Organisms in the class arachnida all possess_ pairs of walking legs
The correct answer and explanation is:
Correct Answer: 4 pairs of walking legs
Explanation (Approx. 300 words):
Organisms in the class Arachnida, such as spiders, scorpions, ticks, and mites, all possess four pairs of walking legs, which totals eight legs. This is a defining feature of arachnids and helps distinguish them from other arthropods like insects (which have three pairs or six legs).
Arachnids are part of the phylum Arthropoda, a large group of invertebrates characterized by jointed limbs, segmented bodies, and exoskeletons made of chitin. Within this phylum, arachnids stand out due to their body structure and limb arrangement.
The typical body plan of an arachnid is divided into two main segments:
- Cephalothorax (prosoma) – This is the front part that contains the head and thorax fused together. It bears the eyes, mouthparts (like chelicerae and pedipalps), and the four pairs of walking legs.
- Abdomen (opisthosoma) – This is the rear part, which may or may not be segmented depending on the species.
Why 4 Pairs of Legs?
Arachnids evolved to have four pairs of walking legs for efficient movement on land. These legs are attached to the cephalothorax and are well-suited for crawling, climbing, hunting, or burrowing—depending on the species. Unlike insects, arachnids do not have antennae, which is another distinctive trait.
Importance in Identification
The number of walking legs is a critical feature in the classification of arthropods. For example:
- Insects have 3 pairs (6 legs)
- Arachnids have 4 pairs (8 legs)
- Crustaceans can have 5 or more pairs
In summary, all arachnids have 4 pairs of walking legs, making a total of 8 legs, and this is one of the key biological characteristics that set them apart from other groups within the arthropods.