The terrestrial Arthropods (Arachnids, Myriapods, Hexapods) represent one of the main examples of the conquest of emerged earth by the living organisms, analogous, as width and importance, to the parallel evolutionary history that involved the kingdom of Plants and the vertebrates of the clade of Tetrapods. The Insects are organisms strongly adapted to terrestrial life even in the most primitive forms. The development of a integument impermeable to gases and water vapour and a respiratory system that replace the ancestral condition of aquatic life, the cutaneous respiration, are included among the basic conditions which permitted the primary adaptation to the terrestrial life. This evolutionary adaptation has allowed to overtake the limits of cutaneous respiration, which exposes the body to the dehydration, however preserving the gas exchanges by diffusion through a liquid substrate.

In Insects, the respiratory function is done by an apparatus that includes separated from the circulatory system. This apparatus is composed of an internal circulatory system, which distributes oxygen to the cells through a capillary network, and some external organs which allow the gas exchange. The tracheal system of Insects is analogous to the respiratory system of tetrapod vertebrates, because it increases the surface needed by the entire body to gas exchanges while limits the loss of water by evaporation. The tracheal system is even analogous to the circulatory system of vertebrates, because its capillary network of tracheoles provides the liquid interface where the diffusion of gases occurs.

To fully perform its biological function, the internal tracheal system is completed by a morphological, anatomical, and physiological interface that allows the gases exchange with the external environment, preventing as far as possible the loss of water through evaporation. In the ancestral condition common to all insects, this interface is given by the spiracular system. In the larvae, the structural and morphological features of these external organs have a great importance because they can be elements of taxonomic diagnosis.

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Adaptations to aquatic habitats

The adults of Diptera are insects whose life takes place in emerged environment, even when they are related to wet habitats. In contrast, the Diptera larvae live often in fluid or semi-fluid substrate so that they have anatomical, morphological and ethological adaptations needed to allow the oxygen supply in environmental conditions incompatible with the breathing. These adaptations are important from a dual point of view, taxonomic and biological sensu lato.

From the taxonomic point of view, the configuration of particular morphological and often ethological adaptations provide important elements of determination of larvae, that usually are more difficult to identify than adults.

From the biological point of view, the larvae living in aquatic habitats (o similar to them) offer a composite rappresentation of how evolution and biodiversity allow the occupation of ecological niches, through both specific adaptations and convergent evolution. Hinton (1953) gave an interesting approach to this phenomenon. Considering that both Apterigotes and Pterigotes, in all postembryonic and imaginal stages, are provided with a tracheal system, that may be secondarily reduced only in small groups of endoparasitic insects, Hinton stated that the insects of present day originate by ancestors already provided with a integument impermeable to gases and a tracheal system completed by external spiracles. This statement extends even to preimaginal stages of Endopterigotes and overcomes any objection of ontogenetic point of view, since the tracheal system and spiracles evolve during the embryogenesis (Courtney et al., 2000). The secondary adaptations to aquatic life of Diptera larvae are ascribable to four types (Hinton, 1953):

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Tracheal system of a segment
Fig. 1 - Tracheal system of a segment.
Ad: dorsal anastomosis; sp: spiracle; c.t: transverse connective; TD: dorsal longitudinal trunks; TL: lateral longitudinal trunk; t.sp: spiracular trachea; Tvs: visceral trachea; Tvt: ventral trachea.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

Internal tracheal system

The tracheal system is composed of a network of tubules of two different types, respectively called tracheae and tracheoles.

The trachee are tubules originated from exoskeleton invaginations with a wall covered by a chitinized cuticle strenghtened with circular o spiral tickening called taenidia (sing. taenidium). These tubules start from the spiracles and branch spreading into the entire body with a progressively smaller diameter, until continuing with the tracheoles (Tremblay, 1985).

The tracheoles are blind-bottomed tubules, even branched, not chitinized and with a microscopic diameter. The end of each tracheole is less than 0,1 μ and penetrates into a cell. The tracheoles are alternately filled and emptied by a fluid in which the oxygen is disperded, allowing its diffusion into the cells (Tremblay, 1985).

