caterpillar damage or Sawfly larvae damage
This is either caterpillar damage or Sawfly larvae damage, which look like small caterpillars. If you have any of these insects left on your courgettes I would advise spraying with the pyrethrum based product; Insect Killer spray [ GPC-027]. This is a natural poison that is safe to use on edible plants and crops. It breaks down in the sunlight and fruit can be eaten 24 hours after use. It is also certified organic. It kills insects on contact, so you have to get good coverage of the pest. You may need to repeat, so I suggest spraying every 7 days while you have pests. It can be used on a wide variety of pests so is a very useful product to have.
I hope this helps.