Refers to arachnids of the order Opiliones, not to the spider family Pholcidae (which is also called daddy longlegs in English).
See also: ガガンボ
I found a harvestman in the garden.
Compound of 盲 (めくら, blind) and 蜘蛛 (くも, spider), literally 'blind spider', referring to the harvestman's long legs and small body.