Scanning and transmission electron microscopic study of the tracheal air sac system in a grasshopper Chrotogonus senegalensis Kraus ФOrthopteraAcrididaePyrogomorphinae.код для вставкиСкачать
THE ANATOMICAL RECORD 223:393-405 (1989) Scanning and Transmission Electron Microscopic Study of the Tracheal Air Sac System in a Grasshopper Chrotogonus senegalensis (Kraus)0rt hoptera: Acrididae: Pyrgomorphinae J.N. MAINA Department of Veterinary Anatomy, Faculty of Veterinary Medicine, University of Nairobi, Nairobi Kenya ABSTRACT The morphology of the trachea-air sac system in a species of grasshopper Chrotogonus senegalensis has been studied by using scanning and transmission electron microscopes. Capacious air sacs were formed as dilatations along the primary tracheal trunks. Narrower secondary trachea arose either directly from the primary trachea that bypassed the air sacs or from the air sacs themselves. At or close to the organ or tissue supplied with air, the secondary trachea gave rise to the notably smaller tertiary trachea that penetrated the tissue, giving rise terminally to the extremely small tracheoles that indent some cells. The trachea and the air sacs were basically made up of an inner cuticular lining, helical taenidial rings, and an overlying epithelial cell cover. The air sacs may be important in efficient ventilation of the respiratory system. The supply of air directly to the tissue cells was viewed as an exemplary efficient design when compared to that prevailing in the nontracheate air-breathing animals, where the vascular system is interposed between the respiratory organ and the target tissue cells. A similarity in the general morphological design of the insect and avian respiratory systems has been observed, mainly in respect to the presence of the air sacs and that of the respiratory shunts. This, together with the reported functional features like the unidirectional mode of ventilation, has been interpreted as a classic case of structural and functional convergent evolution leading to the evolution of similar and comparably efficient respiratory systems capable of providing the large amount of oxygen demanded by flight. In the animal kingdom, active flight is unique to only three groups of animals: insects (class Insecta), birds (class Aves), and bats (order Chiroptera). These animals, through adaptive radiation, have come to dominate the living animals in respect to specific diversity (Pringle, 1957; Smith, 1968; Wimsatt, 1970; Callahan, 1972; Rockstein, 1973; Yalden and Morris, 1975; Eisner and Wilson, 1977). Grossly, these phylogenetically remarkably different groups of animals have acquired comparable aerodynamic morphological configuration in body form, mainly that of having the wings which act as aerofoils in providing and sustaining lift and maneuverability during flight. The energy requirements of flight are remarkably high and largely beyond the reach of most terrestrial animals (Tucker, 1969). The insects, for example, increase their resting metabolic rate by a factor of well over 100 in flight (Krogh and Weis-Fogh, 1951; Weis-Fogh, 1964a) and maintain it entirely aerobically (Miller, 1974). The capacity for flight in some animals is an outstanding example of behavioral convergent evolution that facilitates the exploitation of the aerial ecological niche. This has taken place at different geological times and in response to different selective 01989 ALAN R. LISS, INC. forces. The insects were the first small animals to colonize the dry land with notable success, having established themselves by the late Devonian Period (345 million years ago) (Callahan, 1972). The respiratory apparatus is the organ system most sensitive to changes in energetic demands such as entailed in flight and has to be designed to operate optimally at two remarkably different metabolic levelsresting and flying (Weis-Fogh, 1964b).Pulmonary structural and functional adaptations in the bat and the bird surpassing those of the other air-breathing vertebrates have been elucidated (Dubach, 1981; Jurgens et al., 1981; Maina and King, 1982a,b, 1984; Maina et al., 1982; Maina, 1985; Lechner, 