True Flight Research Articles

Organization of projections from the entorhinal cortex to the hippocampal formation of the Egyptian fruit bat Rousettus aegyptiacus.

The hippocampal formation and entorhinal cortex are crucially involved in learning and memory as well as in spatial navigation. The conservation of these structures across the entire mammalian lineage demonstrates their importance. Information on a diverse set of spatially tuned neurons has become available, but we only have a rudimentary understanding of how anatomical network structure affects functional tuning. Bats are the only order of mammals that have evolved true flight, and with this specialization comes the need to navigate and behave in a three dimensional (3D) environment. Spatial tuning of cells in the entorhinal-hippocampal network of bats has been studied for some time, but whether the reported tuning in 3D is associated with changes in the entorhinal-hippocampal network is not known. Here we investigated the entorhinal-hippocampal projections in the Egyptian fruit bat (Rousettus aegyptiacus), by injecting chemical anterograde tracers in the entorhinal cortex. Detailed analyses of the terminations of these projections in the hippocampus showed that both the medial and lateral entorhinal cortex sent projections to the molecular layer of all subfields of the hippocampal formation. Our analyses showed that the terminal distributions of entorhinal fibers in the hippocampal formation of Egyptian fruit bats-including the proximo-distal and longitudinal topography and the layer-specificity-are similar to what has been described in other mammalian species such as rodents and primates. The major difference in entorhinal-hippocampal projections that was described to date between rodents and primates is in the terminal distribution of the DG projection. We found that bats have entorhinal-DG projections that seem more like those in primates than in rodents. It is likely that the latter projection in bats is specialized to the behavioral needs of this species, including 3D flight and long-distance navigation.

Flying from History, Too Close to the Sun

There is a remarkable trend in contemporary environmentalism that emphasizes ‘accepting responsibility’ for the natural world in contrast to outdated preservationist thinking that shirks such responsibility. This approach is often explained and justified by reference to the anthropocene: this fundamentally new epoch—defined by human domination—requires active human intervention to avert planetary catastrophe. However, in this paper, I suggest this rhetoric encourages a flight from history. This often jubilant, sometimes anxious, yearning for unprecedented human innovation and—ultimately—control in our new millennia mirrors the Futurist movement that took off near the beginning of the last century. Despite the significant differences in the details of how academics have defended this twenty-first-century environmental outlook, they all represent the true flight from history; they too quickly jettison the ideas of historical environmentalists and so misunderstand the environmental values at the heart of preservation that are more salient than ever.

Wing morphology is related to niche specialization and interaction networks in stenodermatine bats (Chiroptera: Phyllostomidae)

Abstract Plant–animal interactions constitute some of the most important ecological processes for the maintenance of tropical forests. Bats are the only group of mammals capable of true flight and have been recognized as important dispersers of pioneer and secondary successional plant species. Although progress has been made in the study of Neotropical bats, morphological variation of the wing and its influence on niche separation between species is unknown. We evaluated relationships among habitat structures of selected Colombian tropical dry forest patches, the diet through interaction networks, and wing morphology of 11 species of bats in the Stenodermatinae subfamily (297 individuals) using geometric morphometry in a phylogenetic context. The results indicate that the phylogenetic signal for wing size is greater than for wing shape, thus providing some evidence for evolutionary convergence. Wing shape variation was associated primarily with the distal anatomical tip of the third finger and the joint between the humerus and the radius and ulna. Species with wide, short wings, as in the genus Artibeus had generalist diets and less nested positions within the interaction networks. In contrast, species with elongated and pointed wings, such as Sturnira and Platyrrhinus, had specialized diets and more nested positions within the interaction networks. We argue that wing shape variation may play an important role as a source of interspecific variation leading to food specialization within tropical bat communities.

REPORT ON THE NUPTIAL FLIGHT OF THE ANT LASIUS NEGLECTUS (HYMENOPTERA, FORMICIDAE) IN DONETSK CITY

For the first time in the invasive population of the ant Lasius neglectus Van Loon, Boomsma et Andrasfalvy, 1990, true nuptial flight was recorded. It has been established that in the conditions of Donetsk it does not occur annually. The ability of the queen to independently establish a colony after the flight has been experimentally proven.

