in the posterior midgut after feeding in response to the blood meal, whereas activity in the anterior midgut is consistent with a nectar-processing role for this midgut region. Ecology Distribution Mosquitoes are cosmopolitan (world-wide): they are in every land region except Antarctica and a few islands with polar or subpolar climates. Iceland is such an island, being essentially free of mosquitoes. The absence of mosquitoes in Iceland and similar regions is probably because of quirks of their climate, which differs in some respects from mainland regions. At the start of the uninterrupted continental winter of Greenland and the northern regions of Eurasia and America, the pupa enters diapause under the ice that covers sufficiently deep water. The imago emerges only after the ice breaks in late spring. In Iceland, the weather is less predictable. In mid-winter it frequently warms up suddenly, causing the ice to break, but then to freeze again after a few days. By that time the mosquitoes will have emerged from their pupae, but the new freeze sets in before they can complete their life cycle. Any anautogenous adult mosquito would need a host to supply a blood meal before it could lay viable eggs; it would need time to mate, mature the eggs and oviposit in suitable wetlands. These requirements would not be realistic in Iceland and in fact the absence of mosquitoes from such subpolar islands is in line with the islands' low insect biodiversity; Iceland has fewer than 1,500 described species of insects, many of them probably accidentally introduced by human agency. In Iceland most ectoparasitic insects live in sheltered conditions or actually on mammals; examples include lice, fleas and bedbugs, in whose living conditions freezing is no concern, and most of which were introduced inadvertently by humans. Some other aquatic Diptera, such as Simuliidae, do survive in Iceland, but their habits and adaptations differ from those of mosquitoes; Simuliidae for example, though they, like mosquitoes, are bloodsuckers, generally inhabit stones under running water that does not readily freeze and which is totally unsuited to mosquitoes; mosquitoes are generally not adapted to running water. Eggs of species of mosquitoes from the temperate zones are more tolerant of cold than the eggs of species indigenous to warmer regions. Many even tolerate subzero temperatures. In addition, adults of some species can survive the winter by taking shelter in suitable microhabitats such as buildings or hollow trees.