptera, Aphididae, Fordini). Each aphid species induces a characteristic gall on a single Pistacia host species. The genus Geoica (Fordini) induce similar spherical closed galls on the lower side of the leaflet's midvein, on different Pistacia species. Two species of Pistacia trees that harbor Geoica galls grow naturally in Israel: P. palaestina and P. atlantica. We analyzed the phylogeny and the genetic structure of the Geoica species complex in Israel, and assessed the genetic differentiation and the level of host plant specificity of the aphids between P. atlantica and P. palaestina. We found that the splitting of the genus between P. atlantica and P. palaestina is estimated to have occurred 24-25 Ma (the Oligocene/Miocene boundary). Five different haplotypes suggesting five different species have been further speciating among Geoica spp., galling on P. atlantica, and an additional three species, on P. palaestina.

High carbohydrate diet ingestion increases post-meal lipid synthesis and drives respiratory exchange ratios above 1.

Locusts have been reported to elevate metabolic rate in response to high carbohydrate diets; this conclusion was based on metabolic rates calculated from CO2 production, a common practice for insects. However, respiratory exchange ratio (RER, CO2 production divided by O2 consumption) can rise above 1 as a result of de novo lipid synthesis, providing an alternative possible explanation of the prior findings. We studied the relationship between macronutrient ingestion, RER and lipid synthesis using South American locusts (Schistocerca cancellata) reared on artificial diets varying in protein:carbohydrate (p:c) ratio. RER increased and rose above 1 as dietary p:c ratio decreased. Lipid accumulation rates were strongly positively correlated with dietary carbohydrate content and ingestion. RERs above 1 were only observed for animals without food in the respirometry chamber, suggesting that hormonal changes after a meal may drive lipid synthesis. Schistocerca cancellata does not elevate metabolic rate on low p:c diets; in fact, the opposite trend was observed.

Plant carbohydrate content limits performance and lipid accumulation of an outbreaking herbivore.

Locusts are major intermittent threats to food security and the ecological factors determining where and when these occur remain poorly understood. For many herbivores, obtaining adequate protein from plants is a key challenge. We tested how the dietary protein : non-structural carbohydrate ratio (p : c) affects the developmental and physiological performance of 4th-5th instar nymphs of the South American locust, Schistocerca cancellata, which has recently resurged in Argentina, Bolivia and Paraguay. Field marching locusts preferred to feed on high carbohydrate foods. Field-collected juveniles transferred to the laboratory selected artificial diets or local plants with low p : c. On single artificial diets, survival rate increased as foods became more carbohydrate-biased. On single local plants, growth only occurred on the plant with the lowest p : c. Most local plants had p : c ratios substantially higher than optimal, demonstrating that field marching locusts must search for adequate carbohydrate or their survival and growth will be carbohydrate-limited. Total body lipids increased as dietary p : c decreased on both artificial and plant diets, and the low lipid contents of field-collected nymphs suggest that obtaining adequate carbohydrate may pose a strong limitation on migration for S. cancellata. Anthropogenic influences such as conversions of forests to pastures, may increase carbohydrate availability and promote outbreaks and migration of some locusts.

Physiological responses to gravity in an insect.

Gravity is one of the most ubiquitous environmental effects on living systems: Cellular and organismal responses to gravity are of central importance to understanding the physiological function of organisms, especially eukaryotes. Gravity has been demonstrated to have strong effects on the closed cardiovascular systems of terrestrial vertebrates, with rapidly responding neural reflexes ensuring proper blood flow despite changes in posture. Invertebrates possess open circulatory systems, which could provide fewer mechanisms to restrict gravity effects on blood flow, suggesting that these species also experience effects of gravity on blood pressure and distribution. However, whether gravity affects the open circulatory systems of invertebrates is unknown, partly due to technical measurement issues associated with small body size. Here we used X-ray imaging, radio-tracing of hemolymph, and micropressure measurements in the American grasshopper, Schistocerca americana, to assess responses to body orientation. Our results show that during changes in body orientation, gravity causes large changes in blood and air distribution, and that body position affects ventilation rate. Remarkably, we also found that insects show similar heart rate responses to body position as vertebrates, and contrasting with the classic understanding of open circulatory systems, have flexible valving systems between thorax and abdomen that can separate pressures. Gravitational effects on invertebrate cardiovascular and respiratory systems are likely to be widely distributed among invertebrates and to have broad influence on morphological and physiological evolution.

Respiratory gas levels interact to control ventilatory motor patterns in isolated locust ganglia.

Large insects actively ventilate their tracheal system even at rest, using abdominal pumping movements, which are controlled by a central pattern generator (CPG) in the thoracic ganglia. We studied the effects of respiratory gases on the ventilatory rhythm by isolating the thoracic ganglia and perfusing its main tracheae with various respiratory gas mixtures. Fictive ventilation activity was recorded from motor nerves controlling spiracular and abdominal ventilatory muscles. Both hypoxia and hypercapnia increased the ventilation rate, with the latter being much more potent. Sub-threshold hypoxic and hypercapnic levels were still able to modulate the rhythm as a result of interactions between the effects of the two respiratory gases. Additionally, changing the oxygen levels in the bathing saline affected ventilation rate, suggesting a modulatory role for haemolymph oxygen. Central sensing of both respiratory gases as well as interactions of their effects on the motor output of the ventilatory CPG reported here indicate convergent evolution of respiratory control among terrestrial animals of distant taxa.

