Arthropods first appeared over 560 million years ago and are now the most abundant and diverse group of multicellular metazoans on Earth with over a million species described, and millions more left to be discovered! Arthropods are bilateral animals with internal and external segmentation. Most have a distinct head region and regional specialization along the rest of the body. Each body segment originally had a pair of segmented appendages. They usually have a pair of compound eyes in addition to one or more simple eyes. Arthropods have a body which is covered by a well-developed exoskeleton, so they need to moult to increase in size. They have an open circulatory syetem and a complete gut. Their nervous system is very similar to that of worms with a dorsal brain and a pair of ventral nerve cords. Most arthropods reproduce sexually, but they may have direct, indirect or mixed development.
This subphylum contains scorpions, spiders and "sea spiders". All of these groups share a similar body architecture, being divided into an anterior prosoma (the cephalothorax) and a posterior opisthosoma (abdomen). Uniramous appendages with many joints are found on each segment. The four pairs of walking legs of chelicerates lack extensor muscles, but possess flexor muscles. All members of this subphylum have chelicerae and pedipalps as their first and second prosomal appendages. The rest of the appendages very from group to group, but all cheilicerates lack antennae. Gas exchange is accomplished by book gills, book lungs, or tracheae, while waste products are excreted by coxal glands and/or Malpighian tubules.
Members of this order are specialized predators. Their compact bodies can be divided into two regions; the gnathosoma (mouth) and the idiosoma (body). Larval mites have a short gnathosoma with stocky pedipalps on either side of paired chelicerae. Their legs have a strong, curved claw to help the larvae stay attached to their host. The gnathosoma of adults is a simply, short channel that leads directly to the esophagus. Their pedipalps are well-developed and are used both as a tactile organ and to hold on to prey.
Mites usually have two pairs of lateral eyes, with a medial eye spot between them. Their eyes detect light intensity, direction and wavelength.
Gas exchange occurs mainly through the integument by diffusion. Larger species have a network of tubes to take environmental gases directly to their internal organs, while mites with thick exoskeletons have pores in the integument to let gases through.
Mites osmoregulate by having large porous areas of their cuticle for the exchange of salts to maintain water balance.
Water mites have an open circulatory system. All organ systems are found in the hemocoel and are bathed in hemolymph. The hemolymph circulates through the hemocoel by body movements.
Undigested food accumulates in the midgut and is absorbed by the hemolymph. The hemolymph deposits these wastes in the excretory tubule as insoluble crystals. They are ultimately discharged through the excretory pore by pressure generated by body movements.
Water mites have a complex life style, with several stages and multiple hosts. Eggs are laid on plants, wood or stones. The six-legged larvae attach themselves to an insect host and feed on its bodily fluids. When they are fully engorged, they detach from their host and attach themselves to plants where they enter an inactive stage. During this time, the larval tissues are resorbed and the body is reconfigured into an eight-legged deutonymph. Deutonymphs are an active, sexually immature stage lacking the completely hardened body of the adult and the final arrangement of the setae on the body. They are predaceous and usually feed on the insect larvae which they parasitized as larvae. The deutonymph stage is the major growth stage of water mites. After reaching maturity, deutonymphs find a plant or soft substrate, embed their chelicerae and transform into another inactive stage, which emerges as an adult in a few days. Adults become sexually mature a few days after emerging. Many species reproduce without touching a member of the other sex. The male drops a spermatophore (a packet of sperm) that the female picks and stores in spermathecae until needed.
Mites only feed on fluids drawn from their hosts or prey. They use their muscular pharynx to suck the fluids out of their prey and into their digestive system.
Mites can have serious impacts on other aquatic invertebrates. They often parasitize 20 - 50% of a population of adult aquatic insects. This parasitism affects the growth, and reproductive ability of the insects. The free-living forms are often major predators on small aquatic organisms such as fish eggs, insect larvae, and small crustaceans. Mites do not have any major predators.
Adults and deutonymphs are predaceous and often feed upon the immature stages of the species that they parasitize as larvae.
Crustaceans are invertebrates belonging to the phylum Arthropoda and include such familiar groups as barnacles, crabs, crayfish, lobster, water fleas and pill bugs. Crustaceans are key players in aquatic food webs. The majority of plankton in freshwater is composed of cladocerans and copepods, who are the major consumers of phytoplankton. Benthic crustaceans are often both scavengers and consumers of plant life found on the lake bottom. Collectively, these crustaceans serve as a key food source for fish, especially during their juvenile stage. Aside from their role in aquatic food webs, the largest species of crustaceans are of considerable economic importance. Lobster, shrimp and even freshwater crayfish each support important fishing industries. They are also an increasingly important target for aquaculture activities. In fact, the value of crustaceans produced in aquaculture is already as great as that of fish!
Adults of the smallest species are less then 0.1 mm in length and weigh less than 1 mg. By comparison, the heaviest crustacean is the mud crab which reaches a peak weight of 40 kg. The Japanese spider crab is the largest living arthropod, with a leg span of 4 m.
Many crustaceans employ standard sexual reproduction, while other crustaceans reproduce by cyclic or obligate parthenogenesis, where males are unknown or rare. Females in parthenogenetic systems produce eggs which do not require fertilization to develop. Aside from this variation in mating system, many crustaceans produce two types of eggs: one which develops immediately, while the other which may diapause for up to several hundred years.
Crustaceans show extraordinary diversity in body shape and form bearing anywhere from 3 to 50 pairs of limbs. However, crustaceans do share common features such as jointed, paired appendages, and two pairs of antennae. All crustaceans are enclosed in a protective exoskeleton made of chitin, which must be shed (or "moulted") to accommodate growth.
Most crustaceans are carnivores or scavengers, though herbivores and detritivores are not uncommon. Cannibalism can occur at very high densities, or when individuals have just moulted and are vulnerable to attack. Food is taken into the mouth and passed to the gastric mill where it is ground into small particles. Digestion occurs in the midgut and waste is passed out of the hindgut.
All crustaceans have an open circulatory system and employ either haemoglobin or haemocyanin as a respiratory pigment. Most crustaceans have a dorsal heart, but some smaller crustaceans simply circulate their hemolymph with body movements. Crustaceans osmoregulate in freshwater by producing copious amount of urine. Most freshwater crustaceans have thoracic and abdominal gills with which they exchange gases while the rest simply diffuse gases across their body integument.
Crustaceans have developed a complex tripartite brain and paired, ganglionated ventral nerve cords. They often possess both compound eyes and a array of simple eyes. Zooplankton show particular sensitivity to light as they undergo daily migrations up and down the water column to stay in the best light conditions. Chemosensory systems have evolved to allow them to locate food and mates while avoiding predators.