1985). The insects have evolved the tracheal system, comparable to the vertebrate circulatory system, which delivers oxygen directly to the tissue cells, a respiratory system which according to Williams (1977) “embodies the refinement of biological Received January 25,1988; accepted June 16,1988. J.N. Maina’s present address is Department of Anatomy, School of Veterinary Medicine, University of California, Davis, CA 95616. Address reprint requests there. J.N. MAINA 394 Ds I SP I , Figs. 1-4 St 395 TRACHEA-AIR SAC SYSTEM IN A GRASSHOPPER engineering almost past belief.” The development, structure, and function of the tracheal system has been extensively investigated and reviewed (see Wigglesworth, 1931, 1953, 1954, 1959, 1972; Maloeuf, 1938; Richards and Anderson, 1942a,b; Whitten, 1955, 1956, 1957, 1962, 1972; Edwards et al., 1958; Buck, 1962; Shafiq, 1963; Weis-Fogh, 1964a,b; Uvarov, 1966; Peterson and Buck, 1968; Peterson, 1970; Miller, 1974; Wigglesworth and Lee, 1982). In spite of the numerous studies, among others, Richards and Korda (19501, WeisFogh (1964a), Uvarov (1966), Whitten (19721, Miller (1974), and Wigglesworth and Lee (1982) have pointed out the diversity of organization of insect tissues. Such immense detailed structural differences are undoubtedly a measure of the remarkable adaptive radiation the insects have undergone. The prevailing incompleteness of the structural and functional information, notably on the respiratory system, was pointed out by Uvarov (1966), who observed that “the knowledge of the insect respiratory mechanisms is still inadequate,” and echoed more recently by Tenney (1979) and Pringle (19831, who, respectively, noted that “the kinds of structural data that are required to deduce states of tissue oxygenation under physiological conditions are simply not in hand” and that “we still have a great deal to learn from them [insects] for a better understanding of biological processes and of the design of efficient machinery.” Recently, Smith (1984) noted that “little is known about the organization of air sacs” and further that “no detailed comparative survey of insect muscle tracheation is available.” The main objective of this study was t o examine, by using scanning and transmission electron microscopes, the topographic anatomy and the structure of the air sacs and trachea in a species of grasshopper as an attempt to illustrate the pertinent structural features of the insect respiratory system which are important in gas exchange. Such studies are apparently lacking in this species. Some similarity in the organization of the insect and the avian respiratory systems was observed and a possible explanation for this conformity is given. MATERIALS AND METHODS Mature specimens of the grasshopper Chrotogonus s e negalensis (Kraus, 1877) were kindly donated by the Department of Zoology, University of Nairobi. They were killed in an air-tight glass jar with chloroform soaked in cotton wool after 2 days of examination for the mode and frequency of respiration. The abdomen and the thorax were opened under a dissecting microscope with the specimens stuck on a board in a supine position, The air sacs and the tracheal system particularly in the flight muscles and the gastrointestinal system were carefully studied. Tissue samples were taken and fixed by immersion in 2.3% glutaraldehyde buffered in potassium phosphate (pH 7.4 and osmolarity 420 mOsm). They were then dehydrated in five changes of absolute alcohol after which they were trimmed to an appropriate size, critical point dried in liquid carbon dioxide, stuck on metal chucks, and sputtered with gold-palladium complex before viewing on a Philips PSEM 275 scanning electron microscope. The remaining tissues were diced into small pieces (about 1 mm3) which were processed for transmission electron microscopy by postfixation in 2% osmium tetroxide for 2 hours, en bloc stained in uranyl acetate with maleic acid, and dehydrated in graded concentrations of ethanol starting at 50% through absolute and acetone before infiltration and embedding in TAAB resin. Blocks were picked; ultrathin sections were cut, mounted on 200-mesh wire