Intricate but tight coupling of spiracular activity and abdominal ventilation during locust discontinuous gas exchange cycles.

Discontinuous gas exchange (DGE) is the best studied among insect gas exchange patterns. DGE cycles comprise three phases, which are defined by their spiracular state: closed, flutter and open. However, spiracle status has rarely been monitored directly; rather, it is often assumed based on CO2 emission traces. In this study, we directly recorded electromyogram (EMG) signals from the closer muscle of the second thoracic spiracle and from abdominal ventilation muscles in a fully intact locust during DGE. Muscular activity was monitored simultaneously with CO2 emission, under normoxia and under various experimental oxic conditions. Our findings indicate that locust DGE does not correspond well with the commonly described three-phase cycle. We describe unique DGE-related ventilation motor patterns, coupled to spiracular activity. During the open phase, when CO2 emission rate is highest, the thoracic spiracles do not remain open; rather, they open and close rapidly. This fast spiracle activity coincides with in-phase abdominal ventilation, while alternating with the abdominal spiracle and thus facilitating a unidirectional air flow along the main trachea. A change in the frequency of rhythmic ventilation during the open phase suggests modulation by intra-tracheal CO2 levels. A second, slow ventilatory movement pattern probably serves to facilitate gas diffusion during spiracle closure. Two flutter-like patterns are described in association with the different types of ventilatory activity. We offer a modified mechanistic model for DGE in actively ventilating insects, incorporating ventilatory behavior and changes in spiracle state.

An experimental evolution study confirms that discontinuous gas exchange does not contribute to body water conservation in locusts.

The adaptive nature of discontinuous gas exchange (DGE) in insects is contentious. The classic 'hygric hypothesis', which posits that DGE serves to reduce respiratory water loss (RWL), is still the best supported. We thus focused on the hygric hypothesis in this first-ever experimental evolution study of any of the competing adaptive hypotheses. We compared populations of the migratory locust (Locusta migratoria) that underwent 10 consecutive generations of selection for desiccation resistance with control populations. Selected locusts survived 36% longer under desiccation stress but DGE prevalence did not differ between these and control populations (approx. 75%). Evolved changes in DGE properties in the selected locusts included longer cycle and interburst durations. However, in contrast with predictions of the hygric hypothesis, these changes were not associated with reduced RWL rates. Other responses observed in the selected locusts were higher body water content when hydrated and lower total evaporative water loss rates. Hence, our data suggest that DGE cycle properties in selected locusts are a consequence of an evolved increased ability to store water, and thus an improved capacity to buffer accumulated CO2, rather than an adaptive response to desiccation. We conclude that DGE is unlikely to be an evolutionary response to dehydration challenge in locusts.

Discontinuous gas-exchange cycle characteristics are differentially affected by hydration state and energy metabolism in gregarious and solitary desert locusts.

The termination of discontinuous gas exchange cycles (DGCs) in severely dehydrated insects casts doubt on the generality of the hygric hypothesis, which posits that DGCs evolved as a water conservation mechanism. We followed DGC characteristics in the two density-dependent phases of the desert locust Schistocerca gregaria throughout exposure to an experimental treatment of combined dehydration and starvation stress, and subsequent rehydration. We hypothesized that, under stressful conditions, the more stress-resistant gregarious locusts would maintain DGCs longer than solitary locusts. However, we found no phase-specific variations in body water content, water loss rates (total and respiratory) or timing of stress-induced abolishment of DGCs. Likewise, locusts of both phases re-employed DGCs after ingesting comparable volumes of water when rehydrated. Despite comparable water management performances, the effect of exposure to stressful experimental conditions on DGC characteristics varied significantly between gregarious and solitary locusts. Interburst duration, which is affected by the ability to buffer CO2, was significantly reduced in dehydrated solitary locusts compared with gregarious locusts. Moreover, despite similar rehydration levels, only gregarious locusts recovered their initial CO2 accumulation capacity, indicating that cycle characteristics are affected by factors other than haemolymph volume. Haemolymph protein measurements and calculated respiratory exchange ratios suggest that catabolism of haemolymph proteins may contribute to a reduced haemolymph buffering capacity, and thus a compromised ability for CO2 accumulation, in solitary locusts. Nevertheless, DGC was lost at similar hydration states in the two phases, suggesting that DGCs are terminated as a result of inadequate oxygen supply to the tissues.

The effect of discontinuous gas exchange on respiratory water loss in grasshoppers (Orthoptera: Acrididae) varies across an aridity gradient.

