The class Insecta comprises more than half of all known species. Not just animal species. Species. Period. That is a fuckton of bugs right there.
When we say 90% of all living things in the entire world are insects, that other 20% is counting all known plants, microorganisms and insect relatives like arachnids and crustaceans. Insects, and JUST insects, may theoretically represent 90 out of 100 living species that we know of. You and your dog and your goldfish and your rubber tree are bullshit compared to insects. You're a blip. A speck. A nothing. You exist only to feed some of the insects. Get used to it.
Now, despite their mind-blowing diversity, insects generally follow the same basic body plan: the head has a set of antennae and a mouth, the thorax has six pairs of legs and frequently two pairs of wings, and the abdomen does all the pooping and fucking. Inside is a brain, a heart, a digestive system and a bunch of weird gooey bug blood called hemolymph. Insects usually breathe through pores along the sides of the abdomen called spiracles, and may have taste buds on the bottoms of their feet, which is why they like to do a little dance routine on your potato salad.
Of course, if there's one thing evolution loves throwing, it's curveballs. Some insects have no wings, some have no eyes, some have lost their legs, some use their antennae as claws...you just never know how they might bastardize their already-alien anatomy.
As if that wasn't enough, some insects see fit to change shape a couple times. While some insect groups undergo "incomplete metamorphosis" and hatch as little baby versions of the adults, other groups undergo "complete metamorphosis," beginning as worm-like larvae geared completely towards eating, undergoing an immobile pupal stage for a while and emerging from the pupa as the adult, geared more towards reproduction. In many species, the larva may represent the majority of the insect's lifespan, with the adults living only just long enough to have sex and lay eggs.
There are many, many different orders of insect. We don't want to put you to sleep here, so we're just going to run down a few of the most major players:
So remember what we said about almost everything on Earth being an insect? Well, at some point during creation God just sort of said "fuck it, I am ALL about beetles now" and never looked back. 40% of insects are beetles. There are fucking trillions of beetles in this world, in every shape, size and color you can imagine, though a few things are usually consistent with these guys.
The surest sign of beetledom are the "elytra;" these are actually the front pair of wings, but evolution has hardened them into a pair of rad flip-up doors to protect the rear pair of fold-out "flight wings." This gives many beetles a sleek, domed appearance. Once again though, there are always exceptions; some degenerate beetles have lost the ability to fly and may have elytra completely fused together or no wings at all...but if it doesn't look particularly like any other sort of insect, you can still assume it's a beetle and be right nearly half the time.
Coleoptera are also one of the complete metamorphers; ALL baby beetles are larvae, appropriately known as "grubs."
"Bugs? You mean all of them?" you ask from the shadows of your sloping brow. No, our poor ignorant reader, the word "bug" is actually supposed to have a meaning as specific as beetle, fly or bee; bugs are insects of the ancient order Hemiptera, sort of the go-bots to Beetle's transformers.
Bugs can look a lot like beetles at first glance, though they only have a half-ass imitation of elytra; the front wings are always leathery and membranous, and the tips fold overtop one another when closed. They're also much older than beetles, a distant second in diversity (roughly 80,000 bugs to 350,000 beetles) and only undergo an incomplete metamorphosis, with babies resembling wingless adults,
Some bugs, however, look absolutely nothing like beetles; they might be fly-like, louse-like or cockroach-like. They really are a sort of all-purpose insect, so the overuse of their name might be a little justified. Stinkbugs, bedbugs, waterbugs, aphids and cicadas are all Hemiptera, so what do these weirdos even have in common?!
The bug's most obvious claim to fame is its neat little switchblade beak. While beetles and most other insects have complex mouths with gnashing mandibles and multiple gripping appendages, bugs have fused all those bells and whistles into a single, sharp spike. Herbivorous bugs use this to suck the juices from plant life. Predatory bugs (like this wheel bug) inject prey with a digestive enzyme and slurp out the guts. Many non-bugs are also liquivores, but tend to have more complex contraptions. The sucking mouth of a mosquito, for instance, consists of several paired drilling appendages.
