Pure playground myth. In America, the arachnid called the daddy longlegs is also known as a harvestman. In Britain, this story refers to the crane fly, an unrelated winged insect that looks like a large mosquito, and is also called daddy longlegs. Both are non-venomous and non-biting and the myth probably started as a schoolyard tale to scare new pupils.
The most venomous creature in the world is generally thought to be the South American poison dart frog or the box jellyfish. If you see either of them in the playground ' run.