[Anonymous]

“Lufenuron, a benzoylurea pesticide, inhibits the production of chitin in insects. Without chitin, a larval flea will never develop a hard outer shell (exoskeleton). With its inner organs exposed to air, the insect dies from dehydration soon after hatching or molting (shedding its old, smaller shell)”.
http://en.wikipedia.org/wiki/Lufenuron

Just look up the ingredients of any flea/tick products or repellents, if you have any doubt about what they are.