grids, and subsequently stained with lead citrate before viewing on a Corinth AEI 275 electron microscope. RESULTS Respiration in C. senegalensis was observed to be aided by abdominal pumping which during rest occurred at a rate of 20 times per minute. The first eight cranial abdominal segments were associated with paired left and right air sacs which lay interposed between the gut and the body wall (Figs. 1-3). These air sacs (Fig. 5), which occurred singly, in pairs, or in groups of three, were embedded in fat tissue. The single air sacs were generally larger than the individual ones that composed the doublets and the triplets. The air sacs were connected directly t o the spiracles and occasionally to the adjacent spiracles by a longitudinal anastomic (intersegmental) tracheal chain (Figs. 3, 4). In this species, the Fig. 1. A longitudinal view of the air sacs (As) and the main tracheal trunks, the dorsal intersegmental trunk (Ds), the ventral tracheal air sacs in general decreased in size cranial-caudally. trunk (Vt), and the lateral intersegmental trunk (St) in Chrotogonus Smaller air sacs (Figs. 2, 4, 10 inset) were observed senegalensis. Sp, spiracle (others shown with arrows). dorsal lateral to the gastrointestinal tract on the proximal segments. Conspicuously large air sacs (about 4 mm Fig. 2. A close-up of the abdominal air sacs (As), the smaller dorsal lateral air sacs, and the main tracheal trunks, these being the dorsal in diameter), which were bilaterally interposed between intersegmental trunk (Ds), the ventral intersegmental trunk (Vt),and the flight muscles, were observed in this species of grassthe lateral intersegmental trunk (St). The arrows show the spiracles. hopper. Like the abdominal air sacs, the thoracic air Gt, outline of the gastrointestinal tract. sacs were buried in fat tissue. The air sacs were made Fig. 3. A gross photograph of the abdomen showing the air sacs up of an epithelial cell cover, spiral taenidia, and an (asterisks) lying next to the gastrointestinal tract (Gt) and the major inner cuticular lining (Figs. 5 inset, 10 inset). tracheal trunks the lateral intersegmental (anastomotic) trunk (Ls) The trachea, like the air sacs, consisted of an inner and the primary trachea (Pt).Sp, spiracle. x 2 . Note that the air sacs cuticular lining underlain by an epicuticle thickened at have been dissected out on one side of the gut to expose the primary intervals to form regularly spaced circular threads, the trachea. taenidia, and to the outside, an epithelial cell cover Fig. 4. A transverse view of the abdomen of Chrotogonus senegalen- (Figs. 6, 8). The size and the frequency of the taenidia sis showing thc main tracheal trunks. Sp, spiracle; Gt, gut; Dt, dorsal and the epithelial cell cover appeared to change as the segmental trachea; Vt, ventral segmental trachea; Ls, lateral interseg- trachea decreased in size, the taenidia and the epithelial mental trunk; Ds, dorsal intersegmental trunk; Dc, dorsal commissure; Vc, ventral commissure. The dashed lines show the outlines of the cell cover being less prominent in the finer passages (cf. Figs. 12, 13). larger air sacs and the much smaller lateral ones relative to the gut. 396 J.N. MAINA Figs. 5 and 6 397 TRACHEA-AIR SAC SYSTEM IN A GRASSHOPPER The tracheal supply to the flight muscles (Figs. 7, 8) and to the gastrointestinal system (Figs. 9, 10) was notably profuse. In general terms, the tracheal system could be divided into a primary set which consisted of relatively large trachea with such trachea in the abdomen dividing to give rise to ventral, dorsal, and the horizontal segmental branches (Fig. 4). The ventral and dorsal branches united through the dorsal intersegmental trachea to form the respective longitudinal trunks. The lateral intersegmental tracheal trunk constituted the lateral longitudinal anastomotic chain. The primary trachea either ran directly to give rise to the smaller secondary trachea near the organ supplied with air or emptied into the air sacs from where the secondary trachea arose (Fig. 5). The secondary trachea divided dichotomously, giving rise t o finer branches-the tertiary trachea-which