The tracheae are part of the integument and their cuticle is renewed by the moulting. The tracheoles are not instead renewed with the moulting but form a capillary network which dynamically adapts independently from the development of the insect, because they can be removed or rebuilt according to the oxygen demand by tissues. Their building is provided by star-shaped cells called tracheoblasts (Tremblay, 1985).

The primitive structure of the tracheal system of the Diptera larvae has been detailed by Teskey (1981a), according to works of previous Authors (Figs. 1 and 2). In the groundplan of Diptera larvae ten pairs of tracheal spiracles are present. An anterior and posterior pairs take place respectively in the prothorax and last abdominal segment. The other eight pairs are distributed laterally on the metathorax and the first seven abdominal segments.

A spiracular trachea starts from each spiracle and connects by a transverse connective to two longitudinal respiratory canals: the dorsal longitudinal trunk and the lateral longitudinal trunk. In each side, the longitudinal trunks converge at both ends and connects to the anterior and posterior spiracles.

The two dorsal trunks are connected by ten dorsal anastomoses and the lateral trunks branch out, in each side, two series of ventral and visceral tracheae, which serve respectively the ventral parts of the body and the internal organs. The first pairs of ventral tracheae are joint by three ventral anastomoses.

The tracheal system of the cephalic region has a different structure because the tracheation of the head starts from the anterior spiracles of the thorax. In the larvae of Diptera, in each side, three cephalic branches start from the thoracic network, connected by a commissure called dorsal cervical anastomosis. Two of these branches are identified by Teskey (1981) as dorsal and ventral cervical tracheae. A third branch, called supraesophageal ganglionar trachea is present in Diptera larvae. Snodgrass (1935) described two pairs of primary branches and his terminology is analogous to names adopted by Teskey, however, the Author referred about the possible occurrence of further tracheae derived from the immediate branching with unknown homology.

The general pattern described may differ, throghout the order, with la reduction of the number of the transverse anastomoses and the possible presence, in some aquatic forms, of vesicular expansions that have a hydrostatic function (Teskey, 1981a).

Tracheal system of Diptera larvae
Fig. 2 - Diagram of the primitive olopneustic tracheal system in Diptera larvae. A: dorsal view. B: lateral view.
Ac: cervical anastomosis; Ad: dorsal anastomoses; Av: ventral anastomoses of ventral tracheae; s.a: anterior spiracles; Ct: transverse connectives; s.ad: abdominal spiracles; s.mp posterior thoracic spiracles; s.p: posterior spiracles; Tcd: dorsal cervical tracheae; Tcg: supraesophageal ganglionic tracheae; Tcv: ventral cervical tracheae; TD: dorsal longitudinal trunks; TL: lateral longitudinal trunks; Ts: spiracular tracheae; Tvs: visceral tracheae; Tvt: ventral tracheae.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

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Tracheal spiracles

The tracheal spiracles are openings in which the spiracular tracheae lead. They are described in detail by several Authors, including Teskey (1981a) and Courtney et al. (2000), and have great taxonomic importance because their structure may differ throughout the order. However differences in structure, morphology and distribution occurs also in the same insect according to the larval stage and the segment. Regardless of structural complexity and its differences, the general and considerable features are the lacki of an internal closing apparatus, the existence of a variable number of openings for each spiracles, the aspect of the peritreme, the structure of the atrium, the consequent aspect of the spiracular area. Another anatomical element of great importance, in order to the functionality of spiracles in wet habitats, is the existence of glands secreting a water-repellent fluid and waterproof hairs variously arranged patterns.

