Here the Red Coried Stink Bug shows its "straw" like proboscis. these creatures are omnivorous, feeding both on nectar and other insects, inserting their mouthparts into their prey. Yikes!
Biologists who name animals and plants are called taxonomists, and they are very particular about what and who gets named who and what. Entomologists, (people who study insects), use taxonomy - like “bug” to refer to a particular type of insect so they can keep them categorised.
A Planthopper, like this one on the right (or the stink bugs above), suck, and that is the key difference between true bugs and other insects: yes, its because of their mouth parts. Most bugs suck fluids from plants, but there are some true bugs, like bed bugs, that feed on animals. The sucking mouth part which looks like a straw is called a proboscis, and it is different than that of other insects.
Of the millions of insect species on earth, about 80,000 are true bugs, which can live almost anywhere in the world and can be found on land or in the water, and can vary tremendously in size.
Other insects - like a bee or butterfly - have a beak-like proboscis, that is retractable, and they can roll it up. The proboscis of a true bug is not retractable. Insects with movable mouthparts allow them to move food from the source to their mouth. The proboscis of a true bug is more rigid and cannot be rolled up.
Here below you can see the retractable tongue of a butterfly hard at work, moving nectar to his mouth. So this is an insect, but not a bug!
So, back to our true bugs: these include insects such as leafhoppers, aphids, cicadas, stink bugs, water bugs and yes those nasty bed bugs. They have many of the same parts as other insects in that they have an exoskeleton, segmented bodies, and 6 legs. All true bugs go through what is called incomplete metamorphosis; which means they hatch as nymphs from their egg, and as we have already found out, a nymph is a miniature version of the adult bug.
True bugs are listed within the order called Hemiptera. Insects in this order are different from other insect orders, such as Hymenoptera (ants and bees), Lepidoptera (butterflies and moths), or Diptera (flies and mosquitoes). Note that these terms and classifications continues to be updated and debated by entomologists.