Ants, bees and wasps all belong to the order "Hymenoptera" ("Membranous Wing") and include the only insects with a venomous stinger as well as the only insects besides termites with a "eusocial" lifestyle, the famous queen/hive arrangement. This doesn't necessarily mean that they're ALL a bunch of swarming, stinging assholes, of course.
Famous for pollinating flowers and making delicious honey out of their spit, only a portion of bee species are actually colonial; many live the solitary lives of more typical insects, and a majority of species don't even have stingers at all. Bees, colonial or otherwise, are of incredible importance to life on Earth as nature's leading experts on pollination. Most modern plant life produces flowers specifically to communicate with bees, enticing them with nectar only to coat them in pollen, the plant equivalent of spunk. By mixing up pollen from one flower to the next, bees allow plants to have long-distance sex and produce seeds. Bees are just remote-control plant dicks.
A colony of bees forming an eskimo, a common defensive posture
Commonly viewed as the rat-bastards of the insect world, Wasps are almost as widely feared as spiders. Besides some of their incredibly nasty stings, the wasp group also includes the majority of Earth's "parasitoid" animals; creatures whose larvae live as parasites before slaughtering and eating their host from the inside-out, directly inspiring the "ALIEN" films. Once again though, most of them are harmless to humans, either unable or uninterested in stinging us and serving a pretty big role in the world as nature's bug police (see: the 5 most hated creatures on the planet don't deserve it)
While beetles have them vastly outnumbered for species, ants may very well be the world's truly dominant animals; their sheer numbers surpass almost anything and everything else, every single species lives in a highly organized and sophisticated colony, and even predators like spiders and mantises will generally avoid them; if you fuck with one ant, you fuck with a thousand, and other insects are so aware of this that many bugs defend themselves simply by looking ant-like.
This is actually an ant-shaped spider. We wish we were kidding.
Your basic ant society begins with a mating flight; a winged male and a young, winged queen. After just a single fit of passion, the male drops dead and the female - knocked up for life - drops her wings and settles down to start a new colony. The eggs she continually pops out will hatch into barely-mobile, maggotlike larvae and eventually develop into either all-purpose worker ants or big, badass soldier ants. All of these little cogs are female and the queen's own pheromones keep them sterile. Combined, the ants function more like a single "superorganism" with thousands of bodies, all working in harmony to maintain the colony, collect food and fend off attackers.
...or build bigger ants.
Like any insect, this basic setup is built upon by all sorts of crazy adaptation, perhaps moreso than any other single group of insect. There's almost no end to their eerily human behavior; there are species that grow crops of fungus and species that "milk" herds of plant-lice. There are ants that enslave and brainwash ants from other colonies. There are parasitic ants that invade another colony, castrate their queen and lay their own eggs to slowly usurp the whole little civilization. The aptly named "Dracula" ants live off their own helpless larva, fattening them up on prechewed food just so they can suck their blood, allow them to heal and do it all over again. Alright, so maybe that one isn't too human. We certainly fucking hope it isn't, anyway.
The insects who seem to offend the fewest people (though this doesn't seem to be saying much), the Lepidoptera include the butterflies, moths and some weird third thing called a skipper, which differs from the other two in little technical ways you probably don't care about. This group is the single most famous example of insect metamorphosis; everybody knows that caterpillars turn into butterflies. It's the subject of the single most-read children's book of all time.
Now, most people probably think of butterflies and moths as "counterparts"...butterflies are the colorful, flapping daytime bugs and moths are the fuzzy, crawly nighttime version. In reality, of the 180,000 some species of Lepidoptera, at least 120,000 are moths, including both day-time and night-time moths. Butterflies are just some hippie weirdos who evolved in more recent eons.
Lepidoptera life cycles are usually completed in under a year, with most of their time spent in the caterpillar stage. The caterpillar exists solely to stockpile nutrients, eating and crapping without interruption for as long as food is in reach (see also: your mom). All this energy storage allows the caterpillar to survive for several months in a pupal cocoon or chrysalis, breaking down into goop that eventually re-organizes into a winged adult. While the larvae are eating machines, the grown-ups are, of course, fucking machines. In some species of moth, the adult doesn't even have a mouth or digestive tract; it rides those last scraps of energy for a few weeks of wild sex shortly before death.