The heart of a flying mammal

The ability of true flight and the great diversity of the order Chiroptera might raise questions about the morphology and functionality of the bats’ heart. Here we aimed to describe anatomical aspects of the heart of the great fruit-eating bat, Artibeus lituratus (Phyllostomidae, Stenodermatinae) and its functional features. Twenty-five individuals had their hearts dissected and described. We found that the A. lituratus heart shares general characteristics of the mammalian and Chiropteran heart, such as the bilateral anterior vena cava, the bulky pulmonary conus, the developed venae cavae sinus, the valve in the posterior vena cava and the papillary muscles attached to the right ventricle septal wall. Some features seem species-specific, like the anomalous membranous structures found in the right ventricle of some individuals, never seen before in other mammals, and also the presence of the Thebesian valve. Some of the features found might be related to flight adaptations as a highly developed set of pericardial ligaments. This complex set of ligaments has never been described before for another mammal, which probably acts by keeping the heart in position while the bat rests upside down. The pericardial ligaments have infiltrations of white fat, which, together with the deposits present on the surface of the heart, possibly act as energy reserves for the heart muscle. In this work, we suggest the presence of two anterior venae cavae has the function of optimizing the blood flow that returns to the heart.

Expansion Tube Experiments of Magnetohydrodynamic Aerobraking for Superorbital Earth Reentry

A set of magnetohydrodynamic (MHD) aerobraking experiments was conducted with the aim of simulating the Earth reentry environment for Mars return trajectories, and to study the effects of MHD flow control on the ionizing shock layer surrounding a planetary entry vehicle. The X2 expansion tube of the University of Queensland was used, which could provide realistic flowfield boundary conditions, where ionization is spontaneously generated within the shock layer as in true flight and the freestream is nonconducting. Suitable flow conditions providing strong MHD interaction at superorbital Earth reentry velocities were developed. Steel ball and neodymium permanent magnet models were tested, both uncoated and coated with electrically insulating high-temperature epoxy. Shock-layer high-speed imaging showed a significant increase in shock standoff distance for the magnetic models. The experimental shock standoff results generally agreed well with analytical and numerical shock standoff predictions. These experiments demonstrate that a strong MHD interaction can be generated for a Mars return trajectory with moderate magnetic field strengths. Furthermore, they provide a promising framework for future MHD aerobraking experiments to investigate MHD drag force and heat-flux mitigation.

Breeding behaviour and diet of Rufous Owls Ninox rufa in the Darwin Botanic Gardens

The breeding cycle of a pair of Rufous Owls Ninox rufa was observed over five breeding seasons, 2016–2020, in the Darwin Botanic Gardens, Northern Territory. A pair failed or did not breed in four of those years and raised one fledgling in 2018. The Owls occupied a hollow 2.5 m above ground in the trunk of an exotic Earpod Tree Enterolobium cyclocarpum. In 2018, laying occurred sometime between 11 June and 2 July, hatching between 21 and 25 July, and fledging (first true flight) on 6 September, with branching on 2–5 September, giving a nestling period of 44–48 days. The juvenile remained dependent until mid November, giving an estimated post-fledging dependence period of 10 weeks. For the following 3 weeks, it roosted alone within its natal home range, increasingly far (270–320 m) from the nest tree and roosting adults. Calling behaviour (including duetting by the pre-laying pair), courtship, copulation and parental behaviour and routines are described. From observations and pellet analysis, the Owls’ diet consisted of 78% mammals [Common Brushtail Possums Trichosurus vulpecula arnhemensis, flying-foxes (probably all Black Flying-foxes Pteropus alecto) and rats (probably Black Rats Rattus rattus)] and 22% birds (passerines and non-passerines up to Orange-footed Scrubfowl Megapodius reinwardt in size) by number.

Algorithmic support of low-altitude aircraft flight based on the hazard analysis of the flight situation

