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The tracheae (T; only a subset labeled) of a small (2.2 g, day 2) fifth instar Manduca sexta caterpillar, exposed via dissection and saline wash. The tracheae are seen as the white-silver branching tubules extending from bundles along the outer body wall. The bundles originate at the spiracle (S; only one labeled) openings and are interconnected by longitudinally running tracheal branches (L; only one labeled). The tan central structure running the length of the larva is the gut (G), which is heavily tracheated. A clear tubular structure in the upper left portion of the picture is a Malphigian tubule (M), and is not a component of the tracheal system.
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Organisms must accommodate oxygen delivery to developing tissues as body mass increases during growth. In insects, the growth of the respiratory system has been assumed to occur only when it molts, whereas body mass and volume increase during the larval stages between molts. This decouples whole body growth from the growth of the oxygen supply syst...
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... increases in tracheal mass and volume within the instar were mirrored by morphological differences between large and small fifth instar caterpillars (Fig. 4, supplementary material Fig. S1). Though the fine details of the tracheal images are difficult to compare quantitatively, there appears to be more tracheae in all body segments in the larger caterpillar than in the small (supplementary material Fig. S1). When comparing the details specifically between homologous sets of tracheae from large and small caterpillars, there appear to be differences in the morphology of the terminal ends of visible tracheae rather than the major tracheal trunks more proximal to the spiracle (Fig. ...
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... increases in tracheal mass and volume within the instar were mirrored by morphological differences between large and small fifth instar caterpillars (Fig. 4, supplementary material Fig. S1). Though the fine details of the tracheal images are difficult to compare quantitatively, there appears to be more tracheae in all body segments in the larger caterpillar than in the small (supplementary material Fig. S1). When comparing the details specifically between homologous sets of tracheae from large and small caterpillars, there appear to be differences in the morphology of the terminal ends of visible tracheae rather than the major tracheal trunks more proximal to the spiracle (Fig. ...
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... all insects, oxygen delivery occurs through a ramifying series of hollow tubules, called tracheae and tracheoles, which branch inward from external openings in the exoskeleton called spiracles ( Fig. 1) (Chapman, 1998). Tracheae develop embryonically from invaginations of the epidermal tissue of the exoskeleton, subsequently decreasing in diameter with each branching so that the smallest tracheoles supply oxygen directly to tissues and cells (Chapman, 1998;Affolter and Caussinus, 2008;Samakovlis et al., 1996). Tracheae and tracheoles develop by distinct developmental mechanisms that drive branching tubulogenesis during embryogenesis and later as a consequence of oxygen deprivation signaling ( Affolter et al., 2003;Uv et al., 2003;Samakovlis et al., 1996;Affolter and Caussinus, 2008;Centanin et al., 2008;Centanin et al., 2010). For simplicity, we collectively refer to the entire tracheal system as 'tracheae', though there are systematic physical and structural differences as tracheae branch and decrease in ...
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Body size and development time are important life history traits because they are often highly correlated with fitness. Although the developmental mechanisms that control growth have been well studied, the mechanisms that control how a species-characteristic body size is achieved remain poorly understood. In insects adult body size is determined by...
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... Moreover, the positive impact of honey and lemon juice on silkworm growth is substantiated by related studies (Thulasi and Sivaprasad, 2015;Madhavi et al., 2018, Madhavi andSiva Prasad, 2022). The study of Helm and Davidowitz (2013) demonstrated that insect structural growth in the tracheal system is positively inuenced by larval nutrition and that the caterpillars reared on a quality nutrient diet invested relatively more mass into their tracheal systems and that the expansion in tracheation occurs in a sizedependent manner. It is also known that the high quality diet can lead to an optimization pattern in growth in tracheal system vis-à-vis growth in oxygen supply and increased metabolic demand (Harrison and Haddad, 2011). ...
