The book lungs of Scorpiones and Tetrapulmonata (Chelicerata, Arachnida): evidence for homology and a single terrestrialisation event of a common arachnid ancestor

The question of whether Arachnida (Chelicerata) conquered terrestrial habitats only once or several times is controversial. The key group in this respect is the Scorpiones. Several authors claim that they became terrestrial independently of other arachnid lineages. This argumentation uses two lines of evidence. One is that book lungs of scorpions and other arachnids are considered non-homologous because they occur on different segments. The other line is based on fossil evidence which suggests that early scorpions were aquatic, together with a putative sister group relationship between scorpions and the aquatic Eurypterida. To address this problem we undertook a comparative scanning electron microscopical and histological study of the book lungs of scorpions, amblypygids, uropygids, and mesothelid spiders. In addition, we included the book gills of a xiphosuran. We found several detailed similarities in the book lungs shared by all arachnid taxa studied. Based on these findings we conclude that arachnid book lungs are homologous. Furthermore, we suggest that the apomorphic book lungs of arachnids indicate a single terrestrialisation event in the stem lineage leading to Arachnida.     

Did the first insects live in water or in air?

We evaluate the arguments and evidence for a terrestrial vs an aquatic origin for the Insecta. The evidence falls into three categories: (1) evidence that does not support one view more than the other; (2) speculative evidence, which should carry little weight; and (3) evidence that does support one view more than the other. Category 1 includes evidence from locomotory and osmoregulatory systems; plausible functions have been proposed for 'protowings' in both aquatic and terrestrial environments, while locomotory and osmoregulatory mechanisms of insects shed little light on their origins. Fossils, phylogenetic speculation, gill structure and life histories fall into category 2, in which, although speculative, the evidence favours a terrestrial origin. The earliest fossil hexapods were apparently terrestrial and unequivocally aquatic hexapods do not appear until 60–70 million years later, while sister-group relationships point to a terrestrial life style from at least the Hexapoda-Myriapoda stem group. The great variation in gill structure, even within orders, suggests convergence, and the more or less completely aquatic life histories are better interpreted as steps towards independence from land, rather than signs of an aquatic origin. Category 3 includes evidence from the tracheal system. In order to have evolved in water, a tracheal system must have first invaginated, then connected with the body wall for gas exchange with the water, and thirdly connected with the internal organs. It is difficult to envisage functions for the first two stages; on the other hand, the system could have readily evolved on land by invagination of respiratory surfaces, and then have been modified to effect gas exchange in water via gills.     

Land‐use effects on terrestrial consumers through changed size structure of aquatic insects

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Proposed habitats of early tetrapods: gills, kidneys, and the water-land transition

Recent finds of early tetrapods have established that the most primitive form, Acanthostega, retained internal gills and other fish-like features; this has led to the conclusion that it was a primarily aquatic animal. Other Late Devonian tetrapods, such as lchthyostega and Tulerpeton, provide no evidence of internal gills, but have also been interpreted as inhabiting an aquatic environment. The probable aquatic habits of a diversity of Devonian tetrapods has led to the suggestion that the entire early tetrapod radiation may have been an aquatic one, with terrestriality having evolved in later forms. However, consideration of the physiology of living amphibious vertebrates suggests that this scenario is unlikely. The use of the gills for the excretion of carbon dioxide and ammonia appears to be a fundamental feature of all primarily aquatic vertebrates. No living fish loses its internal gills, even if it excretes a significant portion of its nitrogenous waste as urea via the kidney in the water. Gills are simply too valuable to be lost by an aquatic animal, even in those air-breathing fishes that no longer use the gills for oxygen uptake. We suggest that the apparent loss of the gills in tetrapods more derived than Acanthostega signals their descent from a more terrestrial phase in tetrapod evolution, following the primary assumption by the kidney of the excretion of nitrogenous wastes. Without this new role of the kidney, loss of the gills would have been impossible. With this new kidney role, loss of the gills may have been advantageous in reducing desiccation on land.     