invaded the tissues (Figs. 7-10), where they gave way to the extremely small tracheoles that interacted and, in some cases, as terminal tracheoles, entered the individual tissue cells (Figs. 11,15).In some instances the elaborate division of the secondary tracheal trunk on the surface of the flight muscle was reminiscent of a motor nerve ending (Fig. 7). The terminal tracheoles (Figs. 11, 13) had a mean diameter of about 0.20 pm and on average lay as close as 0.050 pm to the nearest mitochondrion in the flight muscles of this species of grasshopper. In the intracytoplasmic invaginations of the terminal tracheoles in the gut cells (Figs. 14, 15), some of the nearest mitochondria were as close as 0.014 pm to a tracheole. The mitochondria were seen in such instances to aggregate around the terminal tracheoles, a feature which appears to be an attempt to decrease the diffusional pathway from the tracheole; this relationship was termed by Edwards et al. (1958) the tracheole-mitochondria continuum. The tracheoles, both extracytoplasmic and intracytoplasmic, were invested by tracheoblasts (Figs. 14,151.The intima of the terminal tracheoles was surrounded by three concentric membranes: lying next to the tracheolar cuticle was the intratracheal membrane (Fig. 15). Intracytoplasmically, this innermost layer was separated from the peripheral plasma membrane by a very narrow space of cytoplasm (about 0.03 pm) in width. The outermost layer is separated from the tracheole by a relatively wider space (0.04 pm) which is the invaginated circumtracheolar plasma membrane of the cell (Fig. 15). The Fig. 5. Lateral abdominal air sacs (as) of the grasshopper Chrote gonus senegalensis. The primary trachea (pt) run from the spiracle to the air sac but in some areas run directly to the tissue, decreasing in size to give rise t o the secondary trachea. The inset is an enlargement of the boxed area to show secondary trachea (st) arising from the air sac (as). te, tracheal epithelium; tn, taenidial rings. Figure 5 x33. Inset, ~ 2 3 6 . Fig. 6.Primary trachea (pt) running from the spiracles. The primary trachea are joined by a n anastomotic chain (at). The primary trunks give rise to dorsal and ventral branches which unite with their fellows on the opposite side to constitute the dorsal and ventral tracheal trunks. The anastornotic chain is seen to give rise to smaller trachea (arrows)which supply air to the gastrointestinal system. mt, malpighian tubules. The inset is an enlargement of the boxed area. It shows the taenidial rings that maintain the trachea patent. The epithelial cell cover is shown with the arrow. st, secondary trachea. Figure 6, x30. Inset, x 140. tracheoblasts contained electron-dense granules and mitochondria (Figs. 14, 15). In the flight muscles, the mitochondria, with profuse cristae, virtually run from one Zline to another, contributing about 25% of the volume of the muscle tissue. Haemolymphatic vessels could be discerned notably in the flight muscles (Fig. 11).In some of these vessels haemocytes with large, centrally placed nuclei and numerous cytoplasmic electron dense granules could be discerned (Fig. 11). DISCUSS10N The insect tracheal respiratory system delivers oxygen directly to the mitochondria and takes away carbon dioxide from the individual tissue cells (Snodgrass,1935; Weis-Fogh, 1964a,b; Smith, 1968; Bursell, 1970; Wigglesworth, 1972; Miller, 1974; Bridges et al., 1980). In mechanical terms the tracheal system efficiently acts both as a compressor and as an exhaust pipe (Nachtigall, 1974). It is now well accepted that the tracheal system develops as an invagination of the surface cuticle at the spiracles (Wigglesworth, 1931, 1972; Snodgrass, 1935; Locke, 1957, 1958a,b, 1974; Whitten, 1972; Smith, 1968; Miller, 1974), accounting for the similarity and the reversal of the layers constituting the trachea. The main tracheal trunks from the spiracles branch dichotomously, diminishing in size, ultimately giving rise to the minute tubules-the tracheoles-which are intimately associated with the tissue cells. The doubts expressed by Snodgrass (1935) on the possibility of intracellular