... Body mass utilization continues through larval, pupal and adult stages for maintenance and metabolism. The highest growth rates observed in LBW, LBL and LBPM of silkworm during larval instars are indicative of increased tracheal expansion that meets the increased energy demands during larval-larval transformation as observed in many other lepidopteran insects(Centanin et al., 2010;Helm and Davidowitz, 2013;Nijhout and Callier, 2015). Further, the body perimeter is the only growth variable that shows positive growth uniformly in all the three stages (larval, pupal and adult) indicating increased mass accumulation in the horizontal plane of the body. ...
Synergistic impact of honey and lemon juice-enriched mulberry diets on the growth of Bombyx mori was studied. The body size progressively increased during larval stage, but declined during pupal and adult stages. The nutrient diets promoted larval growth and positively modulated pupal and adult growth. The larval growth curves are typical Gompertz trajectories that reected the growth promoting nature of honey and lemon juice. The log-based growth curves were used to derive critical larval body size determinants that control molting and metamorphosis. The nutrient diets improved critical body size determinants without affecting their time schedules. The compound periodical growth rates showed instar-specic and stage-specic variations. The size specic growth rates in body mass, length and perimeter dimensions indicated the prevalence of an effective mass management mechanism as dened in the Hutchinson's investment principle. The silkworm recorded either higher or lower growth ratios indicating deviation from the Dyar's constancy rule.
... Despite progressive increase in body mass from the third through fth instar, the overall larval growth trajectory of silkworm (Fig. 2), deviates from the exponential growth path and its growth exponent declines at the end of each instar, much like that in Manduca sexta (Sears et al., 2012). Though, the anatomical and physiological bases of body mass growth in silkworm are not examined in the present investigation, the principle is attributable to the selection pressure caused by emerging energy demands, tracheal expansions, fat body depositions and other structural components required for post-embryonic development, silk production and metabolic acceleration (Greenlee et al., 2013;Helm and Davidowitz, 2013;Nijhout and Callier, 2015). Obviously, the larval silkworm augments its respiratory structures during growth by continuously increasing the volume and delivery capacity of the tracheae during larval growth as it is crucial for meeting the energy needs of aerobic metabolism during larval growth and attainment of optimal body size. ...
Quantity-wise and rate-wise growth changes in the larval body mass vis-à-vis log-based growth curves were analyzed in Bombyx mori. The body mass accumulation followed a progressively increasing trend from third to fth instar, registering higher values in growth index (GI) and mean growth rate (MGR). But, the corresponding compound periodical growth rates and mean specic growth rates declined remarkably during the same period. The growth increments in body mass are in consonance with Hutchinson's investment policy and accordingly the silkworm accumulates energy reserves and structures that are necessary for future use during metamorphosis. Higher growth ratios recorded in the present study indicate that the silkworm systematically violates the Dyar's constant rule throughout the growth regime. The logbased growth curves were further analyzed to determine the critical and threshold body sizes that trigger moulting and metamorphosis in the silkworm.
... Lengthening of the tracheal system during body growth does not lead to a reduced gas conductance (Harrison et al., 2014;Lease et al., 2012). However, body mass growth within an insect instar leads to a continuous increase in body tissue mass whereas growth of the exoskeleton lined tracheal tubes is limited to discrete moulting events (Callier & Nijhout, 2011;Chapman, 1998;2013;Klowden, 2007) but see (Helm and Davidowitz, 2013). Consequently, the growing volume of the body tissues compresses the air-filled tracheal system (Lease, 2006) leading to lower tracheal conductance and higher values of CPO2 (Greenlee, 2004). ...
... Lengthening of the tracheal system during body growth does not lead to a reduced gas conductance (Harrison et al., 2014;Lease et al., 2012). However, body mass growth within an insect instar leads to a continuous increase in body tissue mass whereas growth of the exoskeleton lined tracheal tubes is limited to discrete moulting events (Callier & Nijhout, 2011;Chapman, 1998;2013;Klowden, 2007) but see (Helm and Davidowitz, 2013). Consequently, the growing volume of the body tissues compresses the air-filled tracheal system (Lease, 2006) leading to lower tracheal conductance and higher values of CPO2 (Greenlee, 2004). ...
