24.3 Arachnids
KEY CONCEPT
Arachnids include spiders and their relatives.
24.3 Arachnids
Arachnids are the largest group of chelicerates.
• There are three major groups of chelicerates.
– horseshoe crabs
24.3 Arachnids
Arachnids are the largest group of chelicerates.
• There are three major groups of chelicerates.
– horseshoe crabs
– sea spiders
– arachnids
24.3 Arachnids
• Chelicerates share several features.
– no antennae
– Six pair of appendages
• four pairs of walking legs
• one pair each of chelicerae and pedipalps
• Arachnids are a group of chelicerates that live on land.
– Fossils dating 400 m.y.a.
– eight legs
fangs
– fanglike pincers poison gland
that inject venom
spinnerets
– silk glands
24.3 Arachnids
• Arachnids have four different adaptations that reduce water
loss.
– waterproof cuticle
• Water can’t evaporate across skin
– book lungs
• Large surface area for gas exchange
24.3 Arachnids
– Malpighian tubules
• Excretory structure
• Minimizes water loss
– spiracles
• Tiny holes that allow O2
to enter cutilce
24.3 Arachnids
“How do the features of an arachnid allow it to live on land?”
24.3 Arachnids
Arachnids have evolved into a diverse group.
• All spiders make silk and produce venom.
24.3 Arachnids
• Silk is used for:
– Building webs
– Egg cases
– Wrapping prey
– Shelters
– Safety line
24.3 Arachnids
• Venom…
– All are hunters
– Inject by chelicerae
– Neurotoxin or hematoxin
– Black Widow & Brown Recluse
24.3 Arachnids
• Identify the parts of a spider.
24.3 Arachnids
Bell Ringer
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1-What is silk used for?
2-What is venom used for?
3-What are spiracles?
4-How are spiracles an adaptation to reduce water loss?
5-Are all arachnids spiders? If not what else is an
arachnid?
24.3 Arachnids
Arachnids have evolved into a diverse group.
• Spiders make up half of the more than 60,000 known
arachnid species.
24.3 Arachnids
• Arachnids also include mites, ticks, chiggers, and
scorpions.
– Many are parasites to plants & animals
– Needle like mouth parts
24.3 Arachnids
• Scorpions:
– Hunt by night, feeling vibrations as prey moves
– Eats insects, lizards, & even small rodents
• Arachnids are important as prey & predators
• Some mites are serious pests of fruit, cotton & other
crops
• Ticks transmit diseases as Rocky Mountain Spotted
Fever & Lyme disease.
24.3 Arachnids
“How would the loss of many arachnid species affect an
ecosystem?”