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Phylum Arthropoda
Arthropod Taxonomy: Overview
The arthropods evolved along four main lines, which most
zoologists recognize as 4 distinct subphyla
1. Trilobita - extinct trilobites
2. Chelicerata - horseshoe crabs, spiders, ticks, mites, and some
extinct groups
3. Crustacea - crabs, lobsters, shrimps, barnacles
4. Uniramia - insects, centipedes, millipedes
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
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
The Arthropod Exoskeleton
Molting
• In order to grow the
arthropod must shed its
exoskeleton, and secrete a
new and larger one molting or ecdysis.
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
Arthropod joint
Vertebrate joint
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.
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
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
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
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
Arthropod Diversity
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
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.
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
Black widow
•Usually food is PREDIGESTED
Brown recluse
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
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
Malphigian Tubules
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
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
Jumping spider
Grass spider
Sensory Structures
•Most spiders are predaceous and have all
kinds of sensory hairs and relatively welldeveloped eyes for motion detection;
chemoreceptors are also used
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”
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)
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
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
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
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
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.
Crustacean Diversity
Ostracod
Copepod
Fairy shrimp
Water flea
Fish louse
Barnacles are Crustaceans!
The only members of
this class that are
monoecious