Simply put: Not far outside of the Enclave is a forest. Portions of these forests are densely populated with giant spiders. Ecology is a concern of some of the more scholarly of the Enclave. Thus there is a concern as to how this population is sustained. Current theories held consider the orcs provide enough fodder to keep the spider population fairly healthy. There is likely other prey as well out there. The budding University within the enclave may soon be commissioning adventurous souls to find out such details as this.
Spiders in these woods have been reported in various sizes. There are large spiders of web-spinning and roving hunter varieties. The 10' wide (at the legs; body would be about 3' long) spiders will often take 2-3 armed people to bring down. Rumors of larger spiders have been spread. Plenty of fairly decent-sized foraging parties have been lost and never returned. Foraging parties are kindly informed during their training and briefing that a spider 15' or larger should be approached by no less than 5-10 armed and trained individuals.