however, more recently collected genetic evidence reported by fonseca and others suggests a single "c. molestus" form has spread throughout europe and beyond, since populations over a large area share a common genetic heritage. these widely separated populations are distinguished by very minor genetic differences, which suggest the underground form developed recently; a single mtdna difference is shared among the underground populations of 10 russian cities, and a single fixed microsatellite difference occurs in populations spanning europe, japan, australia, the middle east, and the atlantic islands. this worldwide spread might have occurred after the last glaciations or may be even more recent, due to the insects hitchhiking on world trade routes; one possibility is the international secondhand tire trade. the tires retain water in which the larvae can survive, and completely removing water from an old tire can be difficult.