Phylum Arthropoda

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

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

Vertebrate jointArthropod 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

• Usually food is PREDIGESTED

Black widow

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

Grass spider

Jumping spider

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

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

Copepod

Ostracod

Water flea

Fairy shrimp Fish louse

Barnacles are Crustaceans!

The only members of this class that are monoecious