Class Arthropoda

1. The exoskeleton provides a surface for the muscles to work against, protects against predators and injury, and reduces water loss. During ecdysis a new exoskeleton is grown under the old one, with fluid separating them; then the outer skeleton is shed and the body is expanded by blood circulation and air intake, and the soft new skeleton hardens as it is exposed to air or water. The control for this process involves hormones.
2. The arthropod circulatory system is an open one, with a dorsal, longitudinal heart. The blood flows from the anterior end to the head, through internal body spaces toward the posterior end and back in the dorsal vessel. This one-way flow is maintained by valves in the posterior region of the heart. 3. Malpighian tubules are projections from the digestive tract between the midgut and the hindgut. The wastes are processes as follows: fluid is absorbed from the blood through the walls of the Malpighian tubules, nitrogenous wastes are concentrated, then they are emptied into the hindgut and eliminated. Water loss is regulated because water and salts are reabsorbed by the hindgut and returned to the circulation. 4. The two groups are (1) chelicerates, with mouthparts formed from the most anterior pair of appendages; the second pair of appendages are set pincer-like or feeler-like, and the remaining pairs are legs (examples: spiders, mites, scorpions); and (2) mandibulates, with the most anterior appendages, sensory antennae, and jaws formed by the second or third set of appendages (examples: crustaceans, insects, millipedes, and centipedes). 5. The legs are on the abdomen as well as the thorax. 6. Biramous appendages are double-branched, while uniramous are single-branched. 7. Millipedes and centipedes are both members of Uniramia, with bodies that consist of a head region followed by numerous segments with paired appendages. However, centipedes have one pair of legs per segment and are carnivores. Millipedes have two pairs of legs per segment (actually two segments fused to form one) and are herbivores. 8. Most insects possess a tubular digestive tract that is somewhat coiled. Insects that feed on juices low in protein have a greatly coiled tract to provide greater opportunity to absorb nutrients and to allow their digestive enzymes, which are weaker, more length over which to work. 9. An instar in this context refers to stages between molts. The two different kinds of metamorphosis are (1) simple - if wings are present, they develop externally during juvenile stages with no resting stage before the last molt into adulthood (immature stages - nymphs); and (2) complete - wings develop internally during the juvenile stages and appear only during the resting stage prior to the adult (immature stages - larvae; resting stage - pupa or chrysalis).
• Jointed appendages and an exoskeleton greatly expanded locomotive and manipulative capabilities for the arthropod phyla, the most successful of all animals in terms of numbers of individuals, species, and ecological diversification.
• Traditionally, arthropods have been grouped into three subphyla based on morphological characters but new research is calling this classification of the arthropods into question.
• Like annelids, arthropods have segmented bodies, but some of their segments have become fused into tagmata during the course of evolution. All possess a rigid external skeleton, or exoskeleton. • Chelicerates consist of three classes: Arachnida (spiders, ticks, mites, and scorpions); Merostomata (horseshoe crabs); and Pycnogonida (sea spiders).
• Spiders, the best known arachnids, have a pair of chelicerae, a pair of pedipalps, and four pairs of walking legs. Spiders secrete digestive enzymes into their prey, then suck the contents out.• Crustaceans comprise some 35,000 species of crabs, shrimps, lobsters, barnacles, sowbugs, beach fleas, and many other groups. Their appendages are basically biramous, and their embryology is distinctive.  Centipedes and millipedes are segmented uniramia. Centipedes are hunters with one pair of legs per segment, and millipedes are herbivores with two pairs of legs per segment.
• Insects have three body segments, three pairs of legs, and often one or two pairs of wings. Many have complex eyes and other specialized sensory structures.
• Insects exhibit either simple metamorphosis, moving through a succession of forms relatively similar to the adult, or complete metamorphosis, in which an often wormlike larva becomes a usually sedentary pupa, and then an adult.