A hoverfly species from the Burren uses a chemical disguise to trick its way into ant nests. Emma Napper reports.
Hidden under the rocks of the Burren, some of Ireland's rarest bugs are getting away with murder. The hoverfly, Microdon mutabilis, has developed a most unusual way of feeding its offspring.
The fly lays its eggs near the nests of large black ants, Formica lemani. When the eggs hatch, the fly maggots move inside the ant nest where they live by eating the ants' larvae.
The ants, armed with large jaws and acidic spray, would seem to be very able to dispose of their unwanted guests, and yet the maggots are tolerated. The key to this mystery might lie in a form of "chemical disguise". Every ant in a colony is covered in a distinctive combination of chemicals that act like a fingerprint. They use this chemical fingerprint or "nest odour" to identify members of their colony and to recognise interlopers.
"Ants are usually aggressive to any intruder that doesn't have the right nest odour," explains Dr Karsten Schonrögge, senior scientific officer at the Centre for Ecology and Hydrology (CEH) in Dorset, England. It is possible then that the eggs or maggots of the hoverfly are chemically disguised to "smell" like ants so that the ants can't recognise them as foreign to the nest.
Schonrögge leads a research group trying to understand the ants' life-threatening tolerance of the hoverfly larvae - and insects from the Burren are playing a key role in this work. The population of M mutabilis in Co Clare, which is the largest in Europe, helped the researchers to discover that there was not one but two separate species of Microdon.
This work, published in Biological Journal of the Linnean Society (March 2002) showed that Microdon mutabilis is actually two "cryptic" species. "M mutabilis and the new Microdon myrmicae are cryptic as they look very similar," Schonrögge says. However, the two can still be identified because M myrmicae only attack a species of red ant, Myrmica scabrinodis.
Finding this new species could potentially be very good news for the hoverflies.
In the UK, M mutabilis is classified as a UK Red Data Book species, which means it is very vulnerable to any changes in its habitat. In Ireland this hoverfly is not yet classified as in danger.
Now, instead of one quite rare species we have two, which are even rarer.
Schonrögge and his group hope that understanding more about the changes in the populations of the hoverfly and the environment it needs to survive, could ensure that "no changes occur in the environment that might endanger the populations". Ecologists may also be able to "manage sites to encourage the Microdon," and so protect it, he added.
The research group now wants to find out more about the possible chemical disguises used by the Microdon species. This may be important as, although the host ants are widespread throughout Ireland and the UK, the hoverfly is found only in a few small patches. Previous research showed that hoverflies survived better in nests near to where they were born and survived poorly in nests that were several kilometres away.
The group at CEH-Dorset, along with a group at UK government centre, Rothamsted Research in Hertfordshire, now wants to test the theory that populations of the hoverfly may copy the chemical profile not only of a certain species of ant but of certain ant nests within an area.
A single species of ant may contain "nests that are susceptible and nests that are resistant", says Schonrögge. There might also be a difference in any chemical disguises between M mymicae and M mutabilis, as they invade different host ants.
This is not the first time ants have been fooled by sneaky invaders. Chemical disguise has already been found in a few species of butterflies, which have caterpillars that live in ant nests and eat the ants' larvae.
The caterpillars are covered in chemicals that are very similar to those found on the ant larvae and so the ants do not recognise them as an enemy.
Finally, spare a thought for the ants. Schonrögge is keen to point out that any new information we get about ants and their relationships to other animals is very important. "Around 20,000 species worldwide have close relationships with ants," he says.
As well as supporting this huge number of often rare and beautiful flies, butterflies and beetles, ants are also important in almost every environment on earth.
Ants have a large influence on the ecosystems in which they live especially in their role as seed dispersers, says Schonrögge, and it is vital to understand their role in unique and stunning environments like the Burren.
Emma Napper is a research scientist working at CEH-Dorset and Rothamsted Research in Hertfordshire and Southampton University. She is on placement as a British Association for the Advancement of Science Media Fellow