Characteristics of the PhylumArthropoda
The segmented bodies are arranged intoregions, called tagmata (e.g., head, thorax,abdomen). The paired appendages (e.g., legs, antennae)are jointed. They posses a chitinous exoskeletion thatmust be shed during growth. They have bilateral symmetry. The nervous system is dorsal (belly) and thecirculatory system is open and ventral (back).
Arthropod Groups (taxa)
The arthropods are divided into twolarge groups that exist today:
The Chelicerates
and
The Mandibulates
Exoskeletons
• Strength andimpermeability
• Solves difficulties oflife on land:– desiccation– need for structural
support
• BUT: must molt togrow (ecdysis)
Grasshopper molting
Phylum Arthropoda “arthropods”• > 1 million spp., most of them insects (most of those,
beetles)• Most diverse, widely distributed & abundant of all
animal phyla
• Exoskeleton of chitin* Hard & strong* Impermeable to water* Molting
• Segmentation• Jointed appendages
* “jointed feet”• efficient walking &
swimming
* wings in most• one of 2 phyla to fly
Arthropod innovations
Monarch butterfly metamorphosis
Millipede
• Two types, relating to feeding mode
* Uniramous, single branch* Biramous, two branches, sometimes gnathobasic
(like in Limulus and the trilobites)* Some doubt cast recently on this simple scheme!
Arthropod limbs
Trilobite gnathobasic limbs
Further arthropod anatomy
• Extensive cephalization (sense organs & mouth parts)
• Well developedsense organs:* “Taste”* Sight many
with complex,compoundeyes
• Segment fusion
Antennae(sensoryreception)
Head Thorax
Swimmingappendages
Walking legs
Mouthparts (feeding)Pincer (defense)
Abdomen
Some Arthropod groups
Millipedesandcentipedes
Limulus
Trilobite (extinct group)
Class Arachnida“Spiders, mites, ticks, scorpions”
• 6 pairs of appendages• including 4 pairs of walking legs
50 µm
Class Crustacea“Crabs, shrimp, barnacles”
• Mostlymarine
• Carnivores,detritivores,filter feeders
• 2 pairs ofantennae
• 3 or morepairs ofmouthparts
• 3 or morepairs of legs
Cf Fig 33.38
Lobster Goose-neck barnacles
Shrimp on coral
Classes of Crustacea(mostly marine, fresh water, a few terrestrial)
(all have two pair of antennae, five or more pairs of legs,segmented abdominal appendages, head & trunk or
cephalothorax & abdomen body arrangement, have gills)
Isopoda - sowbugs or pillbugs
Amphipoda - sand fleas, amphipods
Cirripedia - barnicles
Decapoda - crabs, lobster, shrimp
several other minor orders
Crayfish cephalothorax(Decapoda)
Sowbug (Isopoda), a terrestrial crustacean
Class Insecta - Insects
Fig 33.35 • Fused segments into 3body regions:* Head* Thorax* Abdomen
• Metamorphosis• Flight
• 26 Orders• By numbers and
influence, the mostimportant phylum oforganisms
Chelicerate Arthropod Characters:
Pincher-like mouthparts -chelicerae - and pedipalps NO antennae Two body regions, usually -cephalothorax & abdomen Four pairs of legs Horseshoe crabs and arachnidsare only living groups
Mandibulate Arthropod Characters:
Mouthparts are mandibles -normally chewing sideways One or two pairs of antennae Various body region arrangements -cephalothorax & abdomen / head &trunk / head, thorax & abdomen Variable leg numbers Insects, crustaceans & myriapods
Orders of Arachnids
Scorpionida - scorpions Pseudoscorpionida - falsescorpions Opiliones - daddy-long-legs orharvestmen Acari - mites & ticks Araneida - spiders
Scorpion Tick(a mite)
Pseudoscorpion
Daddy-long-legs
WolfSpider
Scorpion Anatomy
chelicerae eyes pedipalp
Pseudoscorpion
pedipalps &chelicerae
cephalothorax
abdomen
Mite and Tick Body Regions
American dog tick male
Blacklegged (deer) tick female
American dog tick female laying egg mass (1000-2000 eggs!).
Clover mites
Twospotted spider mites
Predatory mite
Opiliones [=Phalangida] daddy-long-legs
cephalothorax abdomen
pedipalp
chelicera (fang)
cephalothorax
abdomen
narrow waist
Spider Anatomy
Abdomen
Pedipalp
Chelicera (fang)
Cephalothorax
Jumping Spider
Wolf spider with egg case Spitting spider
Tarantula Orbweaving spider
Black widow with egg case
Brown recluse (fiddleback)
Classes of Myriapods(many legged arthropods)
(all have one pair of antennae, a head region, and trunkwith many pairs of legs, use trachea)
Diplopoda - millipedes
Chilopoda - centipedes
Symphyla - garden centipedes
Myriapods
Millipede (Diplopoda)
Two pair of legs per visible segment, attachedunder body.
