Spider supports (or at least allows) INSERT DELAYED but the documentation says the following:
INSERT DELAYED works only with MyISAM, MEMORY, ARCHIVE, and BLACKHOLE tables. If you execute INSERT DELAYED with another storage engine, you will get an error like this: ERROR 1616 (HY000): DELAYED option not supported for table 'tab_name'
Moreover, a Spider table can be considered to be transactional at least when it is backed by a transactional table (e.g., InnoDB table). It doesn't make sense for a transactional storage engine to support INSERT DELAYED.
Based on the two facts above, we conclude that allowing INSERT DELAYED on Spider tables is a bug. Disallow INSERT DELAYED. More precisely, remove HA_CAN_INSERT_DELAYED from Spider's table flag and add a test to make sure that INSERT DELAYED is correctly disallowed. Also, remove the code for handling INSERT DELAYED.