The mango stem borer, Batocera rufomaculata, was accidently introduced into Israel during the 1950's, quickly becoming a serious threat to domesticated and wild Fig trees. A previous study in our lab showed that smaller adult B. rufomaculata that developed from malnourished larvae inside dead host trees, exhibited elevated flight performance compared to larger adults emerging from larvae developing in living host trees. The smaller conspecifics were shown to be capable of flying for longer periods and of covering larger distances. This finding contradicts the classical scaling of cost-of-transport (CoT), where larger animals spend relatively less energy on locomotion compared to smaller ones. The CoT model may not be true when looking at an intraspecific variation in body size, where specimens may adapt mechanisms to alter, or even elevate their locomotion performance according to their life history. Since B. rufomaculata larvae develop, and adults feed and lay eggs, in fig trees the elevated flight capacity of beetles developing in dead trees is likely instrumental in dispersal flight to find a new host tree. In my PhD research I attempted to unveil the mechanisms behind the relationship between body size and prolonged (dispersal) flight in B. rufomaculata. I combined biomechanics analysis, respirometry and telemetry in order to test four hypotheses concerning the ability of smaller beetles to fly better than larger ones.
The first hypothesis was that smaller individuals of B. rufomaculata differ in their forward flight mechanics and aerodynamics compared to larger conspecifics resulting in a more energetically efficient flight. To test this hypothesis, I flew beetles tethered in a wind tunnel and extracted the flapping kinematics of individuals varying in body mass (1-7 gr). From the flapping kinematics I estimated the unsteady aerodynamic forces and mechanical power output required for flight using a quasi-steady blade-element model. I then compared the calculated muscle mass-specific power-output between the small and large beetles. The muscle mass specific power did not differ between small and large individuals flying at the same wind (flight) speed in the wind tunnel. Consequently, the CoT of B. rufomaculata remains similar despite the changes in body mass.
The second hypothesis was that flight metabolic rate (FMR) during forward flight in B. rufomaculata scales with body size differently in small and large beetles making flight less costly for smaller individuals. I tested this hypothesis by measuring the flight metabolic rate of the beetles during tethered and free flight, using the ‘Bolus Injection of Isotopic 13Carbon technique’. I found that flight metabolic rate during free-flight scales with body mass to the power of 0.53, suggesting that larger beetles spend relatively less energy during flight.
The third hypothesis was that the respiratory system of smaller individuals is more effective in delivering oxygen to active muscle tissue enabling them to engage in longer flights. This hypothesis was tested by measuring the tracheal volume and oxygen conductance as a function of body size, sex, and reproductive state. Tracheal volume was measured using the water displacement method and Oxygen conductance was measured using the Critical Partial Oxygen Pressure (CPO2) methods. I found that the tracheal volume scales with body mass to the power of 1.4 in both sexes, however the tracheal volume of gravid females was significantly reduced compared to males and non-gravid females. Surprisingly, the maximal oxygen conductance of the tracheal system was found to be higher in both gravid and non-gravid females compared to males. This elevated conductance may have developed in females to compensate for the decrease in tracheal volume during the development of the female reproductive system.
Finally, I hypothesised that smaller individuals would fly more frequently and larger distances in their natural habitat compared to larger conspecifics. I tested this hypothesis using radio telemetry to monitor the natural movements of the beetles in the field. Miniature radio transmitters were attached to beetles collected in the field and the position of the instrumented beetles was monitored daily. I found that 21% of the tracked beetles never left their host tree and those that did, flew relatively short distances (20 – 900 m). This may be due to the abundance and density of fig trees in the study site, making long distance dispersal flight unnecessary. Among the beetles that moved between host trees only males engaged in daily flights of over 100 m. Some of the tracked beetles disappeared from the study site suggesting that occasional flights outside the range of the radio receiver (~2km) might have occurred. To test this option, I tracked male beetles released in a second study site with no host trees nearby. The males tended to fly in short bouts lasting on average 58 ± 14 s and reaching a mean distance of ~200 m in each flight. Therefore, B. rufomaculata has the capacity to disperse several hundreds of meters daily in a sequence of several short flights.
