Osmoregulation: Arthropod and Water

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aWater loss through evaporation is a major problem for animals that live on land, especially small ones like arthropods, and an array of defenses against desiccation have evolved. Both arachnids and insects possess waxy compounds in the epicuticle, the outer layer of the exoskeleton, which greatly reduce evaporative water loss. Arthropods that lack a waxy epicuticle, such as the pill bugs, and very small arthropods, such as mites and pseudoscorpions, live in leaf mold and soil, beneath logs, under stones, and in other areas where the danger of desiccation is reduced. The waxes in the epicuticle not only reduce water loss but can also act as a water repellent, reducing the danger of submersion in droplets of rain or dew. This resistance to wetting enables aquatic insects, such as beetles, to carry below the surface a film of air, which can then be used in respiration. It also contributes to the ability of water striders to move over the surface of water without breaking through the surface film. Osmoregulation is the regulation of water and ion concentrations in the body. Keeping this regulation precise is critical in maintaining life in a cell. Balance of water and ions is partly linked to excretion, the removal of metabolic wastes from the body. There are three main types of osmoregulatory environments in which animals live: freshwater, marine, and terrestrial. ____________________________________________________________

Crustaceans and arachnids possess paired excretory organs (maxillary, antennal, or coxal glands) that open at the bases of certain appendages. Myriapods, insects, and some arachnids, such as spiders and mites, possess another type of excretory organ, Malpighian tubules, which open into the intestine. Thus in these animals both excretory and digestive wastes exit from the anus Spiders common in warm dry regions, usually terrestrial (Subphylum Chelicerata; Class Arachnida) and insects, most diverse and abundant of all groups of arthropods (Subphylum Haxapoda, Class Insecta) have independently evolved a unique excretory system of Malpighian tubules, which work in conjunction with specialized resorptive cells in the intestinal epithelium. Potassium and other solutes and waste materials are secreted into the tubules, which drain the fluid, or “urine,” into the intestine. Resorptive cells recapture most potassium and water, leaving behind such wastes as uric acid. This recycling of water and potassium allows species living in dry environments to conserve body fluids by producing a nearly dry mixture of urine and feces. Many spiders also have coxal glands, which are modified nephridia that open at the coxa or base, of the first and third walking legs.

Subphylum Myriapoda:
(Chilopoda – centipedes; Diplopoda – millipedes; Pauropods – pauropods; Symphyla – symphylans) Excretion is usually by Malpighian tubules, but these have evolved independently of Malpighian tubules found in Chelicerata.

In Class Chilopoda (centipedes), two pairs of Malpighian tubules empty into the hind part of the intestine.
Subphylum Crustacea:
(mainly marine however there are many freshwater and a few terrestrial species)
Excretory organs of adult crustaceans are a pair of tubular structures located in the ventral part of their head anterior to the esophagus. They are called antennal glands or maxillary glands, depending on whether they open at the base of the antennae or at the base of second maxillae. A few adult crustaceans have both. Excretory organs of decapods are antennal glands also called green glands in this group. Crustaceans do not have Malpighian tubules, the excretory organs of spiders and insects. The end sac of the antennal gland consists of a small vesicle (saccule) and a spongy mass called a labyrinth. The labyrinth connects by an excretory tubule to a dorsal bladder, which opens to the exterior by a pore on the ventral surface of a basal antennal segment. Hydrostatic pressure within the...
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