Mosquitoes can thrive also in underground environments, and as a result of evolution they developed some interesting biological features. It’s the case of Culex pipiens molestus, often known as the London underground mosquito. This insect became very “popular” among Londoners during the Second World War, when it was feeding voraciously on people seeking shelter from the bombings in the underground railway tunnels. However, despite its name, this insect can be found in the underground systems of cities all around the world. But what is so special about it?
Environmental conditions underground are different from the above, and Cu. p. molestus shows traits tuned with the places it colonizes (which include not only underground railway systems, but also sewers and basements). Everything becomes clearer when we compare Cu. p. molestus to the surface populations of Cu. pipiens (the northern house mosquito commonly found in cities, which is an important vector of West Nile virus). While surface populations suspend their growth and development during the winter months (a phenomenon called diapause), the mosquitoes underground are active all year round. Furthermore, females of molestus can produce eggs without a blood meal (autogeny) and they have a biting preference for mammals (rats, mice, humans) rather than birds (the preferred host of pipiens from the surