Respiratory system of arachnids I: morphology of the respiratory system of Salticus scenicus and Euophrys lanigera (Arachnida, Araneae, Salticidae)

The book-lungs and the tracheal systems of two species of jumping spider, Salticus scenicus and Euophrys lanigera, were investigated using gross anatomical, light and electron microscopic methods. Both species possess well-developed book-lungs of similar size and tracheal systems with a basically similar branching pattern. The tracheal spiracle opens into a single atrium, where it gives rise to four thick 'tube tracheae', from which small secondary tube tracheae originate in groups. The secondary tracheae (diameter 1-5 mum) run parallel, without further branching, into the prosoma. In the opisthosoma, they lie ventrolaterally, where they contact muscles and internal organs. In the prosoma, the secondary tracheae may penetrate the gut epithelium and central nervous tissue. The structure of the tracheal walls is very similar to that of insects, consisting of a striated inner cuticular layer with taenidial structures and a surrounding outer hypodermal layer. The wall thickness appears similar in all secondary tracheae, indicating that lateral gas diffusion may be possible through the walls of all small tube tracheae.     

Morphometric analysis of the tracheal walls of the harvestmen Nemastoma lugubre (Arachnida, Opiliones, Nemastomatidae)

The tracheal system of harvestmen consists of two stem tracheae, which give rise to higher order tracheae that supply the extremities and internal organs. In this study, we used stereological morphometric methods to investigate diffusing capacities of the walls ('lateral diffusing capacity') of the tracheae of adult males and females of Nemastoma lugubre. Diffusing barriers of the tracheal walls tend to be thinnest (0.17-0.19 microm) for the smallest tracheae (inner diameter 0.5-2 microm). In other tracheal classes the diffusing barriers increase with increasing diameters. Calculation of the mass-specific diffusing capacity for oxygen (D(O2)) of the walls of all higher order tracheae revealed 10.57 microl min(-1)g(-1)kPa(-1) for the females (mean body mass 3.8 mg) and 25.23 microl min(-1)g(-1)kPa(-1) for the males (mean body mass 1.4 mg). In both animal groups, the main D(O2) (58-67%) lies in the tracheae with an inner diameter of 0.5-2 microm, but also tracheae up to an inner diameter of 20 microm allow gas exchange via the tracheal walls. Stem tracheae are of no importance for lateral diffusion. Our results are consistent with the hypothesis that the functional morphology of the tracheal system of harvestmen represents an 'intermediate state' between the tracheal system of insects in which gas exchange is focused on the distal portions and that of spiders, in which the walls of all tracheae serve in gas exchange.     

Stereological determination of tracheal volume and diffusing capacity of the tracheal walls in the stick insect Carausius morosus (Phasmatodea, Lonchodidae)

First instars of Carausius morosus provide a good model for morphometric evaluation of the diffusing capacity between the tracheal system and hemolymph: air sacs are lacking, tracheoles do not penetrate the organs and muscles, and entire animals can be evaluated electron microscopically without subsampling. The tracheal volume makes up 1.3% of the volume of the whole insect excluding appendages. We calculated the lateral diffusing capacity for oxygen and carbon dioxide for five classes of tracheae according to their diameters, from 0.2 microm to 35 microm. The harmonic mean thickness of the tracheal epithelium is lowest in smallest tracheae and increases with increasing tracheal diameter. Although the smallest tracheae make up 70% (O2) and 60% (CO2) of the total diffusing capacity, the proximal four classes may also be significant in diffusion of oxygen and particularly of carbon dioxide. The suppression of the development of respiratory pigments in the evolution of terrestrial insects may have increased the relative importance of small tracheal elements for local oxygen consumption.     

Corrosion casts as a method for investigation of the insect tracheal system

Summary The tracheal systems of five insect species (two species of ants, worker bee, housefly and the cabbage butterfly) have been studied by scanning electron microscopy of corrosion casts. This technique, which is commonly used for the investigation of vertebrate vasculature, is adapted to demonstrate the ultrastructure of the insect respiratory organ. The problem of filling a blind ending system was solved by injecting the resin Mercox into the evacuated tracheae through a thoracal spiracle. After polymerization of the resin, the tissue was digested enzymatically and chemically. The three-dimensional structure of the tracheal system was investigated by scanning electron microscopy. The technique used here displays for the first time the complex morphology of the entire tracheal system in fine detail, especially the structure of spiracles, airsacs, tracheae and tracheoles. Smooth-walled terminal tracheoles show up in flight muscles. The finest tracheoles that could be identified have diameters of approximately 70 nm. This approaches the finest tracheoles portrayed by transmission electron micrographs.     

The distribution of aeriferous tracheae for the ovaries of insects

Examination of the tracheal supply to the ovaries in insects selected from nine orders shows that the main tracheae are always of the aeriferous type: characterized by a coating of spiral tubules with permeable cuticle which bring the tracheal air into close contact with the haemolymph. The structure of these tracheae is constant, from large tracheae exceeding 50 mum in diameter to small vessels with diameter a small fraction of 1 mum. On the other hand there is great diversity in the methods by which oxygen is delivered to the individual o?cytes, some of which are briefly defined.     

Tracheal development in the Drosophila brain is constrained by glial cells

The Drosophila brain is tracheated by the cerebral trachea, a branch of the first segmental trachea of the embryo. During larval stages the cerebral trachea splits into several main (primary) branches that grow around the neuropile, forming a perineuropilar tracheal plexus (PNP) at the neuropile surface. Five primary tracheal branches whose spatial relationship to brain compartments is relatively invariant can be distinguished, although the exact trajectories and branching pattern of the brain tracheae are surprisingly variable. Immunohistochemical and electron microscopic studies demonstrate that all brain tracheae grow in direct contact with the glial cell processes that surround the neuropile. To investigate the effect of glia on tracheal development, embryos and larvae lacking glial cells as a result of a genetic mutation or a directed ablation were analyzed. In these animals, the tracheal branching pattern was highly abnormal. In particular, the number of secondary branches entering the central neuropile was increased. Wild-type larvae possess only two central tracheae, typically associated with the mushroom body and the antennocerebral tract. In larvae lacking glial cells, six to ten tracheal branches penetrate the neuropile in a variable pattern. This finding indicates that glia-derived signals constrained tracheal growth in the Drosophila brain and restrict the number of branches entering the neuropile.     

Respiratory organs in wolf spiders: morphometric analysis of lungs and tracheae in Pardosa lugubris (L.) (Arachnida, Araneae, Lycosidae)

The respiratory system of the wolf spider Pardosa lugubris consists of a pair of well-developed lungs and four unbranched tube tracheae. We used stereological morphometric methods to investigate the morphological diffusing capacity of the lungs and of the walls of the tracheae ('lateral diffusing capacity'). We examined three groups of female P. lugubris with different mean body masses. The barrier thickness of the gas-exchange epithelium of the lungs was 0.17 microm for the total diffusion barrier and the calculated oxygen diffusing capacity (D(O2)) for the lungs was between 12.9 and 13.4 microl min(-1)g(-1)kPa(-1). Measured metabolic rates compared with the D(O2) of the lungs result in necessary oxygen partial pressure differences of 0.2 kPa during rest and 2.1 kPa during maximum measured activity. The diffusion barrier of the entire tracheal walls was 0.31-0.50 microm and the calculated lateral D(O2) was 0.05-0.2 microl min(-1)g(-1)kPa(-1). Therefore, tracheae are of no importance for the overall oxygen exchange. However, they might be of some importance in local oxygen supply or in overall carbon dioxide release. The comparison with the respiratory system of the jumping spider Salticus scenicus reveals that the lungs have very similar mass-specific D(O2) in both species, and that, in addition, jumping spiders possess a much better developed tracheal system.     

Ontogeny of tracheal system structure: a light and electron-microscopy study of the metathoracic femur of the American locust, Schistocerca americana

Does oxygen delivery become more challenging for insects as they increase in size? To partially test this hypothesis, we used quantitative light and electron microscopy to estimate the oxygen delivery capacity for two steps of tracheal oxygen delivery within the metathoracic femur (jumping leg) for 2nd instar (about 47 mg) and adult (about 1.7 g) locusts, Schistocerca americana. The fractional cross‐sectional areas of the major tracheae running longitudinally along the leg were similar in adults and 2nd instars; however, since the legs of adults are longer, the mass‐specific diffusive conductances of these tracheae were 4‐fold greater in 2nd instars. Diffusive gas exchange longitudinally along the leg is easily possible for 2nd instars but not adults, who have many air sacs within the femur. Mitochondrial content fell proximally to distally within the femur in 2nd instars but not adults, supporting the hypothesis that diffusion was more important for the former. Lateral diffusing capacities of the tracheal walls were 12‐fold greater in adults than 2nd instars. This was primarily due to differences in the smallest tracheal class (tracheoles), which had thinner epidermal and cuticular layers, greater surface to volume ratios, and greater mass‐specific surface areas in adults. Adults also had greater mitochondrial contents, larger cell sizes and more intracellular tracheae. Thus, larger insects do not necessarily face greater problems with oxygen delivery; adult grasshoppers have superior oxygen delivery systems and greater mass‐specific aerobic capacities in their legs than smaller/younger insects. J. Morphol. 262:800–812, 2004. © 2004 Wiley‐Liss, Inc.     

The functional morphology of the respiratory system of the Holothyrida (= Tetrastigmata) Acari: Anactinotrichida

The respiratory system of Holothyrus coccinella Gervais (Holothyridae) and Allothyrus australasiae (Womersley) (Allothyridae) were examined. The stigma-peritreme complex is connected to tracheae and ventilated by indirect muscles. The peritreme provides an alternative route for the entry of air into the tracheal system, should a stigma be occluded by debris and retards water vapour transpiration, the mechanisms of which are compared in the two species.     

条背萤成虫与幼虫发光器的超微结构观察

对水生萤火虫——条背萤Luciola substriata(Gorham)成虫和幼虫发光器的超微结构进行研究。结果表明,成虫发光器由明显的2层组成:反射层和发光层。反射层由排列紧密的“尿酸囊泡”构成,具有发达的气管结构,对光起反射作用;发光层由大量发光细胞构成,内含典型的发光颗粒、线粒体、内质网及大量糖原,该层通过发光细胞胞质内的生化反应而发光。2层均由非细胞层膜包被,间距25~30μm。发光器腹节由外向内依次为表皮、发光层、反射层和内部细胞层。幼虫发光器球形,由背射层和发光层构成,由非细胞层膜包被。背射层由单层柱状细胞构成,内含大量“尿酸囊泡”。发光层细胞膜相互绞缠,含有2种类型的发光颗粒:“致密”型和“凋亡”型,含有大量的线粒体和无定形颗粒,发光细胞之间分布着大量的气管、微气管及神经末梢,可观察到神经突触。与条背萤相比,陆生种成虫反射层和发光层均无非细胞层膜包被,2层间无明显间距,发光颗粒形状不规则,气管通常形成2分支;陆栖种幼虫发光层形状差异较大,背射层由单层或2~4层细胞构成;相似点在于,成虫发光器都由均由反射层和发光层构成,发光细胞内都含发光颗粒、线粒体及大量糖原,都具有发达的气管结构,发光颗粒相似。幼虫发光器都由背射层和发光层构成,都具有发达的气管和直接的神经支配,发光颗粒相似,都由非细胞层膜包被。     

Tracheal branch repopulation precedes induction of the Drosophila dorsal air sac primordium

The dorsal air sacs supply oxygen to the flight muscles of the Drosophila adult. This tracheal organ grows from an epithelial tube (the air sac primordium (ASP)) that arises during the third larval instar (L3) from a wing-disc-associated tracheal branch. Since the ASP is generated by a program of both morphogenesis and cell proliferation and since the larval tracheal branches are populated by cells that are terminally differentiated, the provenance of its progenitors has been uncertain. Here, we show that, although other larval tracheae are remodeled after L3, most tracheal branches in the tracheal metamere associated with the wing disc (Tr2) are precociously repopulated with imaginal tracheoblasts during L3. Concurrently, the larval cells in Tr2 undergo head involution defective (hid)-dependent programmed cell death. In BX-C mutant larvae, the tracheal branches of the Tr3 metamere are also repopulated during L3. Our results show that repopulation of the larval trachea is a prerequisite for FGF-dependent induction of cell proliferation and tubulogenesis in the ASP and that homeotic selector gene function is necessary for the temporal and spatial control of tracheal repopulation.     

Moulting of insect tracheae captured by light and electron-microscopy in the metathoracic femur of a third instar locust Locusta migratoria

The insect tracheal system is an air-filled branching network of internal tubing that functions to exchange respiratory gases between the tissues and the environment. The light and electron-micrographs presented in this study show tracheae in the process of moulting, captured from the metathoracic hopping femur of a juvenile third instar locust (Locusta migratoria). The images provide evidence for the detachment of the cuticular intima from the tracheal epithelial cells, the presence of moulting fluid between the new and old cuticle layers, and the withdrawal of the shed cuticular lining through larger upstream regions of the tracheal system during moulting. The micrographs also reveal that the cuticular intima of the fine terminal branches of the tracheal system is cast at ecdysis. Therefore, the hypothesis that tracheoles retain their cuticle lining at each moult may not apply to all insect species or developmental stages.     