penetration by the terminal tracheoles have now been resolved by Edwards et al. (19581, Tahmisian and Devine (19571, and Wigglesworth and Lee (1982)and in this study. The tracheolar cuticular intima is ensheathed in a tracheoblast which is presumed to be involved in its formation (Keister, 1948; Wigglesworth, 1954; Edwards et al., 1958;Locke, 1958a,b;Shafiq, 1963; Whitten, 1972; Rockstein, 1973).The nature and degree of tracheolization appear to depend on the metabolic demands of a tissue (Buck, 1962; Whitten, 1972; Wigglesworth and Lee, 1982). The tracheoles, besides the obvious difference in their smaller size, are said to differ from trachea in that they are contained in single tracheal cells (Wigglesworth, 19541, have a smooth papillate intima (Edwards et al., 1958;Locke, 1958a)(features both observed here in the grasshopper), in general do not appear to be shed during molting (Keister, 1948; Wigglesworth, 1954), lack taenidial ridges, and are usually less than 1 pm in diameter (Snodgrass, 1935; Wigglesworth and Lee, 1982). The growth of the tracheal system appears to be coordinated such that the crosssectional area of the trachea and the tracheoles largely remains constant (Bursell, 1970; Locke, 1958b),presumably maintaining a uniform or even reducing tracheal air-flow resistance (Weis-Fogh,1964a,b, 1967) until the tracheolar diameter may reach the mean free path of the diffusing oxygen molecules when further reduction in diameter would become a limiting factor to oxygen diffusion (Pickard, 1974). A notable variation apparently exists in the way that the tracheoles interact with the tissue cells (Day, 1951; Hodge, 1955; Edwards et al., 1958; Smith, 1968; Miller, 1974). In the order Odonata the flight muscles of the dragonflies and the damselflies are devoid of intracellular tracheolar invaginations though the dragonflies are considered to be strong fliers (Smith, 1966;Azuma et al., 398 J.N. MAINA Figs. 7 and 8 TRACHEA-AIR SAC SYSTEM IN A GRASSHOPPER 1985).The lack of the tracheolar intracytoplasmic arborizations in this group of insects could be attributed to the rather more primitive synchronous mode of contraction exhibited by their flight muscles, which are structurally similar to those of the vertebrates (Smith, 1966; Pringle, 1983). In the mosquito, tracheal invaginations are lacking in some organs like fat body, papillae, and testes (Edwards et al., 1958) while in the cicadas the invagination was not observed in the tymphal muscle (Smith, 1968).A close relationship between the terminal tracheoles and the mitochondria should increase the tracheolar diffusing capacity. Weis-Fogh (196413) contended that without tracheolar indentation of the tissue cell, oxygen supply would be inadequate in the insect’s flight muscles. Weight for weight, the insect flight muscle is the most energetically active tissue known (WeisFogh, 1964b; Smith, 1977).This is reflected in the high density of mitochondria in flight muscle, constituting up to 3040% of this tissue (Edwards and Ruska, 1955; Pringle, 1957,1983;Smith, 1961a,b, 1962;Wigglesworth and Lee, 19821, values as high as 50% having been reported by Ready (1983) in cricket flight muscle. Haemolymph in the majority of insects is not directly involved in gas transport as it largely lacks the oxygencarrying pigments (Florkin and Jeuniaux, 1974); the insects have successfully thus dissociated the dependence of the respiratory and circulatory system that typifies the vertebrates. The air sacs constitute an integral part of the insect respiratory system. Their structure and development seem to differ from species to species, being absent in the subclass Apterygota and well developed in the Diptera and Hymenoptera (Snodgrass, 1935). In the cicada Fidicina monnifera the air sacs together with the tracheal system constitute 45% of the body volume (Bartholomew and Barnhart, 1984). In the honeybee, where the air sacs are very well developed (Bets, 1923, 1933),the air sacs have no taenidia; such air sacs may respond to a greater extent to the increase and decrease in abdominal pressure and are more important in the ventilation of the tracheal system (Snodgrass, 