Reputation: Yuck. Cockroaches are filthy, immortal scavengers that are unaffected by radiation. In a post-apocalyptic world, it will be these dirty little critters that survive. We would be better off without them.

Reality: There are almost 5,000 species of cockroaches, of which only around 30 have any pest-like tendencies. These few malign a group of insects that boasts an astonishing, enriching diversity of forms. Cockroaches are pretty well toasted by radiation.

The sight of a cockroach scuttling across the kitchen floor is distinctly unsavoury. This emotional truth has led most of us to believe that all cockroaches must be similarly repugnant.

But not George Beccaloni, curator of orthopteroid insects at the Natural History Museum in London, UK. He is on the side of the cockroaches. "People have a very biased view of the group," he says.

Cockroaches are found on all continents apart from Antarctica, from rainforests to deserts

Less than 1% of the 4,800 known species of cockroach cause humans any bother, yet few of us give the 99% a second thought. This is manifestly unfair, says Beccaloni.

He points out that there are about as many cockroach species as there are mammals. So writing off all cockroaches, based on our dislike of 30 or so species, "is like encountering a mouse or a rat and then branding all mammals as disgusting vermin," he says.

I take the point, but remain unconvinced. Mammals are spectacularly diverse. I think back to a childhood holiday in Sardinia, where our flat was plagued by cockroaches. How different can the rest of cockroaches be? Beccaloni takes the next half hour to enlighten me.

For a start, they live in a huge range of habitats.

"Cockroaches are found on all continents apart from Antarctica, from rainforests to deserts," says Beccaloni.

The appealingly-named Megaloblatta blaberoides boasts a whopping wingspan of over 7in (18cm)

The abundance of species is greatest in the tropics, and at low altitudes where temperatures are high. However, there are those can cope with extremes. Eupolyphaga everestiana is a montane specialist that lives on Mount Everest at well over 16,400ft (5,000m) above sea level.

Since they live in so many diverse environments, it is not surprising that cockroaches should come in many different shapes and sizes.

The smallest species on record is the ant cockroach, which lives in the nests of leaf-cutter ants in North America. At just a few millimetres long, it is dwarfed by its hosts.

In contrast, the appealingly-named Megaloblatta blaberoides boasts a whopping wingspan of over 7in (18cm).

Larger still is the giant burrowing cockroach from Queensland, Australia. It is wingless, about 3in (8cm) long, and can weigh over 1oz (30g).

Cockroaches of the Perisphaerus genus can roll up into an armadillo-like defensive ball

This cockroach, which would easily occupy most of your palm, might sound alarming. But it could not care less about humans. "The huge rhinoceros cockroach only feeds on bark and dead leaves," says Beccaloni.

Most cockroaches have taken on similar ecological roles, feeding on decaying organic matter and thus making nutrients available to other organisms. "There are indications that the ecological significance [of cockroaches] is massive," says Beccaloni.

For many species, even some humans, cockroaches are also a sought-after snack. This helps explain why many in the group have evolved nifty ways to avoid being eaten.

The banana cockroaches (Panchlora) have opted for a simple camouflage approach: they are green and this helps them to blend in.

Cockroaches of the Perisphaerus genus can roll up into an armadillo-like defensive ball.

The females churn out perfect clones of themselves without any need for males or copulation

The Prosoplecta species have evolved the same distasteful red-and-black colouration as ladybirds. In order to achieve the rounded shape of a ladybird, Beccaloni says, each of their hind wings rolls up at the ends "like an umbrella around itself".

Some species can fire out a defensive spray, like the Pacific beetle cockroach.

Others, like the Madagascan hissing cockroach, make startling noises when disturbed, presumably to unsettle any would-be predators.

Perhaps inevitably, cockroaches have also come up with a plethora of ways to make more cockroaches.

"Cockroaches as a group are one of the most if not the most varied of all insect groups, in terms of their reproductive biology," says Beccaloni.

A few species appear to be wholly parthogenetic. The females churn out perfect clones of themselves without any need for males or copulation. In others, the females can flip between sexual and asexual modes of reproduction depending on the conditions.

The nymphs have razor-sharp mandibles, which they use to slice into her cuticle and feed on her blood

However, in most species the female produces an egg sac. Some simply lay it and move on, but others incubate the egg case in a brood pouch in their body, effectively giving birth to live young.

Pacific beetle cockroaches have abandoned egg cases altogether. The female deposits eggs directly into her brood pouch. There she nurtures them on a milk-like secretion – "the most nutritious energy-rich protein that's yet been discovered," according to Beccaloni – before giving birth to live, well-developed young.

If this sounds familiar, it should. "It's a very similar situation to the placenta of a mammal," says Beccaloni.

In a few cases, the female even cares for her offspring after birth.

For instance, a Thorax porcellana mother carries her babies huddled beneath her forewing. It sounds almost cute, until you learn that the nymphs have razor-sharp mandibles, which they use to slice into her cuticle and feed on her blood. "They are like little vampires," says Beccaloni.

With so many extraordinary adaptations, it would not come as much of a surprise to find that cockroaches really could survive a nuclear blast. But tolerance of radiation is one talent they lack.

There are about as many cockroach species as there are mammals

"It's mostly an urban myth," says Beccaloni.

A human will usually be killed outright by a dose of 10 Grays. "Cockroaches are only about five times more resistant," says Beccaloni.

At first glance that might sound impressive, but it actually means they "are at the lower end of radiation tolerance for insects," he says. Other species can survive doses of radiation ten times as intense, or even higher.

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