Overall, the results suggest that smaller conspecifics of B. rufomaculata compensate for the decrease in flight efficiency that is associated with the decrease in their body mass. This, however, does not explain the observation that smaller conspecifics engage in longer flights in flight-mills. The explanation may involve behavioural rather than physiological intraspecific differences. Therefore, future studies may find it interesting to test the inner tendency and environmental cues affecting the dispersal flight of the beetles and its duration.
The life history of B. rufomaculata makes it a compelling research model to explore how environmental conditions at larval development affect the flight performance of the dispersing adult.
... During larval growth, the increase in body tissue mass is continuous, but adjustment in the dimensions of the tracheae is limited to the tracheoles (Helm and Davidowitz, 2013;Manning and Krasnow, 1993;Wigglesworth, 1954), whereas the remaining exoskeleton-lined trachea, where convection takes place, can only grow during molt. Indeed, tracheal volume increases hypermetrically with body mass between instars, but decreases with body mass increase within instar (Callier and Nijhout, 2011;Lease et al., 2006). ...
In 1951, a paper authored by the late August Krogh and his then student, Torkel Weis-Fogh, appeared in the Journal of Experimental Biology. It described a study on oxygen consumption and CO2 emission rates in tethered locusts before, during and after flight. It showed that flight metabolic rate was 15-50 fold the resting rate, and that locusts utilize lipids as fuel for their prolonged flight (Krogh and Weis-Fogh, 1951). The attentive reader will find that Krogh and Weis-Fogh write (p. 345): “Female locusts, under laboratory conditions, were disinclined to fly when they were loaded with eggs, and so only males were used”. Why were gravid females reluctant to fly?
In this review, we aim to propose that the increasing volume of the developing eggs compresses the tracheal system of gravid females leading to reduced capacity for active ventilation and therefore a limitation on flight performance. We explore the available evidence in support of this theory and point-out the male-bias in studies examining respiration during flight in insects.
... As insects grow, their body mass increases exponentially and as tissue is being added, it is expected that the cost of maintaining these tissues, reflected in the resting metabolic rate (rmr), increases as well (Greenlee and Harrison, 2005;Kivelä et al., 2016). To increase oxygen supply, insects replace the larger components of the tracheal air supply system during moulting (Callier and Nijhout, 2011) and also show increases in smaller tracheal structures during the inter-moult period of growth (Helm and Davidowitz, 2013;Wigglesworth, 1954). Still, how well the overall size of the oxygen supply system matches tissue growth in developing insects remains surprisingly unclear (Callier and Nijhout, 2011;Snelling et al., 2011a). ...
... Studies of metabolic supply have used several methods to estimate tracheal capacity. These have included light microscopy (Miller, 1966;Wigglesworth, 1930;Snelling et al., 2011b), volume displacement measures (Helm and Davidowitz, 2013), confocal microscopy (Harrison et al., 2018a) and electron microscopy (Snelling et al., 2011c). Lately Xray micro-tomography (µCT) has become an additional tool with which to study tracheal networks (Aitkenhead et al., 2020;Alba-Tercedor et al., 2019;Greenlee et al., 2009;Harrison et al., 2018a;Javal et al., 2019;Raś et al., 2018;Shaha et al., 2013;Wasserthal et al., 2018). ...
