Solution:
Both insects and arachnids are arthropods. However, in the morphology of an insect, there are 6 legs. While in the morphology of an arachnid there are 8 legs. Also, arachnids do not have antennae, their head is not differentiated from the body but is typically divided into two main regions called the cephalothorax (prosoma) and the abdomen (opisthosoma). Spiders belong to the arachnids, therefore a spider is different from an insect. Therefore, a spider cannot be classified as an insect.