1. Arthropods Chapter 18

2. 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.

3. Phylum Arthropoda • Arthropods are: • Multicellular • Bilaterally symmetrical • Triploblastic • Have a true coelom (protostomes) • Segmented

4. General Characteristics of Arthropods • The diversity and success of arthropods are largely related to their segmentation, hard exoskeleton (made of chitin), and jointed appendages.

5. General Characteristics of Arthropods • Segments have combined into functional groups called tagmata. • Tagmata have specialized purposes.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

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

12. Complex Behavior Patterns • Arthropods show complex behavior patterns. • Mostly innate behaviors. • Some learned.

13. Metamorphosis • Intraspecific competition (between members of one species) is reduced because of metamorphosis. • Larval forms may be quite different from adults.

14. Subphylum Trilobita • Early arthropods, such as trilobites showed little variation from segment to segment.

15. Subphylum Trilobita • Trilobites arose during the Cambrian – maybe earlier and lasted for 300 million years.

16. 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. • Pygidium – segments fused into a plate.

17. Subphylum Trilobita • Most could roll up like pill bugs. • Probably benthic scavengers. • Many (especially later species) had large, complex, many-faceted eyes.

18. Subphylum Chelicerata • Chelicerate arthropods include eurypterids, horseshoe crabs, spiders, ticks, mites, scorpions, & sea spiders.

19. Subphylum Chelicerata • They have 6 pairs of cephalothoracic appendages: • Chelicerae (mouthparts) • Pedipalps • 4 pairs of walking legs

20. Class Merostomata • Class Merostomata includes the eurypterids and horseshoe crabs. • Eurypterids were giant water scorpions up to 3 m in length. • Cambrian through Permian. • Predators, some with large crushing claws.

21. Class Merostomata • Three genera of horseshoe crabs live today. • Limulus, found in North America, has existed on earth almost unchanged since the Triassic period.

22. 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.

23. 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.

24. Class Pycnogonida • Sea spiders, class Pycnogonida, have small, thin bodies and usually 4 pairs of walking legs. • Found in all oceans, most common in polar seas. • Some have chelicerae and pedipalps.

25. Class Arachnida • Class Arachnida includes spiders, scorpions, mites, and ticks. 50 µm

26. Class Arachnida • Two tagmata: • Cephalothorax • Chelicerae • Pedipalps • 4 pairs walking legs • Abdomen

27. Class Arachnida • Most spiders – order Araneae – have 8 simple eyes that can detect light and motion. • Some hunting & jumping spiders may form images.

28. Class Arachnida • Many spin a web used for prey capture. • Some chase & catch prey.

29. Class Arachnida • Scorpions – order Scorpiones – 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 – females brood young within their reproductive tract.

30. Class Arachnida • Harvestmen – order Opiliones – 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.

31. Class Arachnida • 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.

32. Phylogeny • Annelids and arthropods share a number of shared derived characters. • Recent molecular evidence has shown them to belong to separate superphyla. • Segmentation arose independently. • Arthropods did not descend from annelids.