The coxal gland is a

gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormones) for release into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland). Structure De ...

found in some

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

s, for collecting and excreting

urine Urine is a liquid by-product of metabolism in humans and in many other animals. Urine flows from the kidneys through the ureters to the urinary bladder. Urination results in urine being excretion, excreted from the body through the urethra. Cel ...

. They are found in all

arachnid Arachnida () is a class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, harvestmen, camel spiders, whip spiders and vinegaroon ...

s (with the exception of some

Acari Mites are small arachnids (eight-legged arthropods). Mites span two large orders of arachnids, the Acariformes and the Parasitiformes, which were historically grouped together in the subclass Acari, but genetic analysis does not show clear evid ...

), and in other

chelicerates The subphylum Chelicerata (from New Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. It contains the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, ticks, and mit ...

, such as

horseshoe crab Horseshoe crabs are marine and brackish water arthropods of the family Limulidae and the only living members of the order Xiphosura. Despite their name, they are not true crabs or crustaceans: they are chelicerates, most closely related to arachn ...

s. The coxal gland is thought to be homologous with the

antennal gland Antennae ( antenna), sometimes referred to as "feelers", are paired appendages used for sensing in arthropods. Antennae are connected to the first one or two segments of the arthropod head. They vary widely in form but are always made of one o ...

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...

s. The gland consists of an end sac (''saccule''), a long duct (''labyrinth'') and a terminal bladder (''reservoir''). There is generally only one pair (two in some

spider Spiders ( order Araneae) are air-breathing arthropods that have eight legs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude silk. They are the largest order of arachnids and rank seventh in total species ...

s), and they open on the coxae of the walking legs. The coxal secretion of adult female ticks of ''Ornithodoros erraticus'' contains a sex pheromone.

Excretory System

The excretory system of the prawn lacks a true kidney. The following parts are found in its excretory system: * Antennal or green glands: A pair of enteric glands are found in the coxa (Grap) of each antenna. They are green in color, hence the name "green gland." The antennal gland consists of four regions: ** End sac: This is the smallest part which is bean-shaped. It is located between the bladder and the labrum. Inside it is a large blood-facuna. It has two levels in the wall. Its walls have numerous blood vessels which are the outer layer, the inner layer is thin and contains large excretory epithelial cells. The end sac functions similarly to the vertebrate glomerular nephron. ** Labyrinth: The labyrinth is highly coiled and is made up of branched excretory tubules. Blood vessels are found in the connective tissue mass. These ducts have a large excretory system are covered by a single layer of cells. The labyrinth is involved with the movement of ions and reabsorption of proteins. ** Bladder: This is the inner side of the last cell and is the largest part. It is made up of a single layer of excretory epithelium. Its inner wall emerges to form a small ureter or ureter. The ureter is enclosed outside by a round renal aperture, which is located on the inner surface of the coxa of the antennae on top of a papilla. The bladder stores urine. A ureter connects the bladder to the nephropore (excretory pore) under the base of the second antennae. ** Lateral ducts: A narrow lateral duct emerges from the bladder of each hyaline gland at the back. The lateral vessels on either side are joined by a transverse connective in front of the brain. After this, moving backward along both the esophagus opens into the renal sac.

Communication

In invertebrates with coxal glands, many forms of communication are conveyed via chemicals in the urine. Several studies have shown that in

crayfish Crayfish are freshwater crustaceans belonging to the clade Astacidea, which also contains lobsters. In some locations, they are also known as crawfish, craydids, crawdaddies, crawdads, freshwater lobsters, mountain lobsters, rock lobsters, mu ...

, these chemicals convey information such as reproductive capability, the identity of individual crayfish, as well as their aggression levels and hierarchical status. For example, when two male crayfish meet, they will engage in combat. Combat determines which individual is "dominant" or "submissive" in the hierarchy. These encounters affect their aggression levels, regulated by

serotonin Serotonin () or 5-hydroxytryptamine (5-HT) is a monoamine neurotransmitter. Its biological function is complex and multifaceted, modulating mood, cognition, reward, learning, memory, and numerous physiological processes such as vomiting and vas ...

, and thus changes the chemical composition of their urine . The chemical changes undergone will mediate further encounters, as they will offer crayfish a certain level of "familiarity" with each other. The chemicals in their urine communicate their rank to other crayfish and while the intensity of the encounter remains the same, the length of the encounter will be much shorter.

Communication Interference

Foreign chemicals introduced into the ecosystem have the potential to interfere with the arthropod's chemical receptors. Insecticide run-off from agricultural applications has been shown to inhibit communication in crayfish.

Neonicotinoids Neonicotinoids (sometimes shortened to neonics ) are a class of neuro-active insecticides chemically similar to nicotine, developed by scientists at Shell and Bayer in the 1980s. The neonicotinoid family includes acetamiprid, clothianidin, din ...

are widely used systemic insecticides that act as a

neurotoxin Neurotoxins are toxins that are destructive to nerve tissue (causing neurotoxicity). Neurotoxins are an extensive class of exogenous chemical neurological insultsSpencer 2000 that can adversely affect function in both developing and mature ner ...

to most arthropods that are water soluble and frequently make it into various waterways. Neonicotinoids bind to

nicotinic acetylcholine receptors Nicotinic acetylcholine receptors, or nAChRs, are receptor polypeptides that respond to the neurotransmitter acetylcholine. Nicotinic receptors also respond to drugs such as the agonist nicotine. They are found in the central and peripheral n ...

(nAChR), which kills insects by overstimulating them. nAChRs can be found in the

interneurons Interneurons (also called internuncial neurons, relay neurons, association neurons, connector neurons, intermediate neurons or local circuit neurons) are neurons that connect two brain regions, i.e. not direct motor neurons or sensory neurons. In ...

of the brain,

neuromuscular junctions A neuromuscular junction (or myoneural junction) is a chemical synapse between a motor neuron and a muscle fiber. It allows the motor neuron to transmit a signal to the muscle fiber, causing muscle contraction. Muscles require innervation to ...

, and the

olfactory bulb The olfactory bulb (Latin: ''bulbus olfactorius'') is a grey matter, neural structure of the vertebrate forebrain involved in olfaction, the sense of odor, smell. It sends olfactory information to be further processed in the amygdala, the orbitof ...

. Neonicotinoids binding to these receptors prevents the chemicals involved in communication from binding to their proper receptors. As a result, crayfish exposed to these insecticides at sublethal doses lose their ability to detect certain cues. Crayfish will fail to recognize individuals that they have had previous encounters with, as well as their hierarchical status. Crayfish also will fail to find food, and lab experiments show that they display no preference for environments with or without food.

References

Arthropod glands Arachnids Xiphosura {{arthropod-anatomy-stub