We tend to use the word bug loosely for any very small creature with legs. However, a true bug is defined as belonging to the order Hemiptera. These creatures characteristically have tough forewings and lack teeth, such as beetles. True bugs have a stylet (a mouth shaped like a straw) that they use to suck juices from plants. Insects belong to the class Insecta and they are characterized by three-part bodies, usually two pairs of wings, and three pairs of legs, (e.g., bees and mosquitoes). Arthropods (spiders, ticks, centipedes, etc.) is a separate phylum from bugs and insects. All of this does not mean that you are wrong to call various insects bugs; because of the common usage of this meaning, it is certainly acceptable.
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