Chapter 38: Arthropods

38-1 Phylum Arthropoda

38-2 Subphylum Crustacea

38-3 Subphyla Chelicerata and Uniramia

I. Characteristics of Arthropods (75% of ALL Animals)

(1) Bilateral symmetry AND true coelom

(2) Segmented body WITH jointed appendages

(3) Exoskeleton AND Cephalization (eyes, feelers, sensory hairs)

(4) Ventral nerve cord AND Open-Circulatory System

38-1 Phylum Arthropoda

NOTE: The EXOSKELETON has THREE layers.

(1) OUTER WAXY layer repels WATER and prevents dehydration.

(2) HARD MIDDLE layer MAIN protection (CaCO3).

(3) INNER layer FLEXIBILITY at joints ~ greater MOVEMENT.

(1) Aquatic arthropods, such as crabs and crayfish, typically have thicker, stronger exoskeletons than do terrestrial arthropods, such as spiders and insects. What advantage does a thick, strong exoskeleton provide in an aquatic environment?

Critical Thinking

(1) Appendages

• JOINTED extensions of BODY, including LEGS and ANTENNAE.

(2) Compound Eyes (in MOST arthropods)• Eyes with MANY individual light detectors, EACH with its own LENS.

II. Molting (hormone INDUCES molting, triggered by GROWTH)• NO growth without periodically SHEDDING its exoskeleton (many times over one’s life).

III. Evolution and Classification (~ 600 m.y.a.)• CLASSIFIED into 4 subphyla using DEVELOPMENT and MORPHOLOGY.

(1) Tagmata (pl. tagma)• Several segments FUSED to perform SPECIFIC functions. (i.e., specialized for feeding, locomotion, and reproduction)

(2) Subphylum Trilobita (e.g., extinct TRILOBITES)• ANCESTRAL arthropods with ONE pair of similar appendages on EACH body segment (i.e., LITTLE to NO specialized tagmata).

(3) Subphylum Crustacea• Branched ANTENNAE and a PAIR of chewing MOUTHPARTS (mandibles); (e.g., shrimps, lobsters, crabs, crayfish, barnacles, daphnia)

(4) Mandibles• CHEWING mouthparts (primitive jaws).

(5) Subphylum Chelicerata• Arthropods WITHOUT antennae BUT have PINCER-like mouthparts (chelicerae).

(6) Suphylum Uniramia (“one-branch,” includes Myriapods and Insects)• Antennae AND mandibles, BUT appendages are UNBRANCHED. (NOTE: Uniramians are ONLY group evolving from TERRESTRIAL habitat)

I. Characteristics and Diversity of Crustaceans (~ 40,000 species)• 2 pairs of appendages on HEAD (ONLY arthropods w/2),

• 1 PAIR on BODY segments;

•Most have GILLS, NAUPLIUS larvae, CaCO3 in exoskeleton (aquatic).

38-2 Subphylum Crustacea

(2) The freshwater flea Daphnia consumes algae for nutrition. It also contains a prominent eyespot. How might the eyespot be connected with the ability of Daphnia to find food?

Critical Thinking

(1) Nauplius (FREE-swimming larvae)

• 3 PAIRS of appendages and a SINGLE EYE in middle of head; (following a series of MOLTS, it takes on ADULT body form).

(2) Plankton (i.e., Zooplankton)• MIXTURE of COPEPODS or DAPHNIA, and ~ a LARGE percentage of BIOMASS in an ecosystem.

(3) Cirrus (cirri, pl.) • 6 pairs of ciliated legs REPLACE swimming parts of barnacle LARVAE and pull FOOD into mouth.

(3) Barnacles are sessile crustaceans. What structural adaptation do barnacles have that enables them to compete with motile organisms for food? What structural adaptation do barnacles have that might protect them from predators?

Critical Thinking

(4) Isopods (“equal-legs”)• Terrestrial AND aquatic with 7 pairs of identical legs

(e.g., sow bugs and pill bugs).

II. The Crayfish• FRESHWATER crustacean (~ lobsters, marine relatives) classified as “DECAPODS” (with shrimps and crabs)

(1) Decapods• “TEN feet;” crustaceans with FIVE pairs of legs.

(A) External Structure• BODY is DIVIDED into 2 major sections:

(1) Cephalothorax

(2) Abdomen

(1) Cephalothorax• Consists of 2 tagma; HEAD (5 segments) and THORAX.

(4) The cephalothorax of a crayfish is covered by the carapace, a single, fused plate of exoskeleton. What are some possible advantages and disadvantages of this fused structure?

Critical Thinking

(2) Thorax• 8 segments found POSTERIOR to head.

(3) Carapace• Tough covering of fused DORSAL segments of cephalothorax.

(4) Abdomen• 7-segmented tagma lies POSTERIOR to cephalothorax.

(5) Telson (NO appendages attached HERE)• 7th segment forms a flat PADDLE at posterior end.

(5) Like other arthropods, crayfish are cephalized, with a variety of specialized sensory structures on their head. However, crayfish also have a high concentration of sensory hairs on the telson. What might be the evolutionary advantage of having so many sensory structures at the posterior end of the animal?

Critical Thinking

(6) Antennae & Antennules• Serve as FEELERS sensitive to touch, taste, and equilibrium.

(7) Maxillae (2 pairs) & Maxillipeds (3 pairs)• Chew food and help with respiration.

(8) Chelipeds• Large pincers used for capturing food and for defense.

(9) Swimmerets (create WATER currents)• Attached to ABDOMINAL segments, used for locomotion & reproduction.

(10) Uropods• On TELSON, help to propel the crayfish during TAILFLIPS.

(B) Digestion, Respiration, Open-Circulation, and Excretion• Gills are attached to walking legs DURING walking, legs circulate water across its GILLS (NOTE: TEETH are in the STOMACH).

(1) Digestive Gland (e.g., associated with STOMACH)• Food is ground up into a PASTE and mixed with ENZYMES; further digestion AND absorption occurs in INTESTINE.

(2) Green Glands (e.g., excretory organs, crayfish live in HYPOTONIC)• Eliminate EXCESS water from tissues through a PORE at base of ANTENNAE.

(6) The American lobster, Homarus americanus, is a NOCTURNAL organism. Marine biologists have discovered that the lobster’s senses of taste and smell are over 1,000 times MORE powerful than those senses in humans. The lobster uses taste and smell both to search for food and to detect mates. What adaptive advantage would these highly developed senses provide for the lobster?

Critical Thinking

(C) Neural Control• Brain with VENTRAL nerve cord connect GANGLIA to muscles.

NOTE: Crayfish sense VIBRATIONS and CHEMICALS with THOUSANDS of small SENSORY HAIRS that project from the exoskeleton.

I. Class Arachnida (Subphylum Chelicerata ~ 70,000 species)• Body divided into CEPHALOTHORAX, an ABDOMEN, CHELICERAE, PEDIPALPS, and FOUR pairs of WALKING legs.

38-3 Subphyla Chelicerata and Uniramia

(1) Pedipalps (on the cephalothorax)• Hold FOOD during chewing, and (in spiders), transfer SPERM to the female during MATING.

(A) Anatomy of a Spider• HOLLOW fangs (modified chelicerae) and 6 to 8 SIMPLE eyes.

(1) Spinnerets (3 pairs of ORGANS on the TIP of the abdomen)• Made of hundreds of TUBES that connect SILK glands to ABDOMEN.

(2) Book Lungs (i.e., spiders are terrestrial)• SACS in abdomen with many PARALLEL FOLDS (resemble a book) and carry out GAS EXCHANGE. (NOTE: MOST spiders have book lungs)

(3) Tracheae (found in place of OR in addition to book lungs)• System of TUBES carry AIR directly to TISSUES from OPENINGS in exoskeleton (i.e., called spiracles).

(4) Spiracles (in exoskeleton)• OPENINGS that bring AIR into the tracheae during GAS EXCHANGE.

(5) Malpighian Tubules (EXCRETORY system; terrestrial adaptation)• HOLLOW TUBES that COLLECT body fluids and WASTES and carry them to INTESTINES, reabsorbing WATER.

(6) Coxal Glands (EXCRETORY organs found in some spider species)• Organs remove WASTES through openings at BASE of legs.

(B) Life of a Spider• STEALTH hunters, most are VENOMOUS, males are smaller mate (and quickly leave) result in a female laying a COCOON of young spider eggs.

(C) Scorpions (nocturnal chelicerates)• Evolved large, pincer-like PEDIPALPS and a STINGER on LAST segment of abdomen (i.e., the telson).

(D) Mites (CHIGGERS-larvae) and Ticks (MOST diverse ~ 30,000 species)• COMPLETELY FUSED cephalothorax and abdomen (i.e., NO separation between them).

NOTE: Although venom is NOT a characteristic, species are PATHOGENIC or PARASITIC and inhabit all types of environments.

II. Myriapods (Subphylum Uniramia; e.g., millipedes and centipedes)• HIGHLY segmented bodies, a single pair of ANTENNAE, no waxy exoskeleton, and ALL are terrestrial.

(A) Class Diplopoda (e.g., millipedes are HERBIVORES)• SLOW moving, ROUND-bodied with TWO pairs of legs on each body segment adapted for BURROWING through vegetation.

(B) Class Chilopoda (e.g., centipede are CARNIVORES)• Flattened body, ONE pair of legs per segment; appendages on first segment ~ modified into a pair of VENOMOUS CLAWS.

Extra Slides AND Answers for Critical Thinking Questions

(1) The lobster’s senses of taste and smell would enable the animal to find food and mates at night, when it is active and its vision is limited by darkness.

(2) Most algae live in lighted areas. The eyespot of Daphnia senses where light is brightest and therefore where most algae are likely to be.(3) Barnacles have long appendages called cirri that sweep small organisms and food particles from the water into the barnacle’s mouth. Barnacles have a very hard shell that completely encloses the body in most species.(4) Fusion of the exoskeleton strengthens a vulnerable joint where the head and thorax join. However, it also eliminates movement at this joint, limiting the flexibility of the animal.

(5) Sensory hairs on the telson can pick up vibrations in the water that may signal the approach of a predator from the rear. Thus, they provide a defensive warning system for the posterior end of the animal.

(6) A thick, strong exoskeleton is better able to prevent an arthropod’s body from collapsing under the pressure exerted by water.