Arachnids produce digestive juices in their stomachs, and use their pedipalps and chelicerae to pour them over their dead prey. The stomach is tubular in shape, with multiple diverticula extending throughout the body. However, mites are variable: It is typically divided into a preabdomen and postabdomen, although this is only clearly visible in scorpions, and in some orders, such as the Acari , the abdominal sections are completely fused. The primary nitrogenous waste product in arachnids is guanine. The digestive juices rapidly turn the prey into a broth of nutrients, which the arachnid sucks into a pre-buccal cavity located immediately in front of the mouth.
However, mites are variable: Complex courtship rituals have evolved in many arachnids to ensure the safe delivery of the sperm to the female. The heart is located in the forward part of the abdomen, and may or may not be segmented. Inside the pit, a small hair touches the underside of the membrane, and detects its motion. The excretory glands of arachnids include up to four pairs of coxal glands along the side of the prosoma, and one or two pairs of Malpighian tubules , emptying into the gut. Behind the mouth is a muscular, sclerotised pharynx , which acts as a pump, sucking the food through the mouth and on into the oesophagus and stomach. Several mites and ticks are parasites , some of which are carriers of disease. Slit sense organs are believed to be involved in proprioception , and possibly also hearing. Scorpions, however, are either ovoviviparous or viviparous , depending on species, and bear live young. The genital opening is usually located on the underside of the second abdominal segment. The median ocelli develop from a transverse fold of the ectoderm. In most species, the male transfers sperm to the female in a package, or spermatophore. Arachnids produce digestive juices in their stomachs, and use their pedipalps and chelicerae to pour them over their dead prey. In some arachnids, the oesophagus also acts as an additional pump. Finally, slit sense organs are slit-like pits covered with a thin membrane. Hemolymph There are characteristics that are particularly important for the terrestrial lifestyle of arachnids, such as internal respiratory surfaces in the form of tracheae , or modification of the book gill into a book lung , an internal series of vascular lamellae used for gas exchange with the air. With the exception of scorpions, which can have up to five pairs of lateral ocelli, there are never more than three pairs present. The endosternite is even calcified in some Opiliones. Extant living arthropods are a monophyletic group and are divided into three main clades: Beneath this is a transparent vitreous body, and then the retina and, if present, the tapetum. Many arachnids have only one or the other type of excretory gland, although several do have both. A consensus emerged from about onwards, based on both morphological and molecular evidence. The primary nitrogenous waste product in arachnids is guanine. In most arachnids only the females provide parental care, with harvestmen being one of the few exceptions. However, arachnids also have two further pairs of appendages that have become adapted for feeding, defense, and sensory perception.
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