We often use the word bug to mean any small creature with legs. That may include insects as well as many other animals that are not insects, such as spiders and centipedes.
A true bug is a type of insect that has a mouth shaped like a straw. Most true bugs, such as spittlebugs and aphids, use this tough mouthpart to pierce plants and drink their sap or other juices. Some true bugs, such as water striders and some shield bugs, use their piercing mouthparts to eat a variety of insects, spiders, and other small animals.
So all true bugs are insects, but not all insects are true bugs—just as all ants are insects, but not all insects are ants.
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