1935).The functional anatomy of the air sacs in the insect respiratory system has been subject to a lot of discussion and even speculation: they were considered to be inadequately studied by Uvarov (1966), and functional analogy of the insect air Fig. 7. Tracheal supply to the flight muscle (m). The secondary trachea (st) give rise to the tertiary trachea (arrows) that invade the muscle bundles, a pattern reminiscent of a motor nerve ending. The figure is an enlargement of the enclosed area in the inset. The inset shows secondary (st) and tertiary (tt)trachea invading the muscle im). The flight muscles were particularly well supplied with air and had an independent supply of air from paired thoracic air sacs. The arrow in the inset shows a n uncoiled taenidium. Figure 7, ~1,044.Inset, x244. Fig. 8. A close-up of a tertiary tracheal (tt)penetrating into the flight muscle (m). The tertiary trachea give rise to the tracheoles, which in some tissues indent the cells as the terminal tracheoles. These minute tubules could not clearly be viewed here, not only because of their small size, but also because only the surface features are technically examined with the SEM. The inset shows the taenidia (arrows)covered by the glistening epithelial cell. The epithelial cell cover becomes less pronounced in the smaller trachea (cf. with the tertiary trachea in the same figure). Figure 8, x 420. Inset, X623. 399 sacs has even been drawn to the avian air sacs (WeisFogh, 1964a).The diverse functions of the air sacs in the insect have been summarized by Wigglesworth (1963, 1972). Further, they are thought to act as bellows in ventilating the tracheal system (Bets, 1933),to guarantee an intensive ventilation of the trachea during breathing (Demoll, 19271, to act as sound resonators (Pringle, 1957; Church, 19601, to act as heat insulators (Weis-Fogh, 1964a1, to assist in egg laying and molting (Clarke, 19571, and to permit the build-up of sufficient pressure to supply air to smaller tracheae (McCutcheon, 1940). In Hymenoptera, there is a good correlation between the development of the air sacs and the size and activity of the various species of insect (Tonapi, 1958). The insects inspire a volume of air equal to two-thirds the total volume of the tracheal system (Krogh, 1920b, 1941). This value was estimated at 5% at rest and at 20% in hyperventilation, with twice as high values expected in gravid females (Weis-Fogh,1967; Miller, 1974). As much as 70% of the total tracheal volume may be ventilated in a respiratory cycle in some insects (Bursell, 1970). The pattern of the flow of the air in the tracheal system has been debated for a long time. This could be attributed to the notable interspecific differences in the mode of air flow. While Snodgrass (1935) asserted that “there is no law governing inhalation and exhalation of the spiracles” and that “the respiratory currents may alternate even in the same individual,” studies by Krogh (1920a1, Lee (1925), McArthur (19291, McGoaan (1931), Fraenkel (19321, Bailey (19541, and Weis-Fogh (1967) have indicated what appears to be a definite pattern of air flow which may change in respect to the prevailing energetic demands. In the desert locust Schistocerca gregaria, during flight the air flows unidirectionally through the thoracic spiracles, supplying the head and the central nervous system and out through the abdominal spiracles, while at rest 50% of the total volume of air ventilated by the abdomen passes unidirectionally (Weis-Fogh,1967).In the honeybee, the tracheal air flow has a strongly unidirectional component during flight (Bailey, 1954). In Sphodromantis (Miller, 19741, 95% of the inhaled air passes unidirectionally while only 5% passes tidally when stimulated. Miller (1974) observed that the synchronized spiracle movements that typify a large number of orders of insects suggest that unidirectional air flow may be a universal feature in the larger insects. The unidirectional mode of ventilation in the respiratory organs is exhibited by diverse groups of animals such as some crustaceans, molluscs, and fish (Miller, 1974), but among the terrestrial species, this is