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 – fanglike pincers fangs poison gland that inject venom – silk glands spinnerets 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 • 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?”.