The systematic position of Arachnida within the animal kingdom can be described as follows:
• Kingdom: Animalia (Animals)
• Phylum: Arthropoda (Arthropods)
o This phylum includes invertebrates with an exoskeleton, segmented bodies, and jointed appendages.
• Subphylum: Chelicerata
o Chelicerata is characterized by the presence of chelicerae (specialized mouthparts) and includes animals that lack antennae.
• Class: Arachnida
o This class encompasses a wide variety of species, all of which have two main body segments (prosoma and opisthosoma), eight legs, and no antennae.
Major Orders within the Class Arachnida
The class Arachnida is further divided into several orders, including but not limited to:
1. Araneae (Spiders): Characterized by having silk-producing spinnerets and chelicerae that function as fangs.
2. Scorpiones (Scorpions): Known for their segmented tails ending in a venomous stinger and large pincers (pedipalps).
3. Acari (Ticks and Mites): Extremely diverse and often microscopic, with a wide range of ecological roles and medical importance.
4. Opiliones (Harvestmen or Daddy Longlegs): Recognized by their fused body segments and long, slender legs.
5. Pseudoscorpiones (Pseudoscorpions): Small arachnids with pincers like scorpions but lacking a tail and sting.
6. Solifugae (Camel Spiders or Wind Scorpions): Known for their large chelicerae and high-speed movement.
7. Thelyphonida (Whip Scorpions or Vinegaroons): Characterized by their whip-like tails and the ability to spray acetic acid as a defense mechanism.
Phylogenetic Relationships
• Arachnida is one of the major classes within the Chelicerata subphylum, which also includes other groups like Merostomata (horseshoe crabs) and Pycnogonida (sea spiders).
• Within Arachnida, evolutionary relationships are studied using both morphological characteristics and molecular data to understand how different orders are related.
This systematic classification helps in understanding the evolutionary lineage of arachnids and their place within the broader spectrum of arthropods.