There are several hundred species of ladybugs (commonly called ladybirds outside the US), which comprise the family Coccinellidae in the order of Coleoptera--that is, beetles. Ladybugs are generally identified by their very VW Beetle like shape and their yellow, red, or orange, spotted elytra. However, colour and patterns vary greatly even among members of the same species, and spotless bugs as well as black bugs with orange or red spots are not uncommon. In Australia, where everything is either upside-down or really weird, they apparently have metallic blue ladybugs that dine on fungi.
Most ladybugs, and this is why we like them, are vicious and voracious carnivores with a predilection for consuming aphids, spider mites, scale bugs, and other widespread pests. Most of them will readily eat plant matter or pollen when hungry but many need a meat diet in order to breed.
In Europe and North America, you're most likely to encounter several of the more widespread species:
Adalia bipunctata, as the name will suggest to the latin-literate among us, is a small member of the family with one red spot on each wing. These are native to the northern hemisphere and very common throughout Europe. These are the fun-sized ones unless you're a blackfly, in which case I suppose that a ladybird of any size is bad news.
Coccinella septempunctata (see the recurring "punctata" theme) is the classic seven-spotted ladybird that illustrators love. This is the most common Eurasian species, which was imported to North America for pest control purposes in the early 1960s and, while not common, is not rare either. C. septempunctata (L.) is itself being displaced by a more vicious intruder both in its native and its adopted range. This one is the ladybug that songs are written about. It is probably the single most sung-to member of the Insecta class.
Coccinella novemnotata has, imagine, nine spots. This bug was widespread in north-eastern North America but, much like the native humans, has been displaced and replaced in its native range first by European, then by Asian invaders.
Hippodamia convergens is the thirteen-spotted beetle native to North America. This one has two triangles of spots at each end of the elytron and a thirteenth where they adjoin the pronotum. The Convergent Lady Beetle, as the name implies, converges and forms clusters. This makes them easy to gather and sell for pest control.
Harmonia axyridis, also known as the Multicolored Asian Lady Beetle, is an East Asian invader (unless you're in eastern Asia) with excellent credentials for aggressiveness and voracity. This beetle is orange-brown to deep red with orange-brown legs and a W-shaped pattern of four black spots on a white pronotum. It has a basic body pattern of nineteen spots, which might be so faint as to display only a subset or even make the beetle appear plain.
This last bug, which our grandchildren may come to identify as the only "ladybug," was unsuccessfully let loose in the US on several occasions for purposes of pest control, starting in 1916. They did not take until they hitched a ride for themselves and appeared out of nowhere, hundreds of miles from major release sites, in Louisiana in the 1980s. Since then they've taken North America by storm and have made some inroads in Europe. They're an extremely fast-breeding and fast-spreading species that eats anything, including ladybug larvae. While their eating habits make them mostly beneficial and welcome, they're on the wine industry's naughty list since they can make do with grape juice after they polish off the pests and crushing them with the grapes makes the wine taste funny, much like happens with millipedes and olive oil. Oddly enough, the funky smell is due to a methoxypyrazine, a chemical directly related to the making of a good Cabernet.
If you see ladybugs on your house in mid to late autumn, this will be H. axyridis looking for shelter for the winter, preferably in your attractive, light-coloured walls. And, if there are cracks and crevices on the outside of the building there will be even more inside. Before you know it, they'll be buzzing around your light and crawling on your window blinds. That is, until the temperature warms up over 50°F/10°C in spring, at which point they vanish for the summer.
In many cultures, the ladybug is associated with spring, good fortune, good weather, and all sorts of beneficence. Rather odd for one of nature's most fearsome little predators. The "lady" in the name is the Virgin Mary of Christian religions, though the ancient Europeans often used similarly divine names. The Russians and Welsh call them names involving cows. Which leads to the question whether Matt Groening knew about it when he came up with the herds of ladybug-like buggalo roaming Mars in several episodes of Futurama. I would not be surprised.
If you go shopping for ladybugs for your garden, your selection is limited. Ladybugs don't breed well in captivity and clustering species are the only ones that are profitable to catch. Most ladybugs sold in North America are H. convergens that were collected in the wild. H. axyridis is not as popular as it used to be since it's come to be seen as a bit of a pest. If you plan on ordering ladybugs for pest control, keep in mind that, unless you have some super tasty plantlife like grape vines, they tend to vanish along with the aphids after polishing them off.
The ladybug is the official insect of seven of the states that feel the need to have one. This list includes Delaware, Massachusetts, New York, Tennessee, New Hampshire (after a tempestuous legislative debate), Ohio, and Pennsylvania. All but New York, which went native with the near-extinct nine-spotted beetle, do not specify the species but use the common name of the whole order.
Ladybugs, though not poisonous, are nasty critters. This is mostly because they react to being handled with reflex bleeding of a smelly, staining, and apparently corrosive substance. I uncovered one report that described 'severe chemical burns' in the oral mucosa of a dog that was stupid enough to eat a bunch of them. The hemolymph of H. axyridis is an irritant and medical literature suggests that the prevalence of allergies to it could be as high as 10%. Mystery allergy sufferers, you have another suspect.
Few ladybugs bite (this seems to be a FAQ), as their mandibles tend to be too small. Unsurprisingly, H. axyridis is an exception to that rule. They do bite spontaneously and humans are on their menu. Regarding this last point, I can offer anecdotal evidence. I recently watched one of the little blighters land on my hand and bite me immediately. It was not content with biting but nibbled on me for over five minutes. It grazed off a millimeter-square patch of epidermis and left enough of a wound to scab over by the next day. It did its damnedest to hang on when I removed it, like some monstrous hybrid of Eric Carle's Very Hungry Caterpillar and Very Grumpy Ladybug. Ladybug bites are like a tiny pinch, and while not entirely painless won't make you flinch.
So, yeah, while they're too pretty and dumb to star in a buggy version of The Birds, THEY EAT PEOPLE! We ought to be running screaming, yet we invite these horrible, man-eating beasts into our garden and sing songs to them. Go figure.