The influence of freshwater-lake subsidies on invertebrates occupying terrestrial vegetation

Studies investigating effects of aquatic-derived resource subsidies have often found large effects on terrestrial systems. Those studies have mostly been performed on effects of subsidies derived from oceanic and riverine systems, and very few have considered effects of subsidies from freshwater lakes. However, since lakes can produce large quantities of emergent aquatic insects that end up on nearby land, it is likely that also freshwater-lake subsidies influence terrestrial systems. We performed sweep-net collections of aquatic and terrestrial invertebrates at varying distances from the shore on vegetation of islands of varying size, in two freshwater lakes in northern Sweden, as well as on the surrounding mainland. We found that the amounts of aquatic insects on terrestrial vegetation decreased with distance from the shore, and that they were the most abundant on small islands, presumably because small islands have a higher perimeter-to-area ratio. Web-building spiders responded positively to the aquatic subsidy by being the most abundant on small islands and by showing a positive relationship with aquatic insect biomass. However, distance from the shore showed no effects on the spiders. Our results strongly support the view that terrestrial systems are subsidized by lakes, and indicate that freshwater-lake subsidies are important for terrestrial invertebrate community structure on adjacent land. Further, our study shows that ecosystems should be treated as interdependent, not as self-contained units, and may as such be important for an increased understanding of the nature and importance of resource flows across ecosystem boundaries.     

Moving in two worlds: aquatic and terrestrial locomotion in sea snakes (Laticauda colubrina,Laticaudidae)

Yellow‐lipped sea kraits (Laticauda colubrina) are amphibious in their habits. We measured their locomotor speeds in water and on land to investigate two topics: (1) to what degree have adaptations to increase swimming speed (paddle‐like tail etc.) reduced terrestrial locomotor ability in sea kraits?; and (2) do a sea krait’s sex and body size influence its locomotor ability in these two habitats, as might be expected from the fact that different age and sex classes of sea kraits use the marine and terrestrial environments in different ways? To estimate ancestral states for locomotor performance, we measured speeds of three species of Australian terrestrial elapids that spend part of their time foraging in water. The evolutionary modifications of Laticauda for marine life have enhanced their swimming speeds by about 60%, but decreased their terrestrial locomotor speed by about 80%. Larger snakes moved faster than smaller individuals in absolute terms but were slower in terms of body lengths travelled per second, especially on land. Male sea kraits were faster than females (independent of the body‐size effect), especially on land. Prey items in the gut reduced locomotor speeds both on land and in water. Proteroglyphous snakes may offer exceptional opportunities to study phylogenetic shifts in locomotor ability, because (1) they display multiple independent evolutionary shifts from terrestrial to aquatic habits, and (2) one proteroglyph lineage (the laticaudids) displays considerable intraspecific and interspecific diversity in terms of the degree to which they use terrestrial vs. aquatic habitats.     

Lyriform slit sense organs on the pedipalps and spinnerets of spiders

Lyriform slits sense organs (LSSO) are a precise assembly of stress detecting cuticular slit sensilla found on the appendages of arachnids. While these structures on the legs of the wandering spiderCupennius salei are well studied in terms of morphology, function and contribution to behaviour, their distribution on pedipalps and spinnerets of spiders is not well explored. A study was therefore carried out to observe the distribution of LSSO on pedipalps and spinnerets of some spider species. Haplogyne spiders belonging to familyPholcidae have a simple complement of LSSOs represented by one or two LSSOs on their femur. The entelegyne spiders possess a complex assembly of LSSOs on the distal segments of their pedipalps. Various types of LSSOs are found on the pedipalps indicating a capacity for analysis of complex cuticular stress. It is suggested that the complexity of LSSOs on pedipalps of entelegyne spiders relates to courtship and spermatophore transfer and may help in reproductive isolation. Lack of LSSOs on the distal segments of pedipalps leads us to infer that unlike legs, pedipalps are less likely to receive vibratory input through their distal segments. Spinnerets have a relatively simple complement of LSSOs. One LSSO is found only on anterior spinnerets and it is a common feature observed among spiders, irrespective of the variations in web building behaviour. The orb-weaving araneidArgiope pulchella, however, has two LSSOs on the anterior spinneret. As non-web builders and orb weavers do not differ markedly in terms of LSSOs on the spinnerets and LSSOs are simple in nature (type A), it is likely that spinning and weaving are not largely regulated by sensory input from LSSOs on the spinnerets.     

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.     