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Terminology and classification

Important elements in the taxonomic determinazione are the number and position of functional spiracles. The basic condition of Diptera larvae is the existence of ten pairs of tracheal spiracles, placed as described in the previous section. In order to identify these spiracles, Teskey (1981a) and Courtney et al. (2000) suggested a terminology an univocal terminology: the prothoracic spiracles would appear as derived from the mesothorax and the eighth pair is often borne on the 7th apparent segment. The apparent position therefore could be derived from spiracles that has moved from the original segments. For this reason, Authors proposed a terminology as follows:

Classification of spiracular systems
Fig. 3 - Classification of spiracular systems based on the number of functional tracheal spiracles in the Diptera larvae.
A: Polypneustic systems: 1: holopneustic; 2: peripneustic; 3: hemipneustic.
B: Oligopneustic systems: 4: amphipneustic; 5: propneustic; 6: metapneustic.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

In most of Diptera larvae, the basic number just reported is reduced, until the extreme condition of the lack of spiracles. While the polypneustic systems have more than two pairs of spiracles, in oligopneustic condition functional spiracles are reduced to one or two pairs. According to the number of functional spiracles, the following adjectives are adopted (Teskey, 1981a; Tremblay, 1985; Courtney et al., 2000, Fig. 3):

As stated above, larvae of most Brachycera are usually included, as general reading, in the amphipneustic type (Colless & McAlpine, 1970; Teskey, 1981a; Matile, 1993a; Tremblay, 2005). However, more detailed studies refer the loss of the functionality of some spiracles. For example, Courtney et al. (2000) include the larvae of most Brachycera in the amphipneustic type but as functional condition, because other spiracles are present but vestigial. Another fact to consider is the difference among the larval stages. Author usually refer to the last instar larva, that is the best known if not the only one. It is however established that the larvae of Cyclorrhapha are metapneustic in first instar and amphipneustic in the second and third stages (Matile, 1993a; Tremblay, 2005; Courtney et al., 2000). In any case, the brachycerous larvae show several specificities, that diverge from the general condition as described (Courtney et al., 2000).

Courtney et al. (2000) has given a detailed comparative dissertation about the morphological and structural features of the anterior and posterior spiracles.

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Anterior spiracles

The anterior spiracles usually take place in a laterodorsal position near the edge of prothorax, but there are also other positions as dorsal in larvae of Agromyzidae or mesothoracic in those of some Syrphidae. The morphology is useful in order to the taxonomic diagnosis, due the variability and specificity. In lower Diptera (Nematocera and Orthorrhapha) the anterior spiracles are generally sessile and have one to several openings. In Cyclorrhapha, they are quite prolonged and retractile, due a protusion of the atrium, and have several openings, usually 5 to 20. In larvae of lower Cyclorrhapha, these openings are borne at the tip of a common stalk, while in those of Schizophora are singly placed at the tip of lobes variously arranged, as tree-like or fan-like.

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Posterior spiracles

Regarding the posterior spiracles must observe that the relation between these spiracles and the segmentation has not great importance, because the secondary segmentation, during the embryogenesis, leads to the fusion of the last primary segments.

The larvae of Nematocera have dorsal or posterodorsal spiracles on the last apparent segment. Usually they are sessile, but may be combined with structures needed to guarantee their functionality. This condition occurs specially in larvae that have aquatic or semiaquatic life but keep the breathing through the posterior spiracles in contact with the water surface. The spiracles of these larvae works as a snorkel for swimmers and this conditions their behavior. For example, le larvae of Dixidae live on the surface fixed to solid substrate with the body bended to U shape; those of several Culicidae live suspended immediately under the surface of backwater to keep the posterior spiracle in contact to the air; those of other culicids breathe piercing taking the air from the air parenchyma of submerged plants after pierce the tissues of submerged plants with their spiracles and take the oxygen from the air parenchyma; those of Ptychopteridae live underwater near the surface because breathe through a long siphon bearing the spiracles at the tip; those of several Tipulidae live near the surface of moist substrates. These behaviors are related with special adaptations of spiracles, in order to protect them from the water ingress and guarantee the continuity with the air. These are, for example, lobes surrounding the spiracular plate (Tipulidae) and patterns of setae or hairs used as waterproof or to break the surface tension (Culicidae). In other groups, however, larvae have a long caudal respiratory tube, called siphon, bearing the posterior spiracles at the apex.

