Head-scratchers (7)
(Other definitions for enigmas that I've seen before include "'Puzzles, riddles (7)'" , "Puzzles - mysteries" , "They're mysterious or difficult to understand" , "Problems" , "'Mysteries, riddles (7)'" .)
(Another definition for toughies that I've seen is " Hard puzzles".)
(Other definitions for lice that I've seen before include "Wingless bloodsucking parasites" , "Crawlers" , "Bloodsucking insects" , "Bloodsucking parasitic insect" , "Parasitic insects" .)