Life styles and patterns of development of gills and muscles in larval cyprinids (Cyprinidae; Teleostei)

A morphometric study of gill structures and of the body musculature during the first weeks after hatching was carried out on larvae of six cyprinid species: Leuciscus cephalus, L. leuciscus, Rutilus rutilus, Alburnus albumus, Chondrostoma nasus and Abramis brama . In all species a unicellular layer of red muscle fibres covers the central muscle mass; this layer is of greatest extent shortly after hatching but diminishes gradually in mass by contracting towards the lateral region of the body until it merges with (or gives rise to) the adult red muscle fibres proper. There is a close relationship between the rate of differentiation of gill structures and the rate at which the larval red muscle layer disappears, the pattern of this relationship reflecting the life style of the species. The longer the larvae delay the start of their free-swimming existence after hatching (which in A. alburnus may be as long as 10 days) the longer does the red layer of muscle fibres serve as the organ of gas exchange and the longer is gill development suppressed. It appears that the metabolism of the swimming muscles is almost entirely aerobic so long as gas exchange takes place across the whole body surface, the glycolytic capacity of the central muscle mass developing only slowly in conjunction with the switch from red layer to gills as the major respiratory organ.     

The tracheal system of the developing primary olfactory pathway of Manduca sexta: tracheae do not play a guidance or targeting role for ingrowing receptor axons

Axons navigate to their targets by detecting signals within the environment through which they are growing. The surfaces of tracheae, which are prominent features of the insect body plan, could be detected as favorable pathways for sensory axons growing toward the brain. The pattern of the tracheal investment of the adult antennal lobe of the moth Manduca sexta suggested two specific possibilities for interaction between tracheae and axons during development: that tracheae might be involved in guiding olfactory receptor axons to their target region of the brain, the antennal lobe; and that tracheae could provide an address system within the lobe that defines the sites of glomeruli, which are olfactory-axon target areas within the lobe. To determine whether tracheae contribute to development of the primary olfactory pathway, the distribution of tracheae in the adult and developing antennal lobes was examined with both confocal and electron microscopes. During the major stages in which axons are growing into the antennal lobe and in which glomeruli are forming, the tracheal investment of the nerve and lobe was found to be minimal. Tracheae thus cannot serve as axon guides or as local address sites for newly forming glomeruli during the initial targeting of receptors onto the antennal lobe.     

Respiratory system of the primitive moth Micropterix calthella (Linnaeus) (Lepidoptera : Micropterigidae)

The 1st thoracic spiracular atrium is closed by anterior and posterior muscle fibres extending between its dorsal and ventral wall. The 2nd thoracic spiracle has only a single (anterior) closing lip, movable by a muscle inserting on the wall below the spiracular aperture; this configuration may be a lepidopteran ground-plan autapomorphy. There are functional spiracles on abdominal segments I – VII, each with a closing “bow” and “lever”. There are intrinsic occlusor muscles in all abdominal spiracles and the 1st spiracle has an extrinsic (ventral) dilator. Dorsal dilator muscles or ligaments are absent. A dorsal and a ventral tracheal trunk extend from the 1st thoracic spiracle into the head; the latter supplies the mouthparts and the antenna; there is no connection between the dorsal and ventral cephalic trunk systems. There is a single series of lateral connectives between the spiracles of each side. There is a ventral tracheal commissure in both pterothoracic segments, but none in the prothorax. In each pterothoracic segment an anterior and a posterior tracheal arch give off branches to the wing and anastomose with each other on their downwards course into the leg. Wing tracheation is greatly reduced. The anterior and posterior tracheae of each wing are independent of each other. There is a dorsal commissure in abdominal segment VIII; ventral abdominal commissures are lacking in Micropterix, although present in other micropterigid genera. The terminalia are partly supplied from tracheae arising in segment VII. Air sacs occur in the tibiae only. Phylogenetic aspects of holometabolan tracheation patterns are discussed.     

Association of tracheal placodes with leg primordia in Drosophila and implications for the origin of insect tracheal systems

Adaptation to diverse habitats has prompted the development of distinct organs in different animals to better exploit their living conditions. This is the case for the respiratory organs of arthropods, ranging from tracheae in terrestrial insects to gills in aquatic crustaceans. Although Drosophila tracheal development has been studied extensively, the origin of the tracheal system has been a long-standing mystery. Here, we show that tracheal placodes and leg primordia arise from a common pool of cells in Drosophila, with differences in their fate controlled by the activation state of the wingless signalling pathway. We have also been able to elucidate early events that trigger leg specification and to show that cryptic appendage primordia are associated with the tracheal placodes even in abdominal segments. The association between tracheal and appendage primordia in Drosophila is reminiscent of the association between gills and appendages in crustaceans. This similarity is strengthened by the finding that homologues of tracheal inducer genes are specifically expressed in the gills of crustaceans. We conclude that crustacean gills and insect tracheae share a number of features that raise the possibility of an evolutionary relationship between these structures. We propose an evolutionary scenario that accommodates the available data.     

