Why are ticks not considered insects?

Ticks are classified as arachnids rather than insects, primarily because they have eight legs in their nymph and adult stages. This is a defining feature of arachnids, which also includes spiders and scorpions. In contrast, insects are characterized by having six legs. The presence of eight legs in ticks is indicative of their evolutionary biology and taxonomic classification, distinguishing them clearly from the insect class.

The other options do not accurately reflect the biological classification of ticks. Size does not determine classification; both insects and arachnids can vary greatly in size. A segmented body is a characteristic of insects, not a defining feature that ticks lack. Additionally, ticks are ectothermic (cold-blooded) rather than warm-blooded, so that option doesn't align with their true biological nature either. Thus, the reason ticks are not considered insects lies in their unique anatomical traits, particularly their eight-legged structure as nymphs and adults.