A book lung is a type of respiration organ used for atmospheric gas exchange present in some arachnids such as scorpions and tetrapulmonates (spiders and whip scorpions). It is a terrestrial adaptation of the book gills seen in aquatic chelicerates such as horseshoe crabs, where the gills are ventral appendages under the abdomen (opisthosoma) arranged into a series of page-like lamellae in order to maximize surface area for diffusion. In arachnids, each of these organs is housed inside an air-filled cavity (atrium) that opens to the surrounding atmosphere through a small opening (spiracle), which provides a controlled environment for respiration.
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In this Dossier
Book lung in the context of Gill
A gill (/ɡɪl/ ) is a specialized respiratory organ that many aquatic animals use for aquatic gas exchange, i.e. to extract dissolved oxygen from water and to excrete carbon dioxide. Branchia (pl.: branchiae, from Ancient Greek βράγχια) is the zoologists' academic name for gills.
The gills of some semi-aquatic clades (e.g. crabs, terrestrial hermit crabs and amphibious fishes), have also adapted to allow air respiration (breathing) on land provided the gills are kept moist. In some terrestrial chelicerates species (tetrapulmonates such as spiders and scorpions), their ancestral gills have evolved into fully air-breathing book lungs. In some planktivorous bony fish species (e.g. silver and bighead carps), the gills are also used as a filter feeding organ via comb-like projections called gill rakers.
Book lung in the context of Haemolymph
Hemolymph or haemolymph is a body fluid that circulates inside arthropod bodies transporting nutrients and oxygen to tissues, comparable with the blood in vertebrates. It is composed of a plasma in which circulating immune cells called hemocytes are dispersed in addition to many plasma proteins (hemoproteins) and dissolved chemicals. It is the key component of the open circulatory system characteristic of arthropods such as insects, arachnids, myriopods and crustaceans. Some non-arthropod invertebrates such as molluscs and annelids also possess a similar hemolymphatic circulatory system.
In insects, the largest arthropod clade, the hemolymph mainly carries nutrients but not oxygen, which is supplied to the tissues separately by direct deep ventilation through an extensive tracheal system. In other arthropods, oxygen is dissolved into the hemolymph from gills, book lungs or across the cuticle and then distributed to the body tissues via the hemocoel.