足長おじさん (ashinaga-ojisan: daddy longlegs) legs + long
Here’s another way to refer to this animal (which must be one of your most pressing needs as you learn Japanese!):
大蚊 (gaganbo or kaganbo: crane fly; daddy longlegs)
big + mosquitoSo, to recap, there are two completely different compounds that mean “daddy longlegs.” And 大蚊 (which is usually represented in hiragana) can refer to either a fly or a spider but breaks down as big + mosquito. Finally, this compound has two yomi. Everything is clear now!