Human flea. Coloured scanning electron micrograph (SEM) of a human flea (Pulex irritans). At bottom centre are two sensory palps. At centre are the mouthparts, which pierce the skin to enable the flea to suck the blood of its host. Fleas are external parasites that may carry and transmit diseases, including the plaque. The itchiness associated with a flea bite is mainly due to an allergic reaction to the flea's saliva or faeces that have been deposited near the bite and rubbed into it by scratching. Magnification: x33 at 10 centimetres wide.