Yeah, jack-jumpers do jump. They look like a large really aggressive bullant, except they leap onto you and attack, and since they both bite and sting, and allergy to the venom is fairly common, they can be a danger. I've been damaged by them several times, including one bite near the navel when one of them got inside the waistband of my trousers - as I said, they are one of the few insects I will actively kill. However, since a bite will just hurt and make me sick, and the bug spray will have me swelling in welts and unable to breathe, I have to resort to several kettles of boiling water down the nest. Cruel but needful.
My youngest son once jumped down from a fence right into the middle of a nest, and had to be rescued. . He was a mess!