... The species was chosen for the current study due to its large size gradient (with some individuals reaching in excess of 12 g), but the large variation among individuals in growth rates suggests it is not perhaps an optimal model for the study of developmental physiology. While we therefore cannot make meaningful interpretation of within-instar developmental constraints (Helm and Davidowitz, 2013), we can investigate the relationship between metabolic demand and supply across the entire size range of our dataspanning at least three instars of development (Callier and Nijhout, 2011;Greenlee and Harrison, 2005). ...
How respiratory structures vary with, or are constrained by, an animal’s environment is of central importance to diverse evolutionary and comparative physiology hypotheses. To date, quantifying insect respiratory structures and their variation has remained challenging due to their microscopic size, hence only a handful of species have been examined. Several methods for imaging insect respiratory systems are available, in many cases however, the analytical process is lethal, destructive, time consuming and labour intensive. Here, we explore and test a different approach to measuring tracheal volume using X-ray micro-tomography (µCT) scanning (at 15 µm resolution) on living, sedated larvae of the cerambycid beetle Cacosceles newmannii across a range of body sizes at two points in development. We provide novel data on resistance of the larvae to the radiation dose absorbed during µCT scanning, repeatability of imaging analyses both within and between time-points and, structural tracheal trait differences provided by different image segmentation methods. By comparing how tracheal dimension (reflecting metabolic supply) and basal metabolic rate (reflecting metabolic demand) increase with mass, we show that tracheal oxygen supply capacity increases during development at a comparable, or even higher rate than metabolic demand. Given that abundant gas delivery capacity in the insect respiratory system may be costly (due to e.g. oxygen toxicity or space restrictions), there are probably balancing factors requiring such a capacity that are not linked to direct tissue oxygen demand and that have not been thoroughly elucidated to date, including CO2 efflux. Our study provides methodological insights and novel biological data on key issues in rapidly quantifying insect respiratory anatomy on live insects.
... To prevent caterpillar suffocation underwater, it should possess a kind of a plastron for effective CO 2 exchange. Assuming that the volume of the tracheal system in M. castrensis caterpillars is similar to that previously studied in Manduca sexta (Lepidoptera: Sphingidae), which is ∼7.5% of the caterpillar volume (Helm and Davidowitz, 2013), we can approximate the tracheal volume for our caterpillars as 72 µl. A plastron is obviously present in M. castrensis caterpillars underwater. ...
The moth Malacosoma castrensis (Lasiocampidae) is commonly found along the Northern Germany coasts whose habitat is mainly represented by salt marshes subjected to sea level variations. Surprisingly, terrestrial caterpillars can withstand many hours being flooded by the seawater. The ability to withstand periods of submersion in a terrestrial insect raises the problem of respiration related to avoiding water percolation into the tracheal system. In the present study, we investigated under laboratory conditions the role of water-repellent cuticle structures in oxygen supply in caterpillars of M. castrensis submerged in water. For this purpose, air-layer stability tests using force measurements, and micromorphology of cuticle structures using SEM and fluorescence microscopy were performed.
A plastron appeared when a caterpillar is under water. Plastron stability, its’ gasses composition, and internal pressure were estimated. The plastron is stabilized by long and rare hairs, which are much thicker than the corresponding hairs of aquatic insects. Thick and stiff hairs with sclerotized basal and middle regions protrude into the water through plastron – water interface, while substantial regions of thin and flexible hairs are aligned along the plastron – water interface and their side walls can support pressure in plastron even below atmospheric pressure. Additional anchoring points between hair's stalk and microtrichia near to the hair base provide enhanced stiffness to the hair layer and prevent hair layer from collapse and water entering between hairs. Advancing contact angle on hairs is more than 90°, which is close to the effective contact angle for the whole caterpillar.
... volume of the tracheal system). This mismatch arises because the tracheal system principally grows at moults (Lundquist, Kittilson, Ahsan, & Greenlee, 2018; but see Helm & Davidowitz, 2013), whereas tissue mass increases between the moults. Consequently, moulting would ultimately be necessary to build a larger respiratory system before oxygen deficiency becomes too severe. ...