The significance of discontinuous gas-exchange cycles (DGC) in reducing respiratory water loss (RWL) in insects is contentious. Results from single-species studies are equivocal in their support of the classic 'hygric hypothesis' for the evolution of DGC, whereas comparative analyses generally support a link between DGC and water balance. In this study, we investigated DGC prevalence and characteristics and RWL in three grasshopper species (Acrididae, subfamily Pamphaginae) across an aridity gradient in Israel. In order to determine whether DGC contributes to a reduction in RWL, we compared the DGC characteristics and RWL associated with CO2 release (transpiration ratio, i.e. the molar ratio of RWL to CO2 emission rates) among these species. Transpiration ratios of DGC and continuous breathers were also compared intraspecifically. Our data show that DGC characteristics, DGC prevalence and the transpiration ratios correlate well with habitat aridity. The xeric-adapted Tmethis pulchripennis exhibited a significantly shorter burst period and lower transpiration ratio compared with the other two mesic species, Ocneropsis bethlemita and Ocneropsis lividipes. However, DGC resulted in significant water savings compared with continuous exchange in T. pulchripennis only. These unique DGC characteristics for T. pulchripennis were correlated with its significantly higher mass-specific tracheal volume. Our data suggest that the origin of DGC may not be adaptive, but rather that evolved modulation of cycle characteristics confers a fitness advantage under stressful conditions. This modulation may result from morphological and/or physiological modifications.

Scorpion speciation in the Holy Land: Multilocus phylogeography corroborates diagnostic differences in morphology and burrowing behavior among Scorpio subspecies and justifies recognition as phylogenetic, ecological and biological species.

Scorpio Linnaeus, 1758 (family Scorpionidae Latreille, 1802) was considered monotypic for over a century, and comprised a single species, Scorpio maurus Linnaeus, 1758, with 19 subspecies, distributed from West Africa, throughout the Maghreb and the Middle East, to Iran. Two parapatric subspecies, Scorpio maurus fuscus (Ehrenberg, 1829) and Scorpio maurus palmatus (Ehrenberg, 1828), have long been recognized in the eastern Mediterranean region. We examined morphological variation, burrow architecture and genetic divergence among 39 populations across the distribution of the two subspecies to assess whether they are conspecific and, if not, how many species might be involved. Cuticle coloration, pedipalp chela digital carina condition, and selected measurements were recorded. Sixty burrows were excavated and examined for burrow structure and depth. A multilocus dataset comprising concatenated fragments of one nuclear (28S rDNA) and three mitochondrial (12S rDNA, 16S rDNA, Cytochrome c Oxidase Subunit I) loci, totaling ca. 2400 base-pairs, was produced for 41 individuals, and a single-locus dataset comprising 658 base-pairs of the COI locus for 156 individuals. Despite overlapping ranges in morphometric characters of pedipalp chela shape, the putative subspecies were easily distinguished by cuticle coloration and condition of the pedipalp chela digital carina, and were also found to differ significantly in burrow architecture and depth. Phylogeographical analyses of the COI and multilocus datasets recovered seven distinct clades. Separate analyses of mitochondrial sequences, and combined analyses of mitochondrial and nuclear sequences support most clades. The two major clades corresponded with the geographical distributions of S. m. fuscus and S. m. palmatus in the region. Specimens from these clades were genetically distinct, and exhibited different burrow structure in geographically-proximate localities, suggesting reproductive isolation. The palmatus clade included two distinct subclades of specimens from localities adjacent to the Dead Sea. Three other clades, comprising specimens from the most northeastern localities, were tentatively assigned to subspecies previously recorded in neighboring Jordan and Syria. The morphological, behavioral and genetic evidence supports previous suggestions that Scorpio maurus is a species complex and justifies the following taxonomic emendations: Scorpio fuscus (Ehrenberg, 1829), stat. nov.; Scorpio kruglovi Birula, 1910, stat. nov.; Scorpio palmatus (Ehrenberg, 1828), stat. nov.; Scorpio propinquus (Simon, 1872), stat. nov.

Anticipatory and reactive crouching of pea aphids in response to environmental perturbations.

Animals use different strategies to deal with changing environmental conditions. While standing and feeding on their host plant, aphids (Hemiptera: Aphididae) may be exposed to detrimental environmental perturbations, such as strong winds. If aphids are forcibly blown off the plant and spend time on the ground, they will face additional dangers by both ground-dwelling predators and detrimental soil temperature. It is therefore adaptive for aphids to behave in a way that lowers the risk of being removed from the plant. We observed that pea aphids (Acyrthosiphon pisum (Harris)) display a specific crouched body posture, previously undescribed, which reduces their chance of being carried off from the plant by sudden winds. We exposed aphids in the laboratory to different cues indicative of a windy environment: wind, plant vibration, and visual stimuli. We found that aphids crouch in two situations: 1) reactively, when they are being pulled by a continuous gust of wind threatening to dislodge them. 2) Anticipatorily, when environmental cues, such as plant vibration or continuous movement near their host plant, may signify that sudden wind gusts are expected. Crouching aphids were less likely to be dislodged by a sudden air stream or plant vibration than were aphids that did not crouch. Crouching thus improves the aphids' chances of remaining on their host plant under unfavorable environmental conditions.