The anatomy of arachnids is fascinating, characterized by a range of adaptations that have enabled them to thrive in diverse environments. Below, you’ll find a detailed overview of both the external and internal anatomy of arachnids.
External Anatomy of Arachnida
1. Body Structure
Segmentation:
Prosoma (Cephalothorax): The anterior part, which combines the head and thorax. It is covered by a hard exoskeleton called the carapace.
Opisthosoma (Abdomen): The posterior segment, which can be segmented (like in scorpions) or unsegmented (like in spiders).
2. Appendages
Legs: Arachnids have eight jointed legs, each typically consisting of seven segments: coxa, trochanter, femur, patella, tibia, metatarsus, and tarsus.
Legs are adapted for various functions, including walking, climbing, and capturing prey.
Chelicerae: The first pair of appendages, modified into fangs in many spiders, are used to grasp and inject venom into prey.
Pedipalps: The second pair of appendages, which function in sensory perception, food handling, and, in males, as reproductive organs.
3. Eyes
Simple Eyes (Ocelli): Most arachnids possess multiple simple eyes (typically eight) that can detect light and movement but do not form detailed images. Some species have reduced numbers of eyes.
4. Exoskeleton
Composition: The exoskeleton is composed of chitin and proteins, providing protection and structural support. It is shed periodically during molting (ecdysis).
5. Silk Glands and Spinnerets (in Spiders)
Silk Glands: Produce silk used for web construction, egg sac creation, and draglines.
Spinnerets: Specialized structures located at the posterior end of the abdomen that manipulate and extrude silk.
6. Respiratory Structures
Book Lungs: Present in many spiders and scorpions; these structures allow for gas exchange and are located in the abdomen.
Tracheae: Some smaller arachnids utilize a tracheal system for direct air transport to tissues.
7. External Features
Coloration and Patterns: Arachnids display a wide range of colors and patterns, which may serve functions in camouflage, warning, or mating displays.