... Within-instar tracheal growth would similarly postpone the point at which oxygen demand surpasses supply, and would thus allow a larva to grow more during the instar. Interestingly, in the sphingid M. sexta, tracheal size increases within the final instar (Callier & Nijhout, 2011;Helm & Davidowitz, 2013) but not within earlier instars (Callier & Nijhout, 2011;Lundquist et al., 2018), which could partially explain the presently observed positive RMIS in Sphingidae. Whether the same mechanism could also explain the last-instar increase in RMI in Drepanidae and Geometridae remains unknown. ...
Juvenile growth trajectories evolve via the interplay of selective pressures on age and size at maturity, and developmental constraints. In insects, the moulting cycle is a major constraint on larval growth trajectories.
Surface area to volume ratio of a larva decreases during growth, so renewal of certain surfaces by moulting is likely needed for the maintenance of physiological efficiency. A null hypothesis of isometry, implied by Dyar's Rule, would mean that the relative measures of growth remain constant across moults and instars.
We studied ontogenetic changes and allometry in instar‐specific characteristics of larval growth in 30 lepidopteran species in a phylogenetic comparative framework.
Relative instar‐specific mass increments (RMI) typically, but not invariably, decreased across instars. Ontogenetic change in RMIs varied among families with little within‐family variation. End‐of‐instar growth deceleration (GD) became stronger with increasing body size across instars. Across‐instar change in GD was conserved across taxa. Ontogenetic allometry was generally non‐isometric in both RMI and GD.
Results indicate that detailed studies on multiple species are needed for generalizations concerning growth trajectory evolution. Developmental and physiological mechanisms affecting growth trajectory evolution show different degrees of evolutionary conservatism, which must be incorporated into models of age and size at maturation.
... This approach assumes no variance in the theoretical slope value but employs the estimated slope and standard error of the empirical data (Zar, 1997). However, while scaling patterns of respiratory structures are expected to vary among tracheated arthropod species, they may also be highly variable across and within stages of the same species due to the discontinuous growth of the tracheal system and, in some cases, complex life histories (e.g., occurrence of diapause) (Helm and Davidowitz, 2013;Vogt and Dillon, 2013;Greenlee et al., 2014). Therefore, no specific allometry model or anatomical scaling hypothesis has been adequately developed yet regarding the scaling relationship of the tracheal volume in this study with body size to test, and we instead sought here to statistically compare the estimated scaling slopes (regression exponents) with other scaling values commonly reported in the literature across a wide range of species (Chown et al., 2007) and between the two main lifestages investigated. ...
Temperature has a profound impact on insect fitness and performance via metabolic, enzymatic or chemical reaction rate effects. However, oxygen availability can interact with these thermal responses in complex and often poorly understood ways, especially in hypoxia-adapted species. Here we test the hypothesis that thermal limits are reduced under low oxygen availability – such as might happen when key life-stages reside within plants – but also extend this test to attempt to explain that the magnitude of the effect of hypoxia depends on variation in key respiration-related parameters such as aerobic scope and respiratory morphology. Using two life-stages of a xylophagous cerambycid beetle, Cacosceles (Zelogenes) newmannii we assessed oxygen-limitation effects on metabolic performance and thermal limits. We complement these physiological assessments with high-resolution 3D (micro-computed tomography scan) morphometry in both life-stages. Results showed that although larvae and adults have similar critical thermal maxima (CTmax) under normoxia, hypoxia reduces metabolic rate in adults to a greater extent than it does in larvae, thus reducing aerobic scope in the former far more markedly. In separate experiments, we also show that adults defend a tracheal oxygen (critical) setpoint more consistently than do larvae, indicated by switching between discontinuous gas exchange cycles (DGC) and continuous respiratory patterns under experimentally manipulated oxygen levels. These effects can be explained by the fact that the volume of respiratory anatomy is positively correlated with body mass in adults but is apparently size-invariant in larvae. Thus, the two life-stages of C. newmannii display key differences in respiratory structure and function that can explain the magnitude of the effect of hypoxia on upper thermal limits.