Low-altitude flight of an aircraft is an effective, but at the same time, a very complex tactical technique, during which the crew does not always have the opportunity to timely recognize the occurrence of an abnormal case, determine the way out of it and counteract an aviation accident development. Despite many advantages of the automatic mode of low-altitude flight performing, its practical implementation is associated with a number of features and disadvantages, which determined the preference for the manual mode of low-altitude flight control. These are the presence of telltale factors, limited ability of performing flights at night and in difficult weather conditions, insufficient reliability etc. The considered features determined the relevance of the of low-altitude flight safety ensuring problem in relation to the manual control mode. As a result of an experimental study of the low-altitude flight performing process in a manual control mode, it was found that when performing manually-controlled low-altitude flight, a hazard assessment of the flight situation becomes pivotal. However the crew being under such conditions is not always able to correctly assess the flight situation hazard due to a combination of objective reasons. The current state of the adaptive and on-board flight safety systems theory makes it possible to increase the safety of the manuallycontrolled low-altitude flight by using adaptive control algorithms based on the flight situation hazard assessment. To solve this problem an adaptive control algorithm is proposed that ensures the formation of a security corridor in the longitudinal control channel, where the upper limit is determined by the critical value of the aircraft detection hazard, and the lower limit is determined by the critical value of the error in maintaining a given flight altitude. For a continuous assessment of the flight situation hazard and the timely formation of control signals the complex information about the current true flight altitude and the foreground is needed. Taking into account the peculiarities of low-altitude flight a digital terrain map containing data on natural and artificial obstacles along the flight route is a more rational source of information, that will make it possible to predict the development of the flight situation hazard. The above reasoning makes it possible to form an aircraft low-altitude flight adaptive control algorithm. A distinctive feature of the proposed algorithm is the implementation of a combined control variety where the pilot is provided with ample manual control opportunities within the security corridor, and the automatic flight control system is assigned the role of a safety subsystem that ensures control and timely return of the flight situation to normal flight conditions. The presented algorithm will allow to increase the crew logical-analytical activity information support during continuous analysis of the existing flight situation due to the formation of protective control actions based on the current flight situation hazard analysis.

Sanjeev Upreti’s Hansa: An Ecocritical Quest

This article explores the idea of symbiosis; the notion of survival and letting nature and other organisms survive. The idea of 'back to nature' has been explored in the novel through the role of Anuj Pandey who makes a journey from East to West at first and from West to East in the end. Prem represents local citizen whereas Anuj Pandey attempts to look cosmopolitan. However, Anuj Pandey’s mission fails. The novelist assumes a person cannot be a true transcendentalist unless he/she experiences materialism through senses. In order to make a true flight, a person needs to move from the world of human to the world of nature. This all is possible through the activation of the sixth sense. The novel depicts the problems of both local and global homes. The locals desire to fly in the global and again the globalized ones desire to fly somewhere else. The novel shows that they intend to fly beyond anthropocentric world which is neither local nor global, but it is the world that is ruled or governed by the presence of nature. A person may feel as a complete man only in the vastness of nature. The article brings theoretical insights from romanticism, transcendentalism, narrative epistemology, anthropocentrism, ecocriticism and biocentrism as the tool or method of analysis.

High Body Temperature is an Unlikely Cause of High Viral Tolerance in Bats.

The global SARS-CoV-2 pandemic and the role of bats in zoonotic spillover have renewed interest in the flight-as-fever hypothesis, which posits that high body temperatures experienced by bats during flight contribute to their high viral tolerance. We argue that flight-as-fever is unlikely to explain why bats harbor more viruses than other mammals on the basis of two lines of reasoning. First, flight temperatures reported in the literature overestimate true flight temperatures because of methodologic limitations. Second, body temperatures in bats are only high relative to humans, and not relative to many other mammals. We provide examples of mammals from diverse habitats to show that temperatures in excess of 40 C during activity are quite common in species with lower viral diversity than bats. We caution scientists against stating the flight-as-fever hypothesis as unquestioned truth, as has repeatedly occurred in the popular media in the wake of the SARS-CoV-2 pandemic.

Modeling joint parameters to make sense of helicopter induced emissions effects: greener rotorcraft

PurposeThis study aims to ascertain the greenest helicopters by modeling joint parameters to make sense of induced emissions effects of helicopters allocated to various categories.Design/methodology/approachEmission indexes of helicopters construct the pillars of the methodology under use. Three different parameters are derived from emission indices: the index showing grams of pollutants that an engine produces per kWh; the index comparing of pollutant mass depending on the energy content of the fuel; and the index expressing the presence of the unreacted hydrocarbon (unburned) released into the atmosphere as a result of the combustion reaction.FindingsVarious helicopters have been designated as sensitive and insensitive to the environment under various conditions and in different categories. Details are in the conclusions section.Practical implicationsThis study includes methods that can be used to select environmentally sensitive helicopters of various categories according to specific pollutants and their combustion efficiency.Originality/valueThe originality of the work lies in the determination of the most sensitive and insensitive to the environment by using true flight data of helicopters operating in various categories during different flight phases. In addition, this paper with an approach to identifying green helicopters has the capability to support studies on regulations for helicopters in some countries by policymakers.

