Appendages are structures like arms, legs and antennae that extend from the body of the animal. In many instances the two appendages join together to form what we call a "joint." Arthropod is a term that literally means "jointed foot." If a prize were given for sheer species numbers, it would go to the arthropods. Approximately 900,000 species have been recorded, and probably at least as many remain to be discovered. While arthropods may be quite different in appearance, they share a number of internal and external characteristics:
Types of animals:
a. insects
b. centipedes / millipedes
c. spiders
d. ticks
e. scorpions
f. crustaceans (shrimp, etc.)
Body segments
a. head - located on an insects head are mandibles, and antennae. The mandibles are adapted for holding, chewing, sucking, or biting the various foods eaten by arthropods; antennae are sensory structures. Some species of arthropods have a fused head and thorax known as a cephalothorax
b. thorax - is composed of three fused segments. Attached to the thorax are three pairs of jointed walking legs, and wings.
c. abdomen - contains reproductive structures
Respiratory structures
1. gills - aquatic arthropods exchange gases through gills, which extract oxygen from water and release carbon dioxide into the water
2. tracheal tubes - branching networks of hollow air passages [spiracles]
3. book lungs - air-filled chambers that contain leaflike plates [spiders]
Other general characteristics:
a. pheromones - chemical [such as odor] signals given off by animals
b. simple eye - contains one lens
c. compound eye - many lenses
d. nervous system - nerve cord branching to form ganglia. The ganglia acts as control centers for the body section in which they are located.
e. open circulatory system - bathes the tissues of the body, and return to the heart through open body spaces.
f. fertilization- primarily sexual
1. oviparous - lay eggs that develop outside the body
2. ovoviviparous - egg development is internal
| Arachnids 1. two body regions 2. book lungs for breathing 3. no appendages used as antennae 4. 4 pairs of appendages used as legs 5. 1 pair of chelicerae [fangs to hold prey] 6. 1 pair of pedipalps [modified to carry sperm in males] 7. spinnerets located at the rear of the spider secretes silk from the silk gland |  |
| Crustaceans 1. two body regions 2. gills for breathing 3. 2 pairs of appendages used as antennae 4. 1 pair of appendages used as mandibles 5. 1 pair of appendages used as maxilla 6. 1 pair, known as the cheliped, is enlarged and serves as a pincher 7. 12 pair of appendages are used for swimming and walking, the last is known as the uropod and serves as a flipper-like structure used in swimming. |  |
| Insects 1. 3 body regions 2. thorax is divided into 3 regions [prothorax, mesothorax, metathorax] 3. 1 pair of legs attaches to each thoracic region, as well as a pair of wings, when present 4. 2 appendages used for mouthparts [mandible and maxilla] / there are four ways in which insects ingest food: I. chewing, II. piercing, III. sponging, IV. sucking 5. 2 lip-like structures [labrum and labium] aid in food handling 6. antennae 7. blood distributes nutrients, but is not important in gas transport 8. ectothermic - temperature gained by outside environment 9. sound is heard by sound waves vibrating antennae, or a tympanic membrane 10. ovipositor used to lay eggs in females, and acts as a stinger |  |
