A book lung is a type of respiration organ used for atmospheric gas exchange that is present in many arachnids, such as scorpions and spiders. Each of these organs is located inside an open ventral abdominal, air-filled cavity (atrium) and connects with its surroundings through a small opening for the purpose of respiration.
Book lungs are not related to the lungs of modern land-dwelling vertebrates. Their name instead describes their structure and purpose as a case of convergent evolution. Stacks of alternating air pockets and tissue filled with hemolymph [lower-alpha 1] give them an appearance similar to a "folded" book. [1]
Their number varies from just one pair in most spiders to four pairs in scorpions. The unfolded "pages" (plates) of the book lung are filled with hemolymph. The folds maximize the surface exposed to air, and thereby maximize the amount of gas exchanged with the environment. In most species, no motion of the plates is needed to facilitate this kind of respiration.
Many arachnids, such as mites and harvestmen, have no traces of book lungs and breathe through tracheae or through their body-surfaces only. Gas exchange is performed by the thin walls inside the cavity instead, with their surface area increased by branching into the body as thin tubes called tracheae. These tracheae may possibly have evolved directly from book lungs because the tracheae in some spiders have a small number of greatly elongated chambers.
The absence or presence of book lungs divides the Arachnida into two main groups:
Tetrapulmonata have two pairs of book lungs found on the second and third abdominal segments (Schizomida have lost a pair, and most advanced spiders have replaced at least one of the pairs with trachea). Scorpions have four pairs of book lungs, found on abdominal segments number three, four, five, and six. [2]
The pulmonate arachnids also appears to be the only members of Arachnida where the respiratory pigment hemocyanin is present in their blood. [3]
One of the long-running controversies in arachnid evolution is whether the book lung evolved from book gills just once in a common arachnid ancestor, [4] or whether book lungs evolved separately in several groups of arachnids as they came onto land. While the third abdominal segment in Tetrapulmonata have book lungs, the scorpions have a pair of sensory organs called pectines instead.
The oldest book lungs have been recovered from extinct trigonotarbid arachnids preserved in the 410 million-year-old Rhynie chert of Scotland. These Devonian fossil lungs are almost indistinguishable from the lungs of modern arachnids, fully adapted to a terrestrial existence. [5]
Book lungs are thought to have evolved from book gills, water-breathing structures among marine chelicerates. Although they have a similar book-like structure, book gills are external, while book lungs are internal. [6] Both are considered appendages rather than conventional internal organs, as they develop from limb buds before the buds flatten into segmented lamellae. [7]
Book gills are still present in the marine arthropod Limulus (horseshoe crabs) which have five pairs of them, the flap in front of them being the genital operculum which lacks gills. Book gills are flap-like appendages that effect gas exchange within water and seem to have their origin as modified legs. On the inside of each appendage, over 100 thin page-like membranes, lamellae, appearing as pages in a book, are where gas exchange takes place. These appendages move rhythmically to drive blood in and out of the lamellae and to circulate water over them. Respiration being their main purpose, they can also be used for swimming in young individuals. If they are kept moist, the horseshoe crab can live on land for many hours.
A gill is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are kept moist. The microscopic structure of a gill presents a large surface area to the external environment. Branchia is the zoologists' name for gills.
The subphylum Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda. Chelicerates include the sea spiders, horseshoe crabs, and arachnids, as well as a number of extinct lineages, such as the eurypterids and chasmataspidids.
Arachnids are arthropods in the class Arachnida of the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, harvestmen, camel spiders, whip spiders and vinegaroons.
Pseudoscorpions, also known as false scorpions or book scorpions, are small, scorpion-like arachnids belonging to the order Pseudoscorpiones, also known as Pseudoscorpionida or Chelonethida.
Amblypygi is an order of arachnids also known as whip spiders or tailless whip scorpions, not to be confused with whip scorpions or vinegaroons that belong to the related order Thelyphonida. The name "amblypygid" means "blunt tail", a reference to a lack of the flagellum that is otherwise seen in whip scorpions. Amblypygids possess no silk glands or venom. They rarely bite if threatened but can grab fingers with their pedipalps, resulting in thorn-like puncture injuries.
Schizomida, also known as sprickets or short-tailed whip-scorpions, is an order of arachnids, generally less than 5 millimetres (0.20 in) in length. The order is not yet widely studied. E. O. Wilson has identified schizomids as among the "groups of organisms that desperately need experts to work on them."
Uropygi is an arachnid order comprising invertebrates commonly known as whip scorpions or vinegaroons. They are often called uropygids. The name "whip scorpion" refers to their resemblance to true scorpions and possession of a whiplike tail, and "vinegaroon" refers to their ability when attacked to discharge an offensive, vinegar-smelling liquid, which contains acetic acid. The order may also be called Thelyphonida. Both names, Uropygi and Thelyphonida, may be used either in a narrow sense for the order of whip scorpions, or in a broad sense which includes the order Schizomida.
Pedipalps are the secondary pair of forward appendages among chelicerates – a group of arthropods including spiders, scorpions, horseshoe crabs, and sea spiders. The pedipalps are lateral to the chelicerae ("jaws") and anterior to the first pair of walking legs.
Aquatic respiration is the process whereby an aquatic organism exchanges respiratory gases with water, obtaining oxygen from oxygen dissolved in water and excreting carbon dioxide and some other metabolic waste products into the water.
Sea spiders are marine arthropods of the order Pantopoda, belonging to the class Pycnogonida, hence they are also called pycnogonids. They are cosmopolitan, found in oceans around the world. The over 1,300 known species have leg spans ranging from 1 mm (0.04 in) to over 70 cm (2.3 ft). Most are toward the smaller end of this range in relatively shallow depths; however, they can grow to be quite large in Antarctic and deep waters.
Solifugae is an order of arachnids known variously as solifuges, sun spiders, camel spiders, and wind scorpions. The order includes more than 1,000 described species in about 147 genera. Despite the common names, they are neither true scorpions nor true spiders. Because of this, it's less ambiguous to call them "solifuges". Most species of solifuge live in dry climates and feed opportunistically on ground-dwelling arthropods and other small animals. The largest species grow to a length of 12–15 cm (5–6 in), including legs. A number of urban legends exaggerate the size and speed of solifuges, and their potential danger to humans, which is negligible.
Eurypterids, often informally called sea scorpions, are a group of extinct arthropods that form the order Eurypterida. The earliest known eurypterids date to the Darriwilian stage of the Ordovician period 467.3 million years ago. The group is likely to have appeared first either during the Early Ordovician or Late Cambrian period. With approximately 250 species, the Eurypterida is the most diverse Paleozoic chelicerate order. Following their appearance during the Ordovician, eurypterids became major components of marine faunas during the Silurian, from which the majority of eurypterid species have been described. The Silurian genus Eurypterus accounts for more than 90% of all known eurypterid specimens. Though the group continued to diversify during the subsequent Devonian period, the eurypterids were heavily affected by the Late Devonian extinction event. They declined in numbers and diversity until becoming extinct during the Permian–Triassic extinction event 251.9 million years ago.
Plesiosiro is an extinct arachnid genus known exclusively from nine specimens from the Upper Carboniferous of Coseley, Staffordshire, United Kingdom. The genus is monotypic, represented only by the species Plesiosiro madeleyi described by Reginald Innes Pocock in his important 1911 monograph on British Carboniferous arachnids. It is the only known member of the order Haptopoda.
The order Trigonotarbida is a group of extinct arachnids whose fossil record extends from the late Silurian to the early Permian. These animals are known from several localities in Europe and North America, as well as a single record from Argentina. Trigonotarbids can be envisaged as spider-like arachnids, but without silk-producing spinnerets. They ranged in size from a few millimetres to a few centimetres in body length and had segmented abdomens (opisthosoma), with the dorsal exoskeleton (tergites) across the backs of the animals' abdomens, which were characteristically divided into three or five separate plates. Probably living as predators on other arthropods, some later trigonotarbid species were quite heavily armoured and protected themselves with spines and tubercles. About seventy species are currently known, with most fossils originating from the Carboniferous coal measures.
Tetrapulmonata is a non-ranked supra-ordinal clade of arachnids. It is composed of the extant orders Uropygi, Schizomida, Amblypygi and Araneae (spiders). It is the only supra-ordinal group of arachnids that is strongly supported in molecular phylogenetic studies. Two extinct orders are also placed in this clade, Haptopoda and Uraraneida. In 2016, a newly described fossil arachnid, Idmonarachne, was also included in the Tetrapulmonata; as of March 2016 it has not been assigned to an order.
The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma (cephalothorax). It is a distinctive feature of the subphylum Chelicerata. Although it is similar in most respects to an abdomen, the opisthosoma is differentiated by its inclusion of the respiratory organs and the heart.
The anatomy of spiders includes many characteristics shared with other arachnids. These characteristics include bodies divided into two tagmata, eight jointed legs, no wings or antennae, the presence of chelicerae and pedipalps, simple eyes, and an exoskeleton, which is periodically shed.
Opiliones are an order of arachnids and share many common characteristics with other arachnids. However, several differences separate harvestmen from other arachnid orders such as spiders. The bodies of opiliones are divided into two tagmata : the abdomen (opisthosoma) and the cephalothorax (prosoma). Unlike spiders, the juncture between the abdomen and cephalothorax is often poorly defined. Harvestmen have chelicerae, pedipalps and four pairs of legs. Harvestmen were traditionally thought to have two eyes, except in the case of eyeless species. Developmental genetic work has shown that living species retain up to six eyes, including one pair of rudimentary median eyes and one pair of rudimentary lateral eyes.
Arthropods are invertebrates in the phylum Arthropoda. They possess an exoskeleton with a cuticle made of chitin, often mineralised with calcium carbonate, a body with differentiated (metameric) segments, and paired jointed appendages. In order to keep growing, they must go through stages of moulting, a process by which they shed their exoskeleton to reveal a new one. They form an extremely diverse group of up to ten million species.
A spiracle or stigma is the opening in the exoskeletons of insects, myriapods, velvet worms and many arachnids to allow air to enter the trachea. In the respiratory system of insects, the tracheal tubes primarily deliver oxygen directly into the animals' tissues. In most species the spiracles can be opened and closed in an efficient manner to admit air while reducing water loss. In various species, this is done by a wide range of mechanisms, such as elastic closure, and closer muscles surrounding the spiracle or kinking the tube. In some the muscle relaxes to open the spiracle, in others to close it. The closer muscle is controlled by the central nervous system, but can also react to localized chemical stimuli. Several aquatic insects have similar or alternative closing methods to prevent water from entering the trachea. The timing and duration of spiracle closures can affect the respiratory rates of the organism. Spiracles may also be surrounded by hairs to minimize bulk air movement around the opening, and thus minimize water loss.
book lung.