Food source of riparian spiders analyzed by using stable isotope ratios

We analyzed the food source of riparian spiders in a middle reach of the Chikuma River, Japan, by using stable isotope ratios of carbon and nitrogen. The carbon and nitrogen isotope ratios of attached algae were higher than those of terrestrial plants, reflecting a large carbon isotope fractionation in terrestrial plants and a difference in nitrogen sources. The carbon isotope ratios of terrestrial insects were similar to those of the terrestrial plants, and the ratios of aquatic insects were scattered between those of the terrestrial plants and the attached algae. The carbon and nitrogen isotope ratios of spiders were intermediate between those of the terrestrial and aquatic insects. The two-source mixing model using the carbon isotope ratio showed that the web-building spiders utilized both the terrestrial and aquatic insects, with large contribution by the aquatic insects (54% on average with a maximum of 92% among spiders taxa collected in each zone), in the riparian area in a middle reach of the Chikuma River. The large contribution of the aquatic insects was often observed for the spiders collected near river channel (<5m) and for the horizontal web-building spiders collected across the riparian area. The relative contribution of the aquatic insects might be related with food availability (distance from river channel) and spiders food preference reflected in their web types (horizontal vs. vertical). Our results showed that organic materials produced in the river channel, in the riparian area, and in the terrestrial area surrounding the riparian area were mixed at the carnivorous trophic level of riparian spiders.     

Allochthonous aquatic insects increase predation and decrease herbivory in river shore food webs

Rivers produce an abundance of aquatic insects that traverse land, where they can have bottom-up effects on predators, who, in turn, can have top-down effects on terrestrial herbivores. This effect can cascade down to plants. These trophic relationships were demonstrated in a field of stinging nettles, Urtica dioica , along a river in Germany. At the shore compared to similar microhabitats 30–60 m away the abundance and biomass of: midges were highest, spiders were also highest, while herbivorous leafhoppers were lowest. At the shore, nettle plants were less damaged by herbivores and thus had less regrowth. Spiders regularly captured both aquatic midges as well as terrestrial leafhoppers and they captured more individuals of both groups at the shore than further away. Midges supported high densities of shore spiders. This was inferred from correlation of distribution and diet in the absence of other environmental gradients. Removal of spiders from experimental plots caused leafhoppers to increase at the shore, causing more plant damage. These effects were not evident at spider-removal sites away from the shore. This demonstrated that spiders depressed leafhoppers and decreased herbivory on plants only at the shore. It is concluded that aquatic insects had a bottom-up effect on spiders and that this subsidy facilitated a top-down effect that cascaded from spiders to leafhoppers to plants. Similar effects would explain the distribution of arthropods along many rivers. Allochthony connects river food webs with shore food webs, making both components essential for each other.     

Does the strength of cross‐ecosystem trophic cascades vary with ecosystem size? A test using a natural microcosm

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Boomerang ecosystem fluxes: organic carbon inputs from land to lakes are returned to terrestrial food webs via aquatic insects

Many ecosystems are linked to their adjacent ecosystems by movements of organisms. For instance, aquatic and terrestrial ecosystems are linked via emerging aquatic insects that serve as prey for terrestrial consumers. However, the role of these organisms in returning recycled carbon to the ecosystem from which it originated is not well known. This is due to the fact that values of carbon isotope signatures from terrestrial leaves and aquatic resources are usually similar and hence results of isotope mixing models need to be considered with caution. We overcame this problem by adding isotopically distinct terrestrial particulate organic carbon (tPOC) as a tracer to the experimental sides of two lakes that were divided in two equal halves with plastic curtains. We focused on aquatic insect larvae (Chironomidae) that fed on maize Zea mays leaves experimentally added to the lakes, and subsequently became prey for terrestrial predators (spiders) after emergence. The carbon isotope values of Chironomidae and spiders were significantly elevated in the lake treatment sides as compared to reference sides, whereas the values of all autochthonous resources were not affected by maize additions. Estimates from stable isotope mixing models indicated a low but demonstrable contribution of maize leaves to the diet of Chironomidae. Overlap between the isotope values of alder leaves, the major natural tPOC source, and autochthonous resources prevented a reliable quantification of allochthony of Chironomidae. However, we qualitatively demonstrated the flow of terrestrial particulate organic carbon to lakes, as leaf fall, and back to terrestrial surroundings via emerging insects. This ‘boomerang’ carbon flux between land and lakes blurs the distinction between autochthonous and allochthonous carbon sources.     