Magnetohydrodynamic drag force measurements in expansion tunnels using an accelerometer-based force balance

An accelerometer-based force balance has been used to measure the magnetohydrodynamic (MHD) drag force which arises when a magnetic field is present in a high-enthalpy ionised flow. This has been conducted using two expansion tunnels, with periods of steady test flow as low as 40μs . The use of this technique for MHD drag measurements is demonstrated and limitations are discussed. The results show that there is a measurable component of MHD drag in a flow-field where electrodynamic boundary conditions are matched to true flight. Furthermore, it is demonstrated that the measured forces can be attributed exclusively to the MHD effect, with good repeatability. The results indicated that at least 100μs of steady flow is required before the MHD force stabilises.

Ecology and Host Identity Outweigh Evolutionary History in Shaping the Bat Microbiome.

Recent studies of mammalian microbiomes have identified strong phylogenetic effects on bacterial community composition. Bats (Mammalia: Chiroptera) are among the most speciose mammals on the planet and the only mammal capable of true flight. We examined 1,236 16S rRNA amplicon libraries of the gut, oral, and skin microbiota from 497 Afrotropical bats (representing 9 families, 20 genera, and 31 species) to assess the extent to which host ecology and phylogeny predict microbial community similarity in bats. In contrast to recent studies of host-microbe associations in other mammals, we found no correlation between chiropteran phylogeny and bacterial community dissimilarity across the three anatomical sites sampled. For all anatomical sites, we found host species identity and geographic locality to be strong predictors of microbial community composition and observed a positive correlation between elevation and bacterial richness. Last, we identified significantly different bacterial associations within the gut microbiota of insectivorous and frugivorous bats. We conclude that the gut, oral, and skin microbiota of bats are shaped predominantly by ecological factors and do not exhibit the same degree of phylosymbiosis observed in other mammals.IMPORTANCE This study is the first to provide a comprehensive survey of bacterial symbionts from multiple anatomical sites across a broad taxonomic range of Afrotropical bats, demonstrating significant associations between the bat microbiome and anatomical site, geographic locality, and host identity-but not evolutionary history. This study provides a framework for future systems biology approaches to examine host-symbiont relationships across broad taxonomic scales, emphasizing the need to elucidate the interplay between host ecology and evolutionary history in shaping the microbiome of different anatomical sites.

Schedule creep – In search of an uncongested baseline block time by examining scheduled flight block times worldwide 1986–2016

Based on a stratified random sampling of airlines’ schedules for 200 heavily travelled directional nonstop airport pairs, this paper examines systematically how scheduled block times in non-stop flights have changed from 1986 to 2016. Three econometric analyses, by way of a 10th percentile quantile regression, 15th percentile quantile regression and ordinary least-squares regression, show that after accounting for the effects of air traffic growth, airport-specific congestion, flight delays, number of seat per flight, aircraft type, flight heading, airport slot policy, other airport-specific anomalies, airline-specific policies and changes in crude oil price, scheduled block times have been growing at a pace between 0.21 and 0.33 min per year depending on the regression model, or a total of between 6.2 and 9.8 min per flight from 1986 to 2016. Over-flying crowded parts of Europe contributes to an increase of 4.1 min of block time in 2016 compared with 1986, while over-flying crowded parts of China contributes to a corresponding increase of 8.9 min of block time. Slot-based practices at one end of an airport pair reduce the scheduled block times between 1.3 and 2.0 min, and this reduction can vary slightly over time in depending on the regression model. Regional influences add to the changes in scheduled block times. Those airport pairs within north-eastern U.S. have scheduled block times between 3.6 and 4.0 min longer than their counterparts in the rest of North America, which in turn grow at 0.10 min per year in the 10th percentile regression. Airline-specific policies also add to further changes to the scheduled block times, with some starting with longer times in 1986 and reducing theirs over the years while others starting with shorter times in 1986 and increasing theirs over the years. Hub-specific adjustments to scheduled block times by individual airlines are also observed. Airlines with increasing frequency share at the departure and arrival airports, or in the non-stop airport pair itself, are shown to reduce the scheduled block times of that route. The overall increase from the projected baseline block times in 1986 to the actual scheduled block times in 2016 is 19.2 min per flight from the sampled nonstop city-pairs, consistent with a previous study on buffers in flight schedules within the U.S. Overall, un-adjusted scheduled block times are not a reliable benchmark for determining true flight delays, but using a percentile statistics from past flight records to determine a minimum feasible block time is a reasonable estimate even if aircraft types are not explicitly accounted for.

