The word ‘bug’ is often used to describe all invertebrates, especially insects. But there is a specific group of insects to which the title of bug is properly applied.
Bugs are in the order hemiptera and all have sucking mouthparts. These include insects such as cicadas, aphids and leafhoppers. The long sucking tube can be seen on the photo above of a dead cicada (it’s the stick-like thing between its legs). Most bugs feed on plants, using their specialised mouthparts to pierce the plants and suck up the sap. However, some feed by sucking the juices out of other insects and a few even feed on humans.
But just to confuse things, not every insect that sucks their food is a bug. Mosquitoes, for instance, suck the blood of vertebrates but they are not bugs. Mosquitoes are actually a type of fly.
As well as having sucking mouthparts, bugs also undergo an incomplete metamorphosis. That means that the young bugs look like bugs when they hatch. As the young bug grows it becomes more and more like the adult form, but it is always identifiable as a bug. See the photos at the bottom of this page for three stages of growth in crusader bugs.
This is in contrast to things like flies that hatch as maggots and pupate to become an adult fly. A young fly looks nothing like the adult form and undergoes a major change while pupating.
While their lifestyle makes bugs sound worthy of extermination, they are all prey items to larger creatures. Birds and spiders eat a lot of bugs. Ladybirds eat aphids. In some areas of the world humans eat cicadas. They are also used in the biological control of other organisms. Looking at the bigger picture, bugs are good things to have around.