Fleas drink blood, but they can go as long as 17 months without nourishment. The young will often stay dormant in their cocoons, patiently waiting for a host animal to walk by. Within a split second, the flea can burst from its cocoon and jump onto the unfortunate host. Fleas rely upon their keen sense of smell and upon heat perception to distinguish one host one host from another. There are 1,200 species of fleas, they usually stick only to their favorite host animal.
Spiders have transparent blood.
They do not eat their victims, they drink them. They are able to take food only by sipping it in liquid form through their tubelike mouths. First, they cover their victims with a special fluid that causes them to dissolve. Then they suck up the dissolved tissue. Size for size, spider silk is stronger than steel.