While the human inhabitants of Rome busy themselves with the commute to work or school, dodging motorini or tourist crocodiles, downing an espresso at a bar and a dozen other daily rituals, most are oblivious to the fact that the capital is abuzz with another, more secretive population. Living discreetly alongside us in our homes, offices, bars and schools, in our parks and among our fountains, museums and historic monuments even making use of our public transport are a vast number of cosmopolitan residents with six legs and an exoskeleton. In fact, as a new exhibition at La Sapienza Universitys Zoological Museum reveals, we share the city with 5,175 of Italys 37,303 insect species.

The number of species we have in Rome rivals many of our national parks, says Professor Augusto Vigna Taglianti, the museums director, standing among cases of specimens and giant models of insects in the exhibition space of the universitys new seat of entomology on Via Tiburtina. The professor is a beetle man. Entomologists fall into two camps, he explains. There are the farfallari the butterfly lovers and the bacarozzari the beetle lovers. Im a bacarozzaro.

The department mainly owes its collections, however, to a farfallaro: the entomologist and explorer Federico Hartig, who donated his specimens to the state in 1940. The new seat, which replaced the old department in Via Catone earlier this year, now holds a lecture hall, the best-stocked entomological library in Italy and, stored in mechanical moving shelving, around a million specimens from all over Europe. Professor Vigna Taglianti turns a wheel to display row upon row of slim cardboard volumes that open to reveal insects arranged like exotic sweets or jewels with chocolate-box perfection.

The new exhibition, entitled Gli Insetti di Roma, fills two rooms with the story of the citys insect life. Glass cases display scores of tiny black flies and butterflies in delicate beiges. Solemn squadrons of beetles in silver-greens stand facing the same direction like a Roman army preparing to advance, each individual the same but different: a head bowed, an antenna askew, a leg a little out of place.

Romes insect life has inevitably changed in the last hundred years as the city has expanded and habitats have been destroyed to make way for buildings. Insects living in and around water are particularly susceptible to environmental changes, Professor Vigna Taglianti explains, referring to the cementation and degradation of the banks of the citys rivers, the Tiber and the Aniene tributary. Species which rely on forest and meadow habitats have also suffered. Around 16 beetles have disappeared, Professor Vigna Taglianti notes, and another 50 or so are on their way out.

Yet other insects continue to flourish. Dragonflies, beetles and flies are still found in force in marshy areas on the outskirts of the city such as Acquatraversa, Insugherata, Infernaccio and Cecchignola. Right in the centre, meanwhile, Villa Borghese is home to the hermit beetle (Osmoderma eremita), one of the few invertebrates protected by the European Community habitats directive. The hermit, which can spend its entire life cycle (including three or four years as a larva) in the hollow of a single old tree, is also known as the Russian leather beetle, since the male releases a pheromone that smells like leather to attract a mate. Professor Vigna Taglianti also points out a tiny fur-covered beetle smaller than a little finger nail called Elytrodon luigionii. This one is special because its the only insect found in Rome and nowhere else in the world, he says. Another group of insects, such as moths and crustaceans, inhabit the 380 underground cavities carved into the citys tufa, including 30 Christian catacombs and 20 pagan tombs.

Less surprising is the army of so-called synanthropic insects, which rely on proximity with humans to survive. Many of these were originally tropical or subtropical and can only live indoors where we keep temperatures warm. Around five per cent of Romes entomological fauna is represented by this truly urban selection, from those that live with us to those that live on us. The exhibition lists a rather disturbing catalogue from ants and silverfish in our food shops and apartments to the wood-eaters in our buildings and fleas and lice that flourish in schools and hospitals. Synanthropic insects came to public attention late last year when passengers on an Intercity Reggio Calabria-Torino train were so badly menaced by bedbugs in the night that they had to pull the emergency brake and be taken by bus to their destinations from Roma Ostiense station.

As with Romes human population, immigrants from all corners of the world have steadily been joining the mix. Foreign species account for around ten per cent of the citys current insect life. We have insects that have come from North and South America, from Asia, from India and Africa and even from Australia, Professor Vigna Taglianti says. Many of them arrive as eggs on imported wood and then carve out their own slot in the citys ecosystem.

For example, the tiny geranium bronze butterfly (Cacyreus marshalli) originally came from South Africa and is thought to have been introduced into Spain in 1990 in larval state on ornamental geraniums. It arrived in Rome in 1997, where it has been happily feeding off the capitals geraniums ever since. The infamous tiger mosquito (Aedes albopictus) came from south-east Asia in the used tyre trade and was first recorded on Italian soil in Genoa and Padua in 1990. By 2000 it had firmly established itself in Rome, where unlike the citys other mosquito species it bites during the day as well as at night and can breed in tiny quantities of water. The tiger is also faster and nimbler than the natives, making it harder to kill.

If, like most of us, you only start thinking about insects when the familiar whine of the first blood-hungry mosquito disturbs your sleep in May, this exhibition provides an eye-opening introduction to Romes other life. Few people realise, for example, that the annual war between man and mosquito is in fact a feminist issue. It is only the female of the species that bothers us, needing blood to lay her eggs the males are harmless nectar-suckers. Take this into consideration next time you stand with slipper poised.

Gli Insetti di Roma: biodiversit in un ecosistema urbano,

29 March-late autumn. Museo di Zoologia dellUniversit La Sapienza, Sede di Entomologia, Piazzale Valerio Massimo 6. Tues-Fri 09.30-12.30. Free entry. Guided tours (also free) are recommended: tel. 0644702813.