Transcript File
What is an arthropod? • A typical arthropod is a segmented, coelomate invertebrate animal with bilateral symmetry, an exoskeleton, and jointed structures called appendages. • An appendage is any structure, such as a leg or an antenna, that grows out of the body of an animal. What is an arthropod? • Arthropods are the earliest known invertebrates to exhibit jointed appendages. • Joints are advantageous because they allow more flexibility in animals that have hard, rigid exoskeletons. Arthropod exoskeletons provide protection • The exoskeleton is a hard, thick, outer covering made of protein and chitin (KI tun). Arthropod exoskeletons provide protection • The exoskeleton protects and supports internal tissues and provides places for attachment of muscles. • In many aquatic species, the exoskeletons are reinforced with calcium carbonate. Why arthropods must molt • A second and more important disadvantage is that exoskeletons cannot grow, so they must be shed periodically. Shedding the old exoskeleton is called molting. Why arthropods must molt • Most arthropods molt four to seven times in their lives before they become adults. • When the new exoskeleton is soft, arthropods cannot protect themselves from danger because they move by bracing muscles against the rigid exoskeleton. Segmentation in arthropods • In most groups of arthropods, segments have become fused into three body sections—head, thorax, and abdomen. Segmentation in arthropods • In other groups, even these segments may be fused. • Some arthropods have a head and a fused thorax and abdomen. Segmentation in arthropods • In other groups, there is an abdomen and a fused head and thorax called a cephalothorax. • Fusion of the body segments is related to movement and protection. Arthropods have efficient gas exchange • Three types of respiratory structures have evolved in arthropods: gills, tracheal tubes, and book lungs. Arthropods have efficient gas exchange • Most insects have tracheal tubes, branching networks of hollow air passages that carry air throughout the body. Arthropods have efficient gas exchange • Muscle activity helps pump the air through the tracheal tubes. • Air enters and leaves the tracheal tubes through openings on the thorax and abdomen called spiracles. Arthropods have efficient gas exchange • Most spiders and their relatives have book lungs, air-filled chambers that contain leaflike plates. Arthropods have efficient gas exchange • The stacked plates of a book lung are arranged like pages of a book. Arthropods have acute senses • Antennae are also used for sound and odor communication among animals. Arthropods have acute senses • Have you ever watched as a group of ants carried home a small piece of food? • The ants were able to work together as a group because they were communicating with each other by pheromones, chemical odor signals given off by animals. Arthropods have acute senses • Antennae sense the odors of pheromones. • Accurate vision is also important to the active lives of arthropods. • Most arthropods have one pair of large compound eyes and three to eight simple eyes. • A simple eye is a visual structure with only one lens that is used for detecting light. Arthropods have acute senses • A compound eye is a visual structure with many lenses. Arthropod nervous systems are well developed. • The nervous system consists of a double ventral nerve cord, an anterior brain, and several ganglia. • Arthropods have ganglia that have become fused. These ganglia act as control centers for the body section in which they are located. Arthropods have other complex body systems • Arthropod blood is pumped by a heart in an open circulatory system with vessels that carry blood away from the heart. • The blood flows out of the vessels, bathes the tissues of the body, and returns to the heart through open body spaces. Arthropods have other complex body systems • Arthropods have a complete digestive system with a mouth, stomach, intestine, and anus, together with various glands that produce digestive enzymes. • The mouthparts of most arthropod groups include one pair of jaws called mandibles. Arthropods have other complex body systems • Most terrestrial arthropods excrete wastes through Malpighian tubules. • In insects, the tubules are all located in the abdomen rather than in each segment. • Malpighian tubules are attached to and empty into the intestine. Arthropods reproduce sexually • Most arthropod species have separate males and females and reproduce sexually. • Fertilization is usually internal in land species but is often external in aquatic species. Arthropods reproduce sexually • Some species, including bees, ants, aphids, and wasps, exhibit parthenogenesis, a form of asexual reproduction in which a new individual develops from an unfertilized egg. • Reproductive diversity is one reason there are more arthropod species than all other animal species combined. Objective 2: Subphylums for Arthropods • -Subphylum Trilobita: Examples include trilobites. All extinct forms; Cambrian to Carboniferous; body • divided by two longitudinal furrows into three lobes; distinct head, trunk, and abdomen, biramous (twobranched) appendages. • -Subphylum Chelicerata: eurypterids, horseshoe crabs, spiders, ticks. First pair of appendages modified to form chelicerae; pair of pedipalps and four pairs of legs; no antennae; no mandibles; cephalothorax and abdomen usually unsegmented. Objective 2: Subphylums for Arthropods • -Subphylum Crustacea: crustaceans. Mostly aquatic, with gills; cephalothorax usually with dorsal carapace; biramous appendages, modified for various functions; head appendages consisting of two pairs of antennae, one pair of mandibles, and two pairs of maxillae; development primitively with Nauplius stage • -Subphylum Uniramia: All appendages currently thought of as uniramous; head appendages consisting of one pair of antennae, one pair of mandibles, and one or two pairs or maxillae. Examples include: centipedes, millipedes, and insects. Objective 3: Classes of Chelicerata • -Class Merostomata: aquatic chelicerates. Cephalothorax and abdomen; compound lateral eyes; appendages with gills; sharp telson; subclass Eurypterida (all extinct) and Xiphosurida, horseshoe crabs. Examples include Limulus. Objective 3: Classes of Chelicerata • -Class Pycnogonida: Sea Spiders. Small (3 to 4 mm). Some reach 500mm; body chiefly cephalothorax; tiny abdomen; usually four pairs on long walking legs (some with five or six pairs); mouth on long proboscis; four simple eyes; no respiratory or excretory system. Example: Pycnogonum. Objective 3: Classes of Chelicerata • -Class Arachnida: scorpions, spiders, mites, ticks, harvestmen. Four pairs of legs; segmented or unsegmented abdomen with or without appendages and generally distinct from cephalothorax; respiration by gills, tracheae, or book lungs; excretion by Malpighian tubules and /or coxal glands; dorsal bilobed brain connected to ventral ganglionic mass with nerves, simple eyes; chiefly oviparous; no true metamorphosis. Examples Argiope, Centruroides. Objective 4: Crustaceans • Antennae: are also used for sound and odor communication among animals. • Mandibles: The mandible of an arthropod is a pair of mouthparts used for either for biting, cutting and holding food. Mandibles are often simply referred to as jaws. • Maxillae: Paired maxillae cut food and manipulate it during mastication. Maxillae can have hairs and "teeth" along their inner margins. Objective 4: Crustaceans • • • • • • Cephalothorax: an abdomen and a fused head and thorax. Gills: respiratory structures for aquatic organisms which extract oxygen from the water. Rostrum: (beak) A snout like projection on the head of an arthropod. Carapace: A shield like plate covering the cephalothorax of certain crustaceans. Telson: A nonsegmented part of tail fan of arthropods. Uropod: The last abdominal segments which make up the tail fan of arthropods.