Trilobites, Chelicerates, and Myriapods

Chapter 19

Phylum Arthropoda

 Two out of every three known species of animals are arthropods.

 Members of the

phylum Arthropoda

are found in nearly all habitats of the biosphere.

Phylum Arthropoda

 Arthropods are:  Multicellular     Bilaterally symmetrical Triploblastic Have a true coelom (protostomes) Segmented

General Characteristics of Arthropods  The diversity and success of arthropods are largely related to their

segmentation

,

hard exoskeleton

(made of

chitin

), and

jointed appendages

.

General Characteristics of Arthropods  Segments have combined into functional groups called

tagmata

.

 Tagmata have specialized purposes.

General Characteristics of Arthropods  As arthropods evolved, the segments fused, and the appendages became more specialized.

 The appendages of some living arthropods are modified for many different functions.

General Characteristics of Arthropods  Arthropods have an open circulatory system in which fluid called

hemolymph

is circulated into the spaces surrounding the tissues and organs.

 A variety of organs specialized for gas exchange have evolved in arthropods.

A Versatile Exoskeleton

 The

exoskeleton

of arthropods is very protective, but still flexible.

 The exoskeleton is made of

chitin

.

 Prevents desiccation.

  Provides places for muscle attachment.

Does not allow for growth, the outer covering must be molted –

ecdysis

.

More Efficient Locomotion

 Usually, each segment bears a pair of jointed appendages.

 The appendages have sensory hairs and may be modified for sensory functions, food handling, or walking & swimming.

Air Piped Directly to Cells

 Most terrestrial arthropods have an efficient

tracheal system

of air tubes, which delivers oxygen directly to the tissues and cells.

  Limits body size.

Aquatic arthropods breathe using internal or external gills.

Highly Developed Sensory Organs  Arthropods have a variety of sensory organs.

Complex Behavior Patterns

 Arthropods show complex behavior patterns.

 Mostly innate behaviors.

 Some learned.

Metamorphosis



Intraspecific competition

(between members of one species) is reduced because of

metamorphosis

.

 Larval forms may be quite different from adults.

Relationships Among Arthropod Subgroups 

Clade Panarthropoda, Phylum Arthropoda

 Divided into subphyla based on relationships between subgroups based on molecular data.

Relationships Among Arthropod Subgroups  Centipedes, millipedes, pauropods, and symphylans are placed in

subphylum Myriapoda

.

 Insects are placed in

subphylum Hexapoda

.

 Spiders, ticks, horseshoe crabs and their relatives form

subphylum Chelicerata

.

 Lobsters, crabs, barnacles, and others form

subphylum Crustacea

.

Relationships Among Arthropod Subgroups  Formerly, insects and myriapods were placed together in uniramia.

 Uniramous appendages.

 The “ mandibulate hypothesis ” suggests all arthropods with mandibles are more closely related to each other than to arthropods with chelicerae.

 Mandibles in each group may or may not be homologous.

 mtDNA data support this hypothesis.

Subphylum Trilobita

 Early arthropods, such as

trilobites

showed little variation from segment to segment.

Subphylum Trilobita

 Trilobites arose during the Cambrian – maybe earlier and lasted for 300 million years.

Subphylum Trilobita

 Trilobites had a trilobed shape.

 Three tagmata: 

Head (cephalon)

with a mouth, compound eyes, antennae, and 4 pairs of leglike appendages.

 

Trunk

with a variable number of segments each with a pair of

biramous

appendages.

 One of the branches of biramous appendage was fringed and may have been a gill.

Pygidium

– segments fused into a plate.

Subphylum Trilobita

 Most could roll up like pill bugs.

 Probably benthic scavengers.

 Many (especially later species) had large, complex, many-faceted eyes.

Subphylum Chelicerata



Chelicerate

arthropods include eurypterids, horseshoe crabs, spiders, ticks, mites, scorpions, & sea spiders.

Subphylum Chelicerata

 They have 6 pairs of cephalothoracic appendages: 

Chelicerae

(mouthparts)  

Pedipalps

4 pairs of

walking legs

 Lack mandibles and antennae.

Class Merostomata



Class Merostomata

includes the eurypterids and horseshoe crabs.



Eurypterids

water scorpions up to 3 m in length.

were giant  Cambrian through Permian.

 Predators, some with large crushing claws.

Class Merostomata

 Three genera of horseshoe crabs live today.



Limulus

, found in North America, has existed on earth almost unchanged since the Triassic period.

Class Merostomata

 Horseshoe crabs have an unsegmented

carapace

(hard dorsal shield), a broad abdomen, and a long

telson

(tail piece).



Cephalothorax

 Chelicerae  Pedipalps  4 pairs walking legs 

Abdomen

 6 pairs of thin appendages  Book gills found on 5.

Class Merostomata

 Horseshoe crabs have simple and compound eyes.

 Feed at night on worms and small molluscs.

 Come to shore in large numbers to mate at high tide.



Trilobite larvae

resemble trilobites.

Class Pycnogonida

 Sea spiders, walking legs.

class Pycnogonida

, have small, thin bodies and usually 4 pairs of  Found in all oceans, most common in polar seas.

 Some have chelicerae and pedipalps.

Class Arachnida



Class Arachnida

includes spiders, scorpions, mites, and ticks.

50 µm

Class Arachnida

 Two tagmata: 

Cephalothorax

   Chelicerae Pedipalps 4 pairs walking legs 

Abdomen

Class Arachnida - Order Araneae  Most spiders –

order Araneae

– have 8 simple eyes that can detect light and motion.

 Some hunting & jumping spiders may form images.

 Sensory setae detect air currents, web vibrations, and other stimuli.

 Spider ’ s vision usually poor, so awareness of environment depends largely on cuticular mechanoreceptors such as sensory setae.

Class Arachnida - Order Araneae  All are predaceous, mostly on insects.

 Many spin a web used for prey capture.

 Some chase & catch prey.

 Injected venom liquefies and digests the tissues which is sucked into spider ’ s stomach.

Class Arachnida - Order Araneae  Two or three pairs of

spinnerets

to contain microscopic tubes that run

silk glands

.

 Liquid scleroprotein secretion hardens as it is extruded from spinnerets.

  Silk threads are very strong and will stretch considerably before breaking.

Silk is used for orb webs, lining burrows, forming egg sacs, and wrapping prey.

Class Arachnida - Order Araneae    Breathe by book lungs and/or tracheae.

 Book lungs unique to spiders - parallel air pockets extend into blood-filled chamber.

 Air enters chamber through a slit in body wall.

Tracheae system is less extensive than in insects.

 Transports air directly to tissues.

Tracheal systems of arthropods represent a case of evolutionary convergence.

Class Arachnida - Order Araneae  In spiders and insects,

Malpighian tubules

serve as excretory structures.

 Potassium, other solutes, and waste are secreted into tubules.

 Rectal glands reabsorb the potassium and water, leaving wastes and uric acid for excretion.

 Conserves water and allows the organisms to live in dry environments.

 Many spiders have

coxal glands

, modified nephridia, at the base of legs.

Class Arachnida - Order Araneae  Reproduction - before mating, male stores sperm in pedipalps.

 A courtship ritual is often required before the female will allow mating.

 Eggs may develop in a cocoon in the web or may be carried by female.

 Young hatch in about two weeks and may molt before leaving the egg cocoon.

Class Arachnida - Order Araneae  Are spiders really dangerous?  Spiders are allies of humans in our battle with insects.

  American tarantulas rarely bite, and bite is not dangerous. Species of black widow spiders are dangerous.

 Venom is neurotoxic.

 Brown recluse spider has hemolytic venom that destroys tissue around the bite.

 Some Australian and South American spiders are the most dangerous and aggressive.

Class Arachnida - Order Scorpiones  Scorpions –

Scorpiones order

– feed on insects & spiders which they seize with their pedipalps.

 The last segment contains a bulbous base and a curved barb that injects venom.

 Scorpions are viviparous or ovoviviparous – females brood young within their reproductive tract.

Class Arachnida - Order Opiliones  Harvestmen –

Opiliones order

– differ from spiders in that the abdomen and cephalothorax are broadly joined rather than constricted.

  Only two eyes Abdomen shows segmentation  Long legs end in tiny claws.

Class Arachnida - Order Acari

 Mites and ticks –

order Acari

– have a fused cephalothorax & abdomen.

 Mites are tiny – 1mm or less.

 Some feed on plant juices and can be major pests.

 Several species of ticks carry diseases such as Lyme disease.

Class Arachnida - Order Acari

 House dust mites free-living and often cause allergies.

 Spider mites - one of many important agricultural pest mites that suck out plant nutrients.

Class Arachnida - Order Acari

 Hair follicle mite

Demodex

- harmless but other species cause mange in domestic animals.

 Human itch mite causes intense itching.

Class Arachnida - Order Acari

 Tick species of

Ixodes

carry Lyme disease.

 Tick species of

Dermacentor

transmit Rocky Mountain spotted fever.

 Cattle tick transmits Texas cattle fever.

Subphylum Myriapoda

  

Subphylum Myriapoda

 Chilopoda includes these classes: (centipedes)  Diplopoda (millipedes)  Pauropoda (pauropods)  Symphyla (symphylans) Use trachea to transport air.

Excretion usually by Malpighian tubules.

Class Chilopoda



Centipedes Chilopoda

–

class

– contain a few or many segments each (except the first behind the head and the last two) with a pair of jointed legs.

 Last pair of legs has a sensory function.

Class Chilopoda

 Head appendages:    One pair antennae One pair mandibles One or two pairs of maxillae.

 Dorsoventrally flattened.

Class Chilopoda

 Sexes are separate.

   Some lay eggs (oviparous).

Some have live young (viviparous).

Young like little adults – no metamorphosis.

Class Chilopoda

 Centipedes live in moist environments.

 They are carnivores, feeding on insects & worms.

 Prey is killed with poison claws on the first segment.

Class Diplopoda



Millipedes

(Class Diplopoda) have two pairs of legs on each segment.

 Head has one pair each of antennae, mandibles, & maxillae.

 Body is more cylindrical.

Class Diplopoda

 Millipedes live in dark, moist places – under rocks or logs.

 Most are herbivores, feeding on decayed plant matter or occasionally living plants.

  Slow moving, coil up when disturbed.

Toxic or repellent fluids secreted when disturbed.

Class Diplopoda

 After copulation, female lays eggs in a nest and guards them.

 Larvae have only one pair of legs per segment.

Class Pauropoda

 Live in moist soil, leaf litter, decaying vegetation, or under bark and debris.

 Least well known of myriapods.

 Soft-bodied, small (2 mm or less).

 Approximately 500 species.

 Head lacks true eyes, has branched antennae, and a pair of sense organs.

Class Pauropoda

 12 trunks segments bear 9 pairs of legs but none on the first or last 2 segments.

 One tergal plate covers two segments.

 Lack tracheae, spiracles, and circulatory system.

 Probably most closely related to diplopods.

Class Symphyla

 Live in humus, leaf mold, and debris.

 Male

Scutigerella

places a spermatophore at end of a stalk.

 Female stores the sperm in special pouches.

 Removes and smears eggs with sperm before attaching them to moss or lichen.

 Young hatch with only 6 or 7 pairs of legs.

Class Symphyla

 Small (2 –10 mm) with centipede-like bodies.

 Soft-bodied with 14 segments - 12 segments bear legs and one bears a pair of spinnerets.

 Antennae are long and unbranched.

 About 160 species are known.

 Eyeless with sensory pits at base of antennae.

Phylogeny

 Relationships between subphyla are debated.

 Taxon of Pancrustacea, which includes hexapods and crustaceans, is well-supported.

 Phylogenies using molecular data rarely support grouping Myriapoda with Pancrustacea.

 There is support for placement of Myriapoda as the sister taxon for Cheliceratae.

Phylogeny

 Biologists assume that the ancestral arthropod had a segmented body with one pair of legs per segment.

 Evolution caused adjacent segments to fuse and to make body regions.



Hox

gene studies indicate that the first five segments fused to form the head tagma in all four extant subphyla.

 In spiders,

Hox

gene studies indicate that the entire prosoma corresponds to the head of other arthropods.

Phylogeny

 Genetic studies have been helpful in understanding the evolution of uniramous and biramous appendages.

 Molecular evidence repeatedly places hexapods with crustaceans even though hexapods have uniramous appendages and crustaceans have biramous appendages.

 Leads to the question: Did uniramous appendage development evolve more than once?

Adaptive Diversification

 In contrast to annelids, arthropods have pronounced tagmatization by fusion of somites.

 Those with primitive characters have appendages on each somite.

 Derived forms are specialized.

 Modification of exoskeleton and appendages allowed variation in feeding and movement.

 Adaptations made possible by cuticular exoskeleton and small size fostered high diversity.

 Subphylum Trilobita

Phylogeny

 Subphylum Chelicerata  Class Merostomata  Class Pycnogonida  Class Arachnida  Subphylum Myriapoda  Class Diplopoda   Class Chilopoda Class Pauropoda  Class Symphyla  Subphylum Crustacea  Subphylum Hexapoda