Centipede (Chilopoda)
Pair of fangs under head, one pair legs per visiblesegment - attached to side of body.
Symphylan (Symphyla)[garden centipede]
No fangs, no eyes, legs attached to side of body.
[one pair of antennae, head & trunk regions, trunk with many pairs of legs]
Millipede (Diplopoda)
Centipede (Chilopoda)
Garden centipede (Symphyla)
Origins of arthropod diversity
• Very controversial! Fossil arthropods fromthe Burgess Shale and similar Cambriandeposits have played a big role
Arthropod relatives
• Onychophorans• Tardigrades
Onychophorans
Onychophoran anatomy
Onychophorans
• Are today a small terrestrial group. They areworm-like and lack sceritisation apart fromin the claws and jaws
Tardigrades
Tardigrades
• Small (< 2mm)• Famous for cryptobiosis
Lobopods
• Tardigrades and onychophorans are oftencalled lobopods because their limbs are lobesrather than being jointed like those ofarthropods
• Relationships to arthropods are clear ingeneral, and unclear specifically.
Fossil lobopods
Microdictyon Aysheaia
Arthropod relationships (oneview)
Burgess Shale arthropods
Naraoia Sidneyia Marrella
Chengjiang arthropods
Fuxianhuia Xandarella Alalcomenaeus
Problems
• None of these animals, although obviouslyarthropods, fit into the modern groups!
• Probably lie in the stem-groups to moderngroups.
The mystery of theanomalocaridids
Anomalocaridids are...
• Large animals (1m or more?) with large eyesand two large “frontal appendages”
• At least have some gnathobasic biramoustrunk limbs
• Relationships are very very controversial
Budd View
• Anomalocaridids show intermediate featuresbetween lobopods and Euarthropods. Theylie in the stem group of the euarthropods (ortactopods)
Slide 77
Other views
• Not the only view possible – some think theyare stem-group chelicerates, and the frontalappendages are homologous to thechelicerae.
Two strange taxa
Kerygmachela Pambdelurion
Kerygmachela and Pambdelurion
• Like the anomalocaridids broadly, but muchmore lobopod like – lack scleritisation andjointed limbs.
• May be even more basasl!
Broader relationships
• Classical view is that arthropods are relatedto, or derived from, annelids
• Shared features include segmentation,coelom, nature of brain and development
Example: arthropod evolution• Internal relationships still enigmatic –
Mandibulata versus Paradoxopoda etc• Extremely rich Cambrian fossil record
What about the stem groups?
Fossil lobopods
Microdictyon Aysheaia
Hallucigenia most famous!
• Should probably not be considered to bestem-group onychophorans…
Some more unusual taxa
Kerygmachela (Budd 1993, 1999)
• Kerygmachela shows distinct lobopodianfeatures…but also some some novelties:large branched frontal appendages, laterallobes.
• It shows intriguing similarities to theanomalocaridids…
Anomalocaridids…
Anomalocaridids
LagganiaAnomalocaris
Collins, 1996
Anomalocaridids• One of the largest
Cambrian animals
• Characterized by apair of frontalappendages, a“Peytoia”, and largeeyes
• Found in the BurgessShale, Chengjiang,Sirius Passet, USAand Australia
Collins, 1996
This specimen of Laggania wasthought to be a sea cucumber
Appendages incorrectly attributed to otheranimals, like Sidneyia or Tuzoia
Hurdia Peytoia
• 32 outer plates• 5 inner rows of teeth each with 4-11 small teeth
Hurdia Peytoia
0.25 cm
How do all these parts fit together?
HurdiaDaley et al. 2009
1 cm
PosteriorAnterior
Oblique lateral view
Gill structures
1 cm
Importance ofHurdia Gills
1. Clarification of themorphology of thisstructure in allanomalocaridids.
• A series of parallellanceolate blades thatattach at one end andhang freely at the other
Importance of Hurdia gills
Pambdelurion…
Pambdelurion shows manysimilarities to Anomalocaris
Pambdelurion - rediviva
Kerygmachela and Pambdelurion
• Like the anomalocaridids broadly, but muchmore lobopod like – lack scleritisation andjointed limbs (Budd 1993, 1997, 1998).
• May be even more basal, thus indicatingtransition from lobopods
Arthropod evolution
