I have no idea! I know that many higly-coloured creatures are poisionous, and they use their colour to deter predators (birds, in the case of the ladybird). Spots might help them in this respect. Spots in insects are also often used to attract mates, but so it might also help different species of ladybird identify each other.
All ladybirds are part of the same family (the Coccinellidae family) but within that family are a number of species – here is a guide to the 23 most common species found in the UK. Within each of those 23 species there will be quite a bit of variation in size, colour and pattern.
As you’ve noticed, ladybirds come in many different colours (including red, orange, yellow, black and cream) and have different numbers of patterns and spots. Apparently the 13 spot ladybird is very rare so well done if you manage to spot one of those! As well as spots ladybirds can have stripes or patterns that look like a human eye or Egyptian hieroglyphs.
But why are they red and spotty? Well it can’t be for camouflage because we always see them on green leaves. Their colours are more likely to be a warning to predators (animals who want to eat them) that the ladybird tastes bad or could even be poisonous. In fact no species of ladybird is poisonous (not even the creepy looking orange ones) but they can excrete (basically squeeze out) some of their own blood to create a bad smell when they’re threatened. Seriously.
Ladybirds have been in the news recently because some of our native British species are being squeezed out by a new species called the harlequin ladybird. These were introduced to Britain deliberately to help control pests as all ladybirds eat aphids, little bugs that like to eat the plants in our gardens. However, harlequin ladybirds have been also eating the baby ladybirds (known as larva) of native species as well as the eggs of butterflies and a number of insects. Click here for a horrible picture of a harlequin eating a larva – it is gross so don’t click if you don’t like that kind of thing:
Harlequins are bigger and rounder than most British species but the two main clues to spotting them are:
1. Look at their legs – if they are brown or reddish it is probably a harlequin
2. Look at their head – harlequins have a white triangle or ‘m’ shaped mark on their head.
If you find a harlequin don’t kill it – it’s not their fault that people brought them here! If you’re into ladybirds you can help track the spread of the ‘killer’ harlequins by helping with this ladybird survey: http://www.harlequin-survey.org/default.htm