The larvae of lower Brachycera show a variability that make difficult a general discussion. The si riscontra una variabilità tale da rendere difficile una descrizione generale. The spiracles are generally placed on the apparent segment VIII. Referring to the apparent segmentation, since it has eight apparent segments in most groups, the position of posterior spiracles is terminal. Within this heterogeneous group, the Asiloidea have a secondary segmentation of abdomen composed of nine apparent segment, thus the position of posterior spiracles is on the penultimate segment. But the larvae of Therevidae and Scenopinidae, that have an additional segmentation, have the posterior spiracles on the antepenultimate segment. Finally, the position on the segment is usually dorsal, but even other positions occur, as lateral (Scenopinidae and Therevidae) o posterior (Tabanomorpha in part and Empidoidea).

As seen in nematocerous larvae, specific morphological elements must be related to the habitat and behavior. In most cases they occur in aquatic forms of some groups.

The larvae of Cyclorrhapha show a substantial homogeneity. In general the posterior spiracles are the main organs involved in the gas exchanges, are borne by the last segment and are joint to the dorsal longitudinal trunks of the internal tracheal system. The spiracles are placed in two chitinized plates surrounded by the peritreme and open usually with three slits. The spiracular openings are often combined with glands secreting a water-repellent substance and four tufts of waterproof hairs. Various features are useful to the taxonomic diagnosis (Courtney et al., 2000), as the number, size, shape and position of the slits, the pattern of the tufts, the position and shape of the spiracular plates and their possible fusion.

The most relevant structures are siphons and respiratory spines.

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Larva of Mesaxymyia
Fig. 4 - Larva of Mesaxymyia Mamaev (Nematocera: Axymyiidae).
s: respiratory siphon; p: anal papillae.
Author: Giancarlo Dessì
(License: Creative Commons BY-NC-SA)

Respiratory siphons

In Zoology, the term siphon is often used to refer to an organ that resembles the hydraulic siphons in structure, shape or function and it connects two internal organs or a internal organ with the outside. Within the specific terminology of Diptera, this word is usually related to the breathing function performed by special adaptation of the spiracles in preimaginal stages of some groups. In the Diptera larvae, when existing, the respiratory siphon is an extension of the atrium of the posterior spiracles, combined with a concrescence of the integument. This complex forms a common appendage that bears the spiracular openings at the tip. The respiratory siphon is an exclusive feature of some larvae living in aquatic habitats o similar but have retained the breathing function.

The respiratory siphon exists in the whole order of Diptera, in families phylogenetically unrelated. Furthermore, in most cases it is not representative of the entire family, but only single small groups within the family, as genera, tribes or subfamilies, are provided. For these reasons the development of a respiratory siphon must be interpreted as secondary character derived from convergent evolution.

Nematocera

Among the Nematocerous, a long respiratory siphon is representative of the larvae belongig to Ptychopteridae and Axymyiidae families, while in larvae of Culicidae and Psychodidae it is short or almost absent (Teskey, 1981a, 1981b; Smith & Ferrar, 2000). Respiratory siphon are also reported by some Authors about Chaoboridae and Corethrellidae (Matile, 1993c; Sæther, 1997a, 1997b). The length of the siphon is the most important element that allows to distinguish the larvae of Axymyiidae and Ptychopteridae from those of other families, being slender and at least as long as the rest of the body (Teskey, 1981b; Smith & Ferrar, 2000).

The larvae of Axymyiidae (Wood, 1981; Krivosheina, 2000) live in galleries dug in the rotten wood on damp soils. These larvae have a siphon chitinized at the apex, mobile and slightly retractable. They can be also distinguished for two or four long anal papillae.

The larvae of Ptychopteridae (Alexander, 1981; Rozkošný, 1997a) live in the slush of backwaters. They have a long siphon fully or partially retractable, used to draw the air from the surface.