Redescription and palaeobiology of Palaeoscorpius devonicus Lehmann, 1944 from the Lower Devonian Hunsrück Slate of Germany

Abstract: Palaeoscorpius devonicus Lehmann, 1944 is known from only a single specimen, found in the Eschenbach Pit near Bundenbach in the Lower Devonian Hunsrück Slate of Germany. It is a key fossil, having been interpreted both as the most basal member of the Scorpiones and as one of the order’s most likely candidates for an aquatic mode of life. Prepared both ventrally and dorsally, some aspects of its morphology remain problematic. Here, with the aid of new techniques, including computed tomography, we present a re‐investigation of this scorpion’s anatomy and a new reconstruction, with a particular focus on the species’ original habitat. On the basis of the environmental interpretation of the Hunsrück Slate and the completeness of the specimen, previous authors concluded that P. devonicus was marine, but none offered convincing morphological evidence. Recent studies of the deposit’s environment suggest that the Hunsrück Sea was part of an intrashelf basin, relatively close to the coastline, and fossils of land plants show that terrestrial wash‐in occasionally occurred. Our revised interpretation of the fossil’s morphology demonstrates that the scorpion was most probably terrestrial. Internal mesosomal organs are interpreted as book lungs, but other terrestrial adaptations are lacking. The absence of both coxapophyses and gnathobases makes determining the scorpion’s feeding mechanism difficult. Interpreting the scorpion’s character states within a phylogenetic framework, especially the possible presence of book lungs, implies either that the plesiomorphic position of P. devonicus is no longer supported or that the development of book lungs had already taken place early in the scorpion lineage.     

THE ORIGIN OF THE SPINNING APPARATUS IN SPIDERS

• 1 Previous attempts to explain the evolution of spider silk have relied heavily on conjecture. The formulation of testable historical hypotheses to replace such speculation is discussed.
• 2 The importance of phylogenetic reconstructions and other historical hypotheses for use in generating and testing hypotheses concerning the evolution of specific adaptations is examined. Recent ideas on arachnid phylogeny are reviewed and their relevance to the problem of silk evolution in spiders is explored.
• 3 Evidence from the analysis of three historical problems (origin of spinnerets, origin of silk glands, original selective pressure favouring evolution of silk) is reviewed from three different frames of reference (in-group analysis, out-group analysis, convergence analysis). Several lines of evidence are found which suggest that silk use originated in spiders due to selective pressures associated with reproduction (specifically, the transfer of sperm or the protection of eggs).
• 4 The prevalence of segmental appendages retained for use in manipulating genital products in both arachnids and non-arachnid arthropods and the probable placement of spinnerets near the genital opening in ancestral spiders suggest that spinnerets represent modified gonopods.
• 5 The most primitive types of silk glands are retained in virtually all spiders, in part, for use in the construction of sperm webs and egg sacs. Similar silk glands are found near the genital opening in many male spiders and used in building a portion of the sperm web.
• 6 The silk of adult arthropods other than spiders is used largely in manipulating or protecting sex cells. If there are multiple functions, use in reproduction is typically one of them. Thus, there is evidence for strong selective pressure favouring the evolution of silk for use in reproduction.
• 7 Two hypotheses are proposed which are consistent with the conclusion that silk in spiders evolved for reproductive needs (the spermatophore-sperm web and egg sac hypotheses). Testable predictions of each hypothesis are proposed.

     

Spinneret Microstructure of the Silk Spinning Apparatus in the Crab Spider, Misumenops tricuspidatus (Araneae: Thomisidae)

The silk spinning apparatus in the crab spider, Misumenops tricuspidatus was studied with the field emission scanning electron microscope (FESEM) and the main microstructural characteristics of the silk glands are presented. In spite of the fact that the crab spiders do not spin webs to trap a prey, they also have silk apparatus even though the functions are not fully defined. The crab spider, Misumenops tricuspidatus possesses only three types of silk glands which connected through the typical spinning tubes on the spinnerets. The spinning apparatus of Misumenops closely corresponds to that of wandering spiders such as jumping spiders or wolf spiders except some local variations. Anterior spinnerets comprise 2 pairs of the ampullates and 48 (±5) pairs of pyriform glands. Another 2 pairs of ampullate glands and nearly 20 (±3) pairs of aciniform glands were connected on the middle spinnerets. Additional 50 (±5) pairs of the aciniform glands were connected on the posterior spinnerets. The aggregate glands and the flagelliform glands which have the function of sticky capture thread production in orb‐web spiders as well as the tubuliform glands for cocoon production in females were not developed at both sexes of this spider, characteristically.     

