Lovely article+illustrations by Kimono Ichiba (via Tanpopo <3), overviewing famous "scary"patterns... which are in fact often auspicious as traditional Japanese patterns ;)
I believe I have them all on the blog somewhere but it's nice having them in one place so let's go!
In ancient China, spider were seen as auspicious messengers connecting Heaven to Earth.
As the spider catches its prey in its web, spiderweb came to signify "grasping happiness".
Apparently during Edo period, prostitutes and geisha used spiderweb patterned items as a good luck charm (meaning something like "this customer will come back").
A very famous spiderweb depiction is the Lady Rokujo ukiyoe [焔 honô (the flame of passion)] by female artist Uemura Shoen. Wisteria caught in the web could mean ``I hope [Prince Genji] will come tonight'' which is pretty sad considering her story T_T
Exact name for human remains pattern is "nozarashi" (lit. "weather beaten") ie bones scattered in a field. This depicts a corpse turned to bones/unveiled from its grave by the elements.
Skulls are thought to ward off evil and bad luck. Bones can also symbolize a do-or-die spirit, or hope for rebirth after death.
OP stresses a theory linking bones pattern to a buddhist saying 色即是空 shikisokuzekū "form is emptiness, matter is void, all is vanity". An interpetation is that we'll all turn to dust one day so we're all equal.
Bones patterns are often seen during Obon (Festival of the dead) season.
Monsters, ogres and ghosts (yôkai / oni / yûrei)
Monsters patterns were then worn to ward off bad luck and evil spirits. Reasoning is: let's repel scary things by wearing an even scarier monster!
Fearsome monsters were especially use by people with dangerous jobs, like Edo period firemen.
Firemen often had the lining of their heavy fire attire (火事装束 kajishouzoku) embelished with lavish designs of brave heroes and fantastic monsters. It was both a talisman and a way to show that they did not fear danger or death.
Another reason behing monsters patterns is the Edo period love for "scary" entertainements, be it ghost stories, parlor or other types of games, art (see for ex. Utagawa Kuniyoshi), etc. And Edo city dwellers were all about being fashionable so a monster pattern would have been considered quite iki!