The larvae of Culicidae (Teskey, 1981b; Stone, 1981; Tremblay, 1991a; Matile, 1993b; Smith & Ferrar, 2000; Minář, 2000) breathe through the posterior spiracles in contact with the water surface or introduced into the tissues of submerged plants. In Culicinae and Toxorhynchitinae subfamilies, the larvae have spiracles borne by a short conical or cylindrical process, more or less elongated. In Anophelinae, the spiracles are sessile, thus the larvae lack a siphon.

Among the larvae breathing from the water surface, the lack of the respiratory siphon conditions the behavior and allows a the recognition of some mosquito larvae according to their position (Tremblay, 1991a; Matile, 1993b. Fig. 5).

Anopheles and Culex larvae
Fig. 5 - Positions taken by larvae of Anopheles Meigen and Culex Linnaeus (Nematocera: Culicidae)
A: larva of Anopheles larva. B: larva of Culex.
Author: Robert Evans Snodgrass (1909)
Modified from the original drawing
(License: Public Domain)

In family Psychodidae there is a strong heterogeneity related to various habitats, including even aquatic or semiaquatic of Horaiellinae, Sycoracinae and most Psychodinae (Quate & Vockeroth, 1981; Wagner, 1987). Larvae of Psychodinae have a short subcylindrical siphon dorsal on the last segment; this character is a determination key within the subfamily proposed by Quate & Vockeroth (1981).

Lower Brachycera

Among the lower Brachycera, short respiratory siphons may be present in aquatic larvae of Tabanidae (Teskey, 1981a; Smith & Ferrar, 2000) and Stratiomyidae (Teskey, 1981a; Courtney et al., 2000; Smith & Ferrar, 2000).

In Tabanidae the respiratory siphon is a conical process that protrudes from the posterior end in larvae of Chrysopsinae and Tabaninae subfamilies (Pechuman & Teskey, 1981). Tremblay (2005) refers to this siphon as similar to a cork.

In Stratiomyidae no explicit references to respiratory tubes has given by James (1981), in the Manual of Nearctic Diptera, and Rozkošný (1997b), in the Manual of Palaearctic Diptera, editors of the chapters about this family. The presence in this family was instead referred by Teskey (1981) and Courtney et al. (2000) in their monographs about the larvae, and by Smith & Ferrar (2000) in the keys to the taxonomic determination. The siphon is evident in some aquatic larvae of Stratiomyinae, specially those of Stratiomys Geoffroy. The body of these larvae is posteriorly tapered and is prolonged in a tubular structure bearing the spiracles at the tip, protected by a fringe of setae (Rozkošný, 1997b; Courtney et al., 2000). The function of these bristles would be to hold an air bubble to allow the breathing underwater. Similar but shorter structures occurs also in aquatic larvae of Oplodontha Rondani (subfamily Stratiomyinae) and Oxycera Meigen (subfamily Clitellariinae).

Cyclorrhapha

The cyclorrhaphous larvae have often posterior spiracles borne by process more or less prolonged, but in the Manual of Nearctic Diptera, Manual of Palaearctic Diptera and other works, only those occurrent in some larvae of Syrphidae, Ephydridae and Aulacigastridae was referred as respiratory tubes or siphons (Teskey, 1981a, 1987; Wirth et al., 1987; Papp, 1998; Mathis & Zatwarnicki, 1998; Courtney et al., 2000; Smith & Ferrar, 2000; Tremblay, 2005). Certain Authors however referred about structures that could resemble siphons, using terms more or less explicit, also for larvae of other families, as Phoridae (Peterson, 1987; Tremblay, 2005), Heleomyzidae (McAlpine, 1987), Canacidae (Wirth, 1987; Mathis, 1998; Smith & Ferrar, 2000), Tachinidae (Smith & Ferrar, 2000).