only encountered in birds and insects. A plausible explanation for this similarity will be risked. The avian pulmonary system consists of compact inexpansile lung which is ventilated by the concerted action of the cranial and caudal groups of air sacs (for review of anatomy see King, 1966; and Duncker, 1971). The capacious air sacs can replenish the intrapulmonary air in a single respiratory cycle. The flow of the air in the parabronchial lumen (paleopulmo)is unidirectional and continuous during the two phases of respiration (see Scheid, 1979; Fedde, 1980; Burger, 1980; and Brackenbury, 1981, for recent reviews). In birds the air flow in the parabronchial lumen is by bulk flow and is entirely by diffusion in the exchange tissue. This compares with 400 J.N. MAINA Figs. 9 and 10 TRACHEA-AIR SAC SYSTEM IN A GRASSHOPPER Figs. 11-13 401 402 J.N. MAINA Figs. 14 and t5 TRACHEA-AIR SAC SYSTEM IN A GRASSHOPPER the nature of air flow in the tracheal system of the larger insects, where the movement of the air is by bulk flow in the trachea and by diffusion in the tracheoles. The spatial relationship between the bulk parabronchial air flow and the centripetal blood flow in the exchange tissue of the bird lung is cross-current(Scheid and Piiper, 1972). This type of gas exchange design is apparently the most efficient respiratory system to have evolved among the air-breathing vertebrates, since for an equivalent degree of ventilation the avian lung extracts more oxygen than the lungs of the other air-breathing vertebrates and the partial pressure of oxygen in the arterial blood may exceed that in the end-expired air (Piiper and Scheid, 1972). The pattern of gas exchange in insect tissue at the tracheolar level, whether these gas exchange tubules are intra- or extracytoplasmic, would appear to be a uniform pool (Piiper and Scheid, 1972) and structurally similar to that of the nonavian airbreathing vertebrates (amphibians, reptiles, and mammals) at the faveolar/alveolar level. This is particularly so as the terminal tracheoles appear to come to a dead end and presumably do not anastomose with each other (Wigglesworth and Lee, 1982). The tracheal system should, however, be highly effkient, as the oxygen is delivered directly to the mitochondria. Weis-Fogh (1964a1, on functional grounds, first drew attention to the similarity between the avian lung and the tracheal system of the insect on the basis of the unidirectional and continuous mode of ventilation. In birds the inhaled air bypasses the gas-exchange part of the lung via the intrapulmonary part of the primary bronchus to the caudal air sacs (King, 1966; Duncker, 1971). In the locust, Weis-Fogh (1964a) observed tracheal shunts bypassing the immediate flight muscles to supply oxygen to parts of the muscle farthest away from the spiracle. Though in the bird lung the primary bronchus is basically a gas conduit, the shunting of the air from the trachea to the posterior air sacs facilitates the continuous and unidirectional ventilation of the paleopulmonic parabronchi which compose the bulk of the respiratory tissue in some birds. Fig. 9. The tracheal supply to the abdominal muscles (m). The secondary trachea (st) that arise from the primary trachea (see Fig. 6) branch profusely, giving rise to the tertiary trachea (arrows) that invade the muscle. The white arrowhead shows the luminal aspect of the gastrointestinal tract. Like the flight muscles, the abdominal muscles were well supplied with air and mt for osmoregulation. The inset (slightly enlarged area enclosed in the figure) shows the terminal parts of the secondary trachea (st) giving rise to the tertiary trachea (arrows) that penetrate the abdominal muscle (m). mt, malpighian tubules. Fig. 9, x27. Inset, ~ 3 7 . 