Flea bites often appear in clusters or rows, especially in areas where the clothes meet the skin, such as along the waistline or where the socks meet the ankle.
This is because it is easy for fleas to attach themselves to clothing and as they travel along the clothing they bite the first area of skin that they come in contact with, leaving a cluster of flea bites that often form a row right along that line.
Sometimes a single flea will bite multiple times in the same area, which is another reason why the bites may be clustered together.
Flea bites frequently cluster around the lower extremities of the body such as the feet and ankles, because those areas are in closest proximity to the ground where the fleas are located, making them more likely to be bitten.