The larvae of Syrphidae are heterogeneous in habitat, behavior and morphology (Vockeroth & Thompson, 1987; Thompson & Rotheray, 1998 Smith & Ferrar, 2000). In most of the family, the posterior spiracles of the larvae are borne by a common dorsal process, usually not retractable (Vockeroth & Thompson, 1987). In most of the family, this respiratory process is also short. In the subfamily Eristalinae, the larvae of Eristalini tribe live in water poor in oxygen and rich in organic matter, often in sewage or other waste water, and have a peculiar adaptation: to take the air from the surface, the process bearing the spiracles is transformed in a long respiratory siphon, provided with intrinsic musculature and telescopic. The exceptional length and the shape of the body of these larvae resemble the shape of rats; for this reason these larvae are known as rat-tailed maggots. The larva of Eristalis (Eristalis) tenax (Linnaeus), a common and cosmopolitan species, can be longer than 2 cm and the siphon can reach 15 cm in lenght, so that it can be easily recognized.

Larva of Eristalis
Fig. 6 - Larva of Eristalis Latreille (Brachycera: Syrphidae).
Author: Jerry Oldenettel
(Home page at Flickr)
Resized from the original picture
(License: Creative Commons BY-NC-SA 2.0)
 

The Aulacigastridae is a small family whose taxonomy is still evolving. The larvae of most species of this family are unknown and only the morphology of Aulacigaster leucopeza (Meigen) is well described (Teskey, 1981a; Rung et al., 2005). The larvae of A. leucopeza live in sap exudes from wounds of various trees and breathe through the spiracles borne at the tip of a long and retractable respiratory siphon (Teskey, 1981a).

The Ephydridae represent one of most important ciclorrhaphous families associated with aquatic or transitional ecosystems, with larvae predominantly aquatic or semiaquatic. An extension bearing the posterior spiracles is a frequent feature of ephydrid larvae (Tremblay, 2005), but the specialist literature on this family (Wirth, 1987; Mathis & Zatwarnicki, 1998) indicates the tribe Ephydrini as the group whose larvae have a typical respiratory siphon. The body of larvae of Ephydrini is posteriorly tapered and prolonged by a thin caudal appendage, relatively long and bifidus at the apex.

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Respiratory spines

In literature the term respiratory spines refers to shapes of posterior spiracles that allows to pierce the tissues of aquatic plants. This name was used by Snodgrass (1935) to describe the structure of the posterior spiracles in the larvae of Donacia Fabricius (Coleoptera: Chrysomelidae). In these larvae, the piercing organ is formed by an extension of the peritreme in which there are the sacs formed by the spiracles. In works about the Diptera, the use of this term is sporadic. Teskey (1981a) defined as "respiratory spines" the piercing organs of some larvae of Tabanidae, Syrphidae and Ephydridae. Courtney et al. (2000) use this term to indicate generically piercing shapes of posterior spiracles in aquatic or endoparasitic cyclorraphous larvae that take the air from the tissues of submerged plants or the tracheal system of the host. The term was used even by Steyskal (1987) referring to some larvae of Micropezidae. However the larvae of this family are not aquatic and the function of these organs is unknown in larvae of Micropezidae and other families (Teskey, 1981a).

In literature various Authors have reported about the breathing of aquatic larvae, through the air taken from internal tissues of submerged plants, by means particular patterns of the spiracles able to piercing. This morphological and ethological condition occurs in various families, both nematocerous and brachycerous, but involves species of small groups, often single genera. Furthermore these adaptations are mentioned contextually to the presence of respiratory tubes in larger groups of the same family. Also in this case these adaptations should be derived from convergent evolution. Among the Diptera whose larvae have this condition, the following has been often cited:

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Gills

In Entomology the word gill should be used with caution because it can be misleading. In this regard, see the definition of the lemma given by the Encylopedia Britannica[1]. The gill sensu stricto is a organ of strictly aquatic animals derived from an extension of the integument containing blood, into a capillary network or coelomatic cavity. Its properly respiratory function is to allow the gas exchanges by diffusion between the blood and the liquid outside. In order to perform this function, the gills have an adeguate surface and a permeable integument. Organs of this type are present in various groups of the Animalia kingdom, as result of the evolution from aquatic ancestor, among both vertebrates and invertebrates, included some arthropods (e.g. the Crustaceans). In all these organisms, the gas exchange through the gills is supposedly an apomorphic condition within the various single groups.