Prey use by web-building spiders: stable isotope analyses of trophic flow at a forest-stream ecotone

A forest-stream trophic link was examined by stable carbon isotope analyses which evaluated the relationship of aquatic insects emerging from a stream to the diets of web-building spiders. Spiders, aquatic and terrestrial prey, and basal resources of forest and stream food webs were collected in a deciduous forest along a Japanese headwater stream during May and July 2001. The ¹³C analyses suggested that riparian tetragnathid spiders relied on aquatic insects and that the monthly variation of such dependence is partly associated with the seasonal dynamics of aquatic insect abundance in the riparian forest. Similarly, linyphiid spiders in the riparian forest exhibited ¹³C values similar to aquatic prey in May. However, their ¹³C values were close to terrestrial prey in both riparian and upland (150m away from the stream) forests during June to July, suggesting the seasonal incorporation of stream-derived carbon into their tissue. In contrast, araneid spiders relied on terrestrial prey in both riparian and upland forests throughout the study period. These isotopic results were consistent with a previous study that reported seasonal variation in the aquatic prey contribution to total web contents for each spider group in this forest, implying that spiders assimilate trapped prey and that aquatic insect flux indeed contributes to the energetics of riparian tetragnathid and linyphiid spiders.     

Relating body size to the role of aquatic subsidies for the riparian spider <Emphasis Type="Italic">Nephila clavata</Emphasis>

We examined the relationship between body size of the riparian spider Nephila clavata and the contribution of allochthonous (aquatic insects) and autochthonous (terrestrial insects) sources to its diet using stable isotope analysis. During the study period from July to September, the body size of the females increased remarkably (about 60-fold) but that of males remained small. The biomass of both aquatic and terrestrial insects trapped on the spider webs increased with spider size, with the biomass of the former ranging between 30 and 70% of that of the terrestrial insects. The average relative contribution of aquatic insects to the diet of the spiders, calculated from δ¹³C values, was 40–50% in spiders in the early juvenile and juvenile stages, 35% in adult males and 4% in adult females. There was a significant negative relationship between the relative contribution of aquatic insects and body size of the female spiders. We conclude that aquatic insects might be an important seasonal dietary subsidy for small spiders and that these allochthonous subsidies may facilitate the growth of riparian spiders, which may in turn enable the spiders to feed on larger prey.     

Fine Structural Analysis of the Silk Apparatus in the Funnel-web Spider, Agelena limbata (Araneae: Agelenidae)

ABSTRACT Silk apparatus of the funnel-web spider, Agelena limbata was located at the ventral end of the abdominal part, and was composed of internal silk glands and external spinnerets. Among the three pairs of spinnerets, the posterior pairs were highly elongated along the body axis. By the light and electron microscopic inspections, it was found that four types of silk glands were connected through the typical spinning tubes of each spinneret. Anterior spinnerets comprise 2 pairs of the ampullate and 125 to 150 pairs (female) or 110 to 114 (male) of pyriform glands. Another 2 pairs of ampullate glands in both sexes, 5 to 8 pairs of tubuliform glands in females, and 20 to 26 pairs (female) or 15 to 17 pairs (male) of aciniform glands were connected on the median spinnerets. Additional 8 to 10 pairs of tubuliforms in female and 41 to 53 pairs (female) or 27 to 32 pairs (male) of aciniform glands were on the posterior spinnerets, respectively. While the ampullate and tubuliform glands were connected with the spigot-type spinning tubes, the pyriform and aciniform glands with that of spool-type tubes. It has been also revealed that the tubuliform glands were only observed in female spiders, however the flagelliform and aggregate glands which had the function of adhesive thread production in orb-web spiders were not observed at both sexes of this spiders.     

Microstructure of the silk apparatus of the comb-footed spider, Achaearanea tepidariorum (Araneae: Theridiidae)

The microstructural organization of the silk‐spinning apparatus of the comb‐footed spider, Achaearanea tepidariorum, was observed by using a field emission scanning electron microscope. The silk glands of the spider were classified into six groups: ampullate, tubuliform, flagelliform, aggregate, aciniform and pyriform glands. Among these, three types of silk glands, the ampullate, pyriform and aciniform glands, occur only in female spiders. One (adult) or two (subadult) pairs of major ampullate glands send secretory ductules to the anterior spinnerets, and another pair of minor ampullate glands supply the median spinnerets. Three pairs of tubuliform glands in female spiders send secretory ductules to the median (one pair) and posterior (two pairs) spinnerets. Furthermore, one pair of flagelliform glands and two pairs of aggregate glands together supply the posterior spinnerets, and form a characteristic spinning structure known as a “triad” spigot. In male spiders, this combined apparatus of the flagelliform and the aggregate spigots for capture thread production is not apparent, instead only a non‐functional remnant of this triad spigot is present. In addition, the aciniform glands send ductules to the median (two pairs) and the posterior spinnerets (12–16 pairs), and the pyriform glands feed silk into the anterior spinnerets (90–100 pairs in females and 45–50 pairs in males).