The properties of the lining membrane of the insect tracheal system

In the surface layer of the lining cuticle of the tracheae of adult Calliphora there is no sign of any waterproofing layer of cuticulin (sclerotin + lipid) as seen in the surface of the general body cuticle. In a few insects: Calliphora adult thorax, Rhodnius adult tracheae serving the ovary, Periplaneta abdominal tracheae, it has been possible to introduce silver hydroxide solution into the lumen of tracheae in the living insect. In each case the silver hydroxide reacted at room temperature with the argentaffin structures in the cuticle, as happens in the soft surface cuticle of Rhodnius larva before moulting or after gentle abrasion. In the thorax of Calliphora the taenidia of the tracheae are stiffened by argentaffin cuticulin. but immediately upon entering the cleft in the flight muscle the taenidia disappear and are replaced by simple folds, so that no stiff taenidia enter the muscle and there is no argentffin material deeper in the flight muscle system.     

Ontogeny of tracheal dimensions and gas exchange capacities in the grasshopper, Schistocerca americana

How does body size affect the structure and gas exchange capacities of insect tracheae? Do insects become more oxygen-limited as they grow? We addressed these questions by measuring the dimensions of two transverse tracheae within the abdomen of American locusts of different ages, and evaluating the potential for diffusion or convection to provide adequate gas exchange. The grasshopper abdomen has longitudinal tracheae that run along the midgut, heart, nerve cord, and lateral body wall. Transverse tracheae run from each spiracle to the longitudinal tracheae. Dorsal air sacs attach near each spiracle. In both transverse tracheae studied, diffusive capacities increased more slowly than metabolic rates with age, and calculated oxygen gradients necessary to supply oxygen by diffusion increased exponentially with age. However, surgical studies demonstrated that transport of gas through these transverse tracheae occurred by convection, at least in adults. Convective capacities paralleled metabolic rates with age, and the calculated pressure gradients required to sustain oxygen consumption rates by convection were independent of age. Thus, in growing grasshoppers, tracheal capacities matched tissue oxygen needs. Our morphological and physiological data together suggest that use of convection allows older grasshoppers to overcome potential limitations on size imposed by diffusion through tracheal systems.     

The tracheal supply and muscle metabolism during muscle growth in the puparium of Calliphora vomitoria

Thirty hours after puparium formation in Calliphora, the larval tracheal system is replaced by an air-filled pupal system. This is characterized initially by many tufts of tracheae and coiled tracheoles lying in the blood. Between the third and fourth day, the sixth dorsal longitudinal flight muscles are practically without attached tracheae and their longitudinal growth can partially occur when oxygen uptake is inhibited with potassium cyanide. Sodium iodoacetate prevents muscle growth. After the fifth day of development the pupal tracheoles spread out over the surface of the developing adult muscles. Between the seventh and ninth day, longitudinal growth and increases in the diameter of the myofibrils are halted by cyanide and iodoacetate. Some longitudinal growth and an increase in the total protein content of the muscles can occur in 1% oxygen. Air filling of the adult tracheae takes place 2–3 hr before the emergence of the adult and is accompanied by an increase in oxygen consumption of the thorax. The metabolism and growth of the muscles is discussed with respect to their changing oxygen supply.     

Ontogeny of tracheal dimensions and gas exchange capacities in the grasshopper, Schistocerca americana

How does body size affect the structure and gas exchange capacities of insect tracheae? Do insects become more oxygen-limited as they grow? We addressed these questions by measuring the dimensions of two transverse tracheae within the abdomen of American locusts of different ages, and evaluating the potential for diffusion or convection to provide adequate gas exchange. The grasshopper abdomen has longitudinal tracheae that run along the midgut, heart, nerve cord, and lateral body wall. Transverse tracheae run from each spiracle to the longitudinal tracheae. Dorsal air sacs attach near each spiracle. In both transverse tracheae studied, diffusive capacities increased more slowly than metabolic rates with age, and calculated oxygen gradients necessary to supply oxygen by diffusion increased exponentially with age. However, surgical studies demonstrated that transport of gas through these transverse tracheae occurred by convection, at least in adults. Convective capacities paralleled metabolic rates with age, and the calculated pressure gradients required to sustain oxygen consumption rates by convection were independent of age. Thus, in growing grasshoppers, tracheal capacities matched tissue oxygen needs. Our morphological and physiological data together suggest that use of convection allows older grasshoppers to overcome potential limitations on size imposed by diffusion through tracheal systems.