Explain how to distinguish between these arthropod: spiders, insect, millipedes, centipedes. Explain how to distinguish between these arthropod: spiders, insect, millipedes, centipedes. Solution By simply looking at them, you can make the distinction. Spiders have four pairs of legs and a cephalothorax, means head and thorax are fused together. Insects have head, thorax and abdomen; they have 3 pairs of legs. Millipedes\' body has many segments. Each segment has one pair of legs. Centipedes\' body also has many segments. And each segment has two pairs of legs..