403 The design specialization in the respiratory system of insects and birds indicates the large demands to which the system is subject because of their unique capacity for flight. The structural and functional similarity in insect and avian respiratory systems, in animals so phylogenetically remarkably different, could be viewed as a classic case of systemic convergent evolution that has given rise to highly efficient respiratory systems that have enabled these animals to exploit the aerial ecological niche. ACKNOWLEDGMENTS I wish to thank Prof. E.R. Weibel, the director of the Institute of Anatomy, University of Bern, Switzerland, in whose laboratory the SEM part of this work was carried out. I am indebted to Dr. P. Gehr and Mr. K. Babl for numerous courtesies during this time, without which this work would not have been possible in the present form. This study received generous financial help from the Leverhulme Trust of London, for which I am grateful. LITERATURE CITED Azuma, A.S., I. Azuma, 0. Watanabe, and T. Furuta 1985 Flight mechanics of a dragon fly. J. Exp. Biol., 116t79-107. Bailey, L. 1954 The respiratory currents in the tracheal system of the adult bee. J. Exp. Biol., 31:589-595. Bartholomew, G.A., and C.M. Barnhart 1984 Tracheal gases, respiratory gas exchange, body temperature and flight in some tropical cicadas. J. Exp. Biol., 111:131-144. Bets, A.D. 1923 The Practical Bee Anatomy. The Apis Club. New York. Bets, A.D. 1933 How bees fly. The Bee World, 14:50-55. Brackenbury, J.H. 1981 Air flow and respired gases within the lung air sac system of birds. Comp. Biochem. Physiol. [A], 68:l-8. Bridges, C.R., P. Kestler, and P. Scheid 1980 Tracheal volume in the pupa of the saturniid moth Hyalophoru cecropia determined with inert gases. Respir. Physiol., 40:281-291. Buck, J. 1962 Some physical aspects of insect respiration. Annu. Rev. Entomol., 7:27-56. Burger, R.E. 1980 Respiratory gas exchange and control in the chicken. Poult. Sci., 59:2654-2665. Bursell, E. 1970 An Introduction to Insect Physiology. Academic Press, London. Callahan, P.S. 1972 The Evolution of Insects. Holiday House, New York. Fig. 12. A high-power view of a tertiary trachea (Tt) in the flight muscle showing the characteristic tacnidia (Tnl overlaid by an epithelial cell (arrows). ~ 6 8 , 3 3 0 . Fig. 13. A high-power view of a terminal tracheole (Tr)in the flight muscle. Note the reduced sporadicity of the taenidia and the mitochondria (Mt) lying close to the tracheole. The arrows show the invagination of the sarcoplasmic membrane as the tracheole invades the muscle tissue. ~ 1 2 6 , 7 0 0 . Fig. 14. A high-power view of tracheoles (Tr),in the gastrointestinal tract, contained in a tracheoblast. Note the numerous electron-dense granules in the tracheoblast. The inset shows a single tracheole contertiary trachea (arrows) that penetrate the intestinal wall giving rise tained in a tracheoblast. These cells are associated with the develop. ~352,000. to tracheoles (t); mt, the osmoregulatory malpighian tubules that sim- ment of the trachea. ~ 1 7 6 , 2 4 0Inset, ilarly invade the gut wall (asterisk).The open arrow shows the luminal Fig. 15. An intracytoplasmic extension of a tracheole (Tr)into a cell aspect of the gut. The inset shows the smaller single air sacs (as) located dorsal to the gastrointestinal tract. The arrows show the epi- of the gastrointestinal system. The membranes that surround the tracheole in this very-high-power view are evident. From the inside thelial cell cover and mt. Figure 10, x340. Inset, x50. the tracheole is surrounded by an intratracheal membrane (It), a tracheal membrane (Tm), and the unit membrane (the plasma membrane) Fig. 11. A transmission electron micrograph of the flight muscle of of the invaginated cell (Pm). Note the mitochondrion (Mt) in the cytoChrotogonus senegalensis showing a haemocyte (Hc) contained in a plasm of the tracheoblast (Tb).The arrows show mitochondria in the haemolymphatic duct running next to the muscle. The terminal tra- cytoplasm (Ct) of the invaded cell lying in close proximity t o the chea gives rise to the tracheoles (Tr)that invade the muscle and come terminal tracheole. The inset shows an invading tracheole transected to lie next to the mitochondria (Mt). Sr, sarcoplasmic reticulum. longitudinally. The plasma membrane of the invaded cell (arrow) can ~37,700. be seen. ~ 3 2 2 , 4 2 4 Inset, . ~105,882. Fig. 10. A transverse view of the gastrointestinal tract showing the tracheal air supply. 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