phylum arthropoda. arthropod taxonomy: overview the arthropods evolved along four main lines, which...
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Phylum Arthropoda
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Arthropod Taxonomy: Overview
The arthropods evolved along four main lines, which most zoologists recognize as 4 distinct subphyla
1. Trilobita - extinct trilobites2. Chelicerata - horseshoe crabs, spiders, ticks, mites, and some extinct groups3. Crustacea - crabs, lobsters, shrimps, barnacles4. Uniramia - insects, centipedes, millipedes
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Arthropod Respiratory Advances
• Special respiratory structures allow the arthropods to metabolize more efficiently and thus move rapidly
• High metabolic rates require rapid oxygen delivery, and arthropods can accomplish this with respiratory organs that have a large surface area for collecting oxygen quickly
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Structure of the Exoskeleton
• Composed of the polysaccharide chitin and protein – glycoprotein
• Secreted by the Hypodermis• Outer surface called the
epicuticle; contains waxes • The thicker portion is called
the procuticle:• exocuticle • endocuticle
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The Arthropod Exoskeleton
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Molting• In order to grow the
arthropod must shed its exoskeleton, and secrete a new and larger one - molting or ecdysis.
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Jointed Appendages• Exoskeleton divided into a number of plates and cylinders
• At the junction point between plates and cylinders, the exoskeleton remains thin and flexible; these are the joints
• Jointed appendages allows arthropods to move efficiently and quickly• Muscles are integral to arthropod movement; they attach to the inner side of
the exoskeleton; they often function as a lever system
Vertebrate jointArthropod joint
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Specialized Arthropod Segments: Reduction in Metamerism
• The evolution of the arthropods witnessed a reduction in metamerism
• The arthropods evolved modified groups of segments (e.g., segments became lost, some fused together
• The fusion of groups of segments into functional groups is called tagmatization
• In so doing, various appendages on segments became specialized for functions other than locomotion, e.g. prey capture, filter feeding, sensing various kinds of stimuli, gas exchange, copulation, etc.
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Tracheae • Gas exchange organs among terrestrial arthropods is usually
internal; invaginations of the integument
• Insects have tracheae, branching networks of hollow air conducting tubes such that air is sent to every cell in every tissue
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Acute Senses
• Arthropods have a well-developed nervous system that is of the same overall design as the annelids; anterior brain and a double, ventral hollow nerve cord.
• The sensory receptors of arthropods are usually associated with modifications of the chitinous exoskeleton
• The head usually bears various kinds of sense organs (e.g. antennae) with extreme sensitivity
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Acute Senses cont.
• Many arthropods have compound eyes - eyes that are composed of many visual units called facets (ommatidia); capable of color vision and detecting the slightest movements of prey or predators
• Some eyes are simple eyes with only a few photoreceptors; however, they are capable of forming crude images
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Reproduction
• Dioecious; fertilization is external in aquatic forms, internal among the terrestrial forms. Most are oviparous (external development of young from eggs)
Circulatory System
• Open circulatory system
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Arthropod Diversity
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Subphylum Trilobita• A group of extinct marine
arthropods. • Ranged in size from a few
millimeters up to 75 centimeters.
• Body divided into three longitudinal regions:
• Compound eye, composed of radially arranged visual units
• They would have curled up like a woodlouse for protection against predators.
• Two-lobed appendages
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Subphylum Chelicerata• Lack antennae• Body is usually composed of two regions: cephalothorax (prosoma)
and abdomen• Cephalothorax is usually covered dorsally by the carapace• Six pairs of appendages: first pair are modified feeding structures -
chelicerae • Second pair are called pedipalps
• Four additional pairs of appendages are walking legs
• No abdominal appendages• Some have compound eyes,
usually have simple eyes capable of forming crude images.
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Class Meristomata (e.g., Horeshoe crabs)
• Marine chelicerates, common off of the Atlantic and Gulf coasts• Large dorsal carapace bearing compound eyes• Possess chelicerae, pedipalps, and 4 pair of walking legs (all but
the last pair are chelate• Abdomen terminates in a long tail called the telson; used to turn
the animal right side up• Possess a series of gill plates called book gills
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Class Arachnida• Respiration is accomplished via book
lungs, tracheae, or both
• Most are harmless or very beneficial to humans
• Usually have 8 simple eyes; at the very least they detect motion; for some of the predatory forms, they are capable of forming crude images
• Many species have evolved poison glands associated with the chelicerae
• Usually food is PREDIGESTED
Black widow
Brown recluse
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ORDER SCORPIONIDA (Scorpions)
• About 2000 species worldwide. Nocturnal animals, all relatively large. Associated with deserts, but also common in tropics.
• Fluoresce under UV light. • Small chelicera• Two segments, prosoma and opisthosoma• Most stings are harmless, similar to a wasp
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Reproduction
• Courtship rituals; “scorpion waltz”• Most are Oviviparous• Sometimes young are “piggybacked”
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ORDER Araneae (Spiders)• Largest Group of Arachnids. Eight-eyed!• Prosoma have chelicerae with large fangs and
poison glands• Pedipalps are leg-like; in males modified for
sperm transfer• Cephalothorax and abdomen shows no
external segmentation; tagma are joined by a narrow pedicel
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Malphigian Tubules
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Respiration and Circulation • Arachnids have book lungs, chambers with leaf-like plates for
exchanging gases; air flows over the plates and blood flows through them
Blood contains hemocyanin and has ameoboid cells that are similar to human immune cells and platelets
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Prey capture among the spiders
• Some species are cursorial predators, those that stalk and ambush their prey; they usually have well-developed eyes
• Others are web building spiders, those that construct various kinds of webs made of silk to trap their prey
• Eyes are not as well developed as cursorial predators, but they have a battery of sensory hairs for detecting vibrations
Grass spider
Jumping spider
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Sensory Structures
• Most spiders are predaceous and have all kinds of sensory hairs and relatively well-developed eyes for motion detection; chemoreceptors are also used
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Reproduction
• Can use chemical, tactile or visual signals• Females use pheromones to attract males• Courtship behavior, like “web-plucking” is
common• Male pedipalps have penislike “embolus”
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Other Relatives
Class Acarina – Ticks and Mites• Herbivores, scavengers or ectoparasites• Can transmit disease
Class Pycnogonida – Sea Spiders
Males protect the young, “stuck together”
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Class Malacostraca• Possess 2 pair of antennae: First pair is homologous to those of
insects; second pair is unique to the crustaceans• Second antennae have various functions, including sensory,
locomotion or feeding.• The head bears a pair of compound eyes and 3 pairs of mouthparts: a
pair of mandibles (chewing/grinding) and 2 pairs of maxillae (food handling)
• Primitively, the first three pairs of thoracic segments are maxillipeds; (handling food/sensory)
• Also, there are usually 5 pairs of appendages strengthened for walking (walking legs) and protection (chelipeds, pincer-like claws)
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SubPhylum Crustacea cont.
• Abdomen is also highly variable, but it is primitively large• Groups with a well-developed abdomen usually possess six pairs
of appendages: Five pairs of structures called swimmerets (=pleopods); one pair of structures called uropods,
• Uropods together with the terminal telson form a tail fan than can serve as rudders during locomotion
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Gills
• Many aquatic arthropods (crabs and lobsters) have gills, which are typically modifications of appendages or outgrowths of the body wall - folds of tissue with a large surface area
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Sensory Structures
• Compound eyes on moveable eyestalks
• Crayfish have statocysts present for balance
• Tactile receptors on the appendages and at joints
• Mouthparts and Antennae lined with chemoreceptors (setae)
Reproduction
• Male turns female on back and deposits sperm; after fertilization, eggs are shed; they are sticky and attach to the pleopods
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SubPhylum Crustacea cont.
• Primitively many of the appendages of the crustaceans are
biramous: there is an outer exopod and an inner endopod• They usually have an extremely hardened exoskeleton, which is
impregnated with calcium carbonate - carapace
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SubPhylum Crustacea cont.
• The primitive larva of the crustaceans is called the nauplius larva• It has an unsegmented body, a frontal eye, and 3 pairs of
appendages, representing the 2 pairs of antennae and the mandibles
Crabs and their relatives have a second larval stage called a zoea. They have Postlarva is a sexually immature miniadult.
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Crustacean Diversity
Copepod
Ostracod
Water flea
Fairy shrimp Fish louse
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Barnacles are Crustaceans!
The only members of this class that are monoecious