Only what I read in the wiki. Following some canon lore they are independent creatures who consist of real flesh and bones instead of dough and paste (but hey... it depends on the artist). They are able to impregnate yukkuri so they bear their offspring. However this often leads to the lynching of the yukkuri mother by her own kind. Yurrikus are widely ignored by factories due to the low demand resulting from their strange looks.
"Yurrikus are widely ignored by factories due to the low demand resulting from their strange looks." I like the part where talking, moving disembodied heads that live and die are considered "normal" now.
Though I'll admit, these are kind..wait, no. Really freaky with the backwards talk.
Seeing this again makes smile of the irony of now the yukkuri now knows how it feels to get their house invaded. Not that they'll learn anything from it mind you... Does anyone know a good story(ies) on the Yurrikus?