As stated in the introduction, the Insects are organisms derived from ancestors perfectly adapted to the terrestrial life and the breathing. The successive colonization of aquatic habitats, through convergent evolution, led to adaptations in the anatomy, physiology, and behavior that are not related to the branchial respiration. If we want to make an analogy, we could compare the adaptations of the aquatic and semiaquatic insects with the changes that involved the ancestors of the aquatic and semiaquatic mammals, as whales, manatees, and seals, respectively related to deers, elephans, and dogs.

The aquatic insects show three possible type of organs that may resemble, by relevancy or analogy, the true gills: the blood gills, spiracular gills, and tracheal gills (Servadei et al., 1972; Tremblay, 1985).

The blood gills are anatomically analogous to the gills sensu stricto of animals primary aquatic. They are derived from extensions of the integument which have lost the ancestral impermeability to gases and water and inside they contain hemolymph but not tracheae. Several Authors referred about the presence of these organs in larvae of holometabolous insects, even Diptera included (Servadei et al., 1972), however the blood gills of Insects have not anatomical relationships with the tracheal system. The permeability of the integument could allow gas exchanges, but this respiratory functions should be secondary to other functions, while these organs would perform an osmotic function thus they are primary involved in the regulation of the hydrosaline balance (Servadei et al., 1972; Teskey, 1981a; Tremblay, 1985; Courtney et al., 2000).

The spiracular gills are derived from extroversions of closed spiracles whose interior is connected to the coelom (Servadei et al., 1972; Tremblay, 1985). Their presence among the Diptera is reported about aquatic forms of pupae, but the larvae lack this type of organs.

The tracheal gills are extensions of the integument containing a dense network of tracheae that have not connections with outside (Servadei et al., 1972; Tremblay, 1985). The analogy with the gills sensu stricto is only functional, because these organs perform an exclusive respiratory function but have not relationships with the circulatory system. From an evolutionary point of view, these organs the tracheal gills could be assumed the highest level of the secondary adaptation to the aquatic life. Tracheal gills have been widely reported in literature but their presence in the Diptera is however a rare condition because it is confined to larvae of some Chironomidae and all the Blephariceridae (Teskey, 1981a; Courtney et al., 2000). Both these families have larvae strongly adapted to the aquatic life and have an apneustic tracheal system. Various Authors formerly referred as tracheal gills also the anal papillae, but Teskey (1981a) and Courtney et al. (2000) reiterated that this statement is unfounded.

Hogue (1981) and Courtney (2000) provided a detailed description of the tracheal gills of blepharicerid larvae. These structures are fingerlike and gathered in tufts of 3-7. The tufts are placed in two ventral series anterolateral to each sucker except those of the cephalothorax. Thus a larvae is provided of 10 tufts of tracheal gills. The last segment bears also a tuft of four similar appendages behind the last sucker; Courtney (2000) identifies these as anal papillae, thus they should be osmoregulatory organs. The first instar larvae of all Blephariceridae and the second instar larvae of some genera lack the tracheal gills (Courtney, 2000).

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Other adaptations

In order to complete the context, we can mention the existence of particular adaptations which occurs in extreme conditions, specially in habitats poor in oxygen due the difficulty of change. These adaptation involve often larvae of Chironomidae. Larvae of this family are apneustic but can have even other adaptations that represent a high degree of specialization. These adaptations include, for example, the cutaneous respiration (Hinton, 1958; Darvas & Fónagy, 2000; Hövemeyer, 2000), the presence of hemoglobin in the hemolymph (Tremblay, 1991b; Courtney et al., 2000; Hövemeyer, 2000) and perhaps an anaerobic metabolism (Tremblay, 1981b). Analogous adaptations or of a different type may be concern larvae of endoparasitic species (Darvas & Fónagy, 2000; Hövemeyer, 2000).

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Bibliografia

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