... Still, many terrestrial animals pass through immature stages in which oxygen supply depends on a poorly developed respiratory system (Woods and Hill 2004;Smith et al. 2015). For example, the total metabolic demand of grasshoppers (Schistocerca americana) and caterpillars (Manduca sexta) increases by approximately 50% during a single instar, without a proportional increase in the size of the tracheal system (Kirkton et al. 2012;Helm and Davidowitz 2013;Lundquist et al. 2018). As a result, insects at the end of the instar are more susceptible to oxygen limitation than those at the beginning (Greenlee and Harrison 2004;Harrison et al. 2005;Callier and Nijhout 2011;Kivelä et al. 2016). ...
... We expected oxygen supply to limit the heat tolerance of grasshoppers more severely at the end of the instar, given their proportionally smaller tracheal systems Harrison 2004, 2005;Harrison et al. 2005;Kirkton et al. 2012;Helm and Davidowitz 2013). This hypothesis stems from three assumptions: ...
Thermal physiology changes as organisms grow and develop, but we do not understand what causes these ontogenetic shifts. According to the theory of oxygen- and capacity-limited thermal tolerance, an organism’s heat tolerance should change throughout ontogeny as its ability to deliver oxygen varies. As insects grow during an instar, their metabolic demand increases without a proportional increase in the size of tracheae that supply oxygen to the tissues. If oxygen delivery limits heat tolerance, the mismatch between supply and demand should make insects more susceptible to heat and hypoxia as they progress through an instar. We tested this hypothesis by measuring the heat tolerance of grasshoppers (Schistocerca americana) on the second and seventh days of the 6th instar, in either a normoxic or hypoxic atmosphere (21% or 10% oxygen, respectively). As expected, heat tolerance decreased as grasshoppers grew larger. Yet contrary to expectation, hypoxia had no effect on heat tolerance across all stages and sizes. Although heat tolerance declines as grasshoppers grow, this pattern must stem from a mechanism other than oxygen limitation.
... Our combined results identify the trachea and the IPCs as two targets of 20E action to regulate insulin signalling in the third larval instar. The trachea and oxygen supply had previously been shown in the tobacco hornworm Manduca sexta and in Drosophila to be important determinants of growth and metamorphosis (Callier and Nijhout, 2011;Callier et al., 2013;Helm and Davidowitz, 2013). The IIS phenotypes we observe when perturbing 20E signalling using Dilp2-GAL4 R during L3 (Fig. 3D-G) were reminiscent of those previously reported for hypoxic animals, i.e. reduced Dilp3 and Dilp5 transcription and a retention of Dilp2 (Wong et al., 2014). ...
Growth and maturation are coordinated processes in all animals. Integration of internal cues, such as signalling pathways, with external cues such as nutritional status is paramount for an orderly progression of development in function of growth. In Drosophila, this involves insulin and steroid signalling, but the underlying mechanisms and their coordination are incompletely understood. We show that bioactive 20-hydroxyecdysone production by the enzyme Shade in the fat body is a nutrient-dependent process. We demonstrate that under fed conditions, Shade plays a role in growth control. We identify the trachea and the insulin-producing cells in the brain as direct targets through which 20-hydroxyecdysone regulates insulin-signaling. The identification of the trachea-dependent regulation of insulin-signaling exposes an important variable that may have been overlooked in other studies focusing on insulin-signaling in Drosophila Our findings provide a potentially conserved, novel mechanism by which nutrition can modulate steroid hormone bioactivation, reveal an important caveat of a commonly used transgenic tool to study IPC function and yield further insights as to how steroid and insulin signalling are coordinated during development to regulate growth and developmental timing.
