Pinworms. Pinworms are nematodes. The biggest difference from most of the others is that they are so small, usually just a 1/4 to 1/2 inch in length. The primary symptom they cause is anal itch. You can diagnoses them yourself by applying a 2" strip of scotch tape to your rectum/anus at night when you have the itching. You will see one or more 1/4 to 1/2 inch long filaments looking like white thread.