Temperature control of cryogenic wind tunnel with a modified L1 adaptive output feedback control

Background: Temperature is one of the main variables need to be regulated in cryogenic wind tunnel to realize the true flight Reynolds number. A new control methodology based on L1 output feedback adaptive control is deployed in the temperature control. Methods: This design is composed of three parts: linear quadratic Gaussian baseline control, L1 adaptive control and nonlinear feedforward control. A linear quadratic Gaussian controller is implemented as the baseline controller to provide the basic robustness of temperature control. A L1 output feedback adaptive controller with a modified piecewise constant adaptive law is deployed as an augmentation for the baseline controller to cancel the uncertainties within the actuator’s bandwidth. The modified adaptive law can guarantee better steady-state tracking performance compared with the standard adaptive law. A global nonlinear optimization process is carried out to obtain a suboptimal filter design for the L1 controller to maximize the performance index. The nonlinear feedforward control is to cancel the coupling effects in control of the tunnel. Results: With these design techniques, the augmented L1 adaptive controller improves the performance of the baseline controller in the presence of uncertainties of dynamics. The simulation results and analysis demonstrate the effectiveness of the proposed control architecture. Conclusion: The modification of adaptive law plus the global nonlinear optimization of the filter in the L1 adaptive control architecture helps the controller achieve good control performance and acceptable robustness for the temperature control over a wide range of operations.

Temperature trajectory control of cryogenic wind tunnel with robust L1 adaptive control

Temperature control in a cryogenic wind tunnel is the key to realizing finely controlled Reynolds number close to true flight. This study deploys the L1 adaptive control methodology to ensure the total temperature profile of the cryogenic wind tunnel tracks a specified reference trajectory. After introducing a non-linear model of a cryogenic wind tunnel and a linear temperature model, a linear–quadratic–Gaussian (LQG) controller is implemented as the baseline controller. The L1 adaptive controller with piecewise constant adaptive law is used as an augmentation to the baseline controller to cancel the matched and unmatched uncertainties within the actuator’s bandwidth. By introducing two modifications to the standard L1 adaptive controller, which are the transportation delay modelling in the state predictor and the non-linear state dependent filter, the L1 adaptive controller improves the performance of the baseline controller in the presence of uncertainties in temperature control, guaranteeing proper stability and delay margin. The simulation results and analysis demonstrate the effectiveness of the proposed control architecture. The main contribution of this paper lies in the first applications of L1 adaptive control to the wind tunnel control problem and the non-linear state dependent filter in L1 adaptive control structure.

Design and Analysis of Winged Hovercraft

A flying hovercraft and ground effect vehicle design. The flat double-wing design for small, medium, and large cargo or military transport vehicles allows travel from very slow to medium to high speeds over water or land. The highly elongated, flat wing of very low aspect ratio provides a vehicle that is capable of traveling in ground effect flight at a safe distance above the water. Primarily a high-speed (150-200 kts), over-water, ground-effect, vehicle that is augmented with hovercraft capabilities for acceleration to flight speed, deceleration, and slow to moderate speed (0-75 kts.) operations. The vehicle design is adaptable from one-man units to large, ocean-going high-speed cargo ships of 400 feet and over, with carrying capacities over five million pounds. A vehicle of this design is 250% more efficient than modern aircraft, 15 times faster than cargo ships, and capable of going into true flight to overfly land masses for ocean-to-ocean and inland lake or river access. The design can be adapted to a multitude of fast, military, over-water vehicles of any size capable of travel over any terrain.

Sensory rewiring in an echolocator: genome-wide modification of retinogenic and auditory genes in the bat Myotis davidii.

Bats comprise 20% of all mammalian species and display a number of characteristics, including true flight, echolocation, and a heightened ability to resist viral load that uniquely position this group for comparative genomic studies. Here we searched for evidence of genomic variation consistent with sensory rewiring through bat evolution. We focused on two species with divergent sensory preferences. Myotis davidii is a bat species that echolocates and possesses dim- but not daylight-adapted vision whereas the black flying fox (Pteropus alecto) has highly developed day vision but does not echolocate. Using the naked mole rat as a reference, we found five functional genes (CYP1A2, RBP3, GUCY2F, CRYBB1, and GRK7) encoding visual proteins that have degenerated into pseudogenes in M. davidii but not P. alecto. In a second approach genome-wide codon usage bias (CUB) was compared between the two bat species. This CUB ranking systematically enriched for vision-related (CLN8, RD3, IKZF1, LAMC3, CRX, SOX8, VAX2, HPS1, RHO, PRPH2, and SOX9) and hearing-related (TPRN, TMIE, SLC52A3, OTOF, WFS1, SOD1, TBX18, MAP1A, OTOS, GPX1, and USH1G) machinery in M. davidii but not P. alecto. All vision and hearing genes selectively enriched in M. davidii for which orthologs could be identified also were more biased in the echolocating M. lucifugus than the nonecholocating P. vampyrus. We suggest that the existence of codon bias in vision- and hearing-related genes in a species that has evolved echolocation implies CUB is part of evolution’s toolkit to rewire sensory systems. We propose that the two genetic changes (pseudogene formation and CUB) collectively paint a picture of that incorporates a combination of destruction and gain-of-function. Together, they help explain how natural selection has reduced physiological costs associated with the development of a smaller eye poorly adapted to day vision but that also contribute to enhanced dim light vision and the hearing adaptations consonant with echolocation.

Tipe Echolokasi serta Struktur Larynx pada Miniopterus dan Rhinolophus

A bat is the only mamal with true flight ability and have capability to perform echolocation. Miniopterus and Rhinolopus are bat species that have some different morphological characteristics but both of them commonly live in the cave. The research were purposed to study the differences and connection of echolocation call type and larynx’s structure of Miniopterus and Rhinolopus The two bat species were collected from 20 th and 23 th cave of Japanese Cave Complex, Kaliurang, Sleman, Yogyakarta. Specimens were taken to the laboratory for further analysis. Echolocation signals of both bat species were recorded using ultrasonic microphone and were analyzed with Batscan 9.0 software. Specimens were subjected for anatomical and histological structure analysis of the larynx. Anatomical structure observations were carried out with specimens dissection and they were fixed in 20% formol solution. Some samples of larynx of each species were processed and stained with Alizarin Red’s-Alcian Blue staining for larynx’s bones observation. Histological preparation of larynx was performed using paraffin method and stained with Hematoxylin-Eosin and MAF. The results showed that echolocation signals of Minipterus have lower frequency (peak frequency between 44.1-51.6kHz) compared to that of Rhinolopus (peak frequency between 54.4-64.1 kHz). However, Miniopterus has better ability to modulate the signal frequency than Rhinolopus Both species are able to produce a long-duration signal but Rhinolopus can produce longer signal duration. These results can be related to the differences of larynx’s anatomical and histological structure of both species which are equally able to keep the intensity of echolocation signals but have different work mechanisms.

Unique expression patterns of multiple key genes associated with the evolution of mammalian flight

Bats are the only mammals capable of true flight. Critical adaptations for flight include a pair of dramatically elongated hands with broad wing membranes. To study the molecular mechanisms of bat wing evolution, we perform genomewide mRNA sequencing and in situ hybridization for embryonic bat limbs. We identify seven key genes that display unique expression patterns in embryonic bat wings and feet, compared with mouse fore- and hindlimbs. The expression of all 5'HoxD genes (Hoxd9-13) and Tbx3, six known crucial transcription factors for limb and digit development, is extremely high and prolonged in the elongating wing area. The expression of Fam5c, a tumour suppressor, in bat limbs is bat-specific and significantly high in all short digit regions (the thumb and foot digits). These results suggest multiple genetic changes occurred independently during the evolution of bat wings to elongate the hand digits, promote membrane growth and keep other digits short. Our findings also indicate that the evolution of limb morphology depends on the complex integration of multiple gene regulatory networks and biological processes that control digit formation and identity, chondrogenesis, and interdigital regression or retention.