Arthropoda
• Monophyly Now Accepted
• Similarity between insects & myriapods
thought to be due to convergent adaptations
to terrestrial habitat
Molecular data refute these classic hypotheses
Myriapods Insects Crustaceans Chelicerates
Crustaceans
Chelicerates
Myriapods
Insects
Uniramia
(Cisne)
What is the sister group to Pancrustacea?
Hexapoda
Chelicerata
Moore textbook, page 279 & most schemes suggest myriapods + chelicerates. Note,
however, that the node is poorly supported
The latest: mtDNA sequenceBased phylogeny
• This study controlled for
misleading effects of “multiple
changes in pattern and
substitution rates (as well
as long branch effects)
• Chelicerate-Myriapod sister
group and Pancrustacea
group supported (both
Hexapoda& Crustacea
not clades)
• Sea spiders confuse the
arachnid story
Hassanin (2006) Molecular Phylogenetics
and Evolution 38 100–116
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Diversity of arthropod body plans.
Anomalocaris – a Cambrian arthropod predator up to 6 ft in length
http://www.bogleech.com/pokemon/pkmn-anomalocaris.jpg
http://universe-review.ca/I10-27-anomalocaris.jpg
Arthropoda on Tree of Life
http://www.tolweb.org/Arthropoda/2469
Chelicerata
• No head; instead an anterior prosoma (8
segments usually) and posterior abdomen
• 1st segment – no appendages;
• 2nd has chelicerae (feeding & grasping
structures);
• 3rd – has pedipalps (sensory, prehensile or
reproductive)
• 4-7 bear 4 pairs of walking legs (often with
chelae at end)
2
Chelicerates cont…
• No antennae and no jaw like chewing
mandibles
• Mostly terrestrial species
• Ignore “Merostomata” in Moore
Class: Pycnogonida
sea spiders
1st segment of chelicerates
having no appendages
(considered as labrum in most
extant arthropods):
Side story about
pycnogonid chelifore
homology with chelicerae
Neuro-anatomy evidence
from Budd & Telford Nature
437: 1100 (2005)
3
Jager et al (2006)
Nature: 506-508
HOX gene
data refute
the previous
hypothesis
Class: Xiphosura (horseshoe crabs)
http://www.dnr.sc.gov/marine/sertc/images/photo%20gallery/Limulus.jpg
Class: Eurypterida
http://dictionary.laborlawtalk.com/prev_wiki/images/2/25/Eurypterid.jpg
http://users.pandora.be/biologie/images/dierensys_42.jpg
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Could eurypterids have represented a 2nd arthropod
invasion of land via freshwater to form the terrestrial
arachnids?
• scorpion-like ancestor? ( scorpions or whip
scorpions are closely related to Xiphosura
inmtDNA and nuclear DNA – based
phylogenies )
Class: Arachnida
(short-tailed whip scorpions)
(hooded tick spiders)
http://www.tolweb.org
Arachnida
• Unsegmented prosoma, usually covered
by carapace
• Abdomen segmented in many
• Appendages arise from prosoma (1 pair
each of chelicerae and pedipalps; 4 pairs
of legs)
• Primarily carnivores
• Book lungs
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Order: Araneae (spider)
Opiliones (harvestmen)
http://insects.ummz.lsa.umich.edu/Images/Arachnida/opiliones2.jpg
Acarina:
mites and ticks
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Pseudoscorpions
http://www.bugwise.net.au/images/
250/r_pseudoscorpion.jpg
Sofugida (sun spiders)
http://myrmecos.net/images/Solpugid1.html
Scorpiones
http://entomology.unl.edu/images/scorpions/barkscorpion1a.jpg
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Uropygi (whip scorpions)
http://popper.cs-i.brandeis.edu:8088/ecolib/dbtest2.servlet
Myriapoda
•
•
•
•
•
•
•
Appendages unbranched
Mandibles
Antenna on 2nd head segment
Mandibles on 4th
Tracheal system
Malpihgian tubules
Ganglion in each segment
Chilopoda (centipedes)
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Diplopoda (millipedes)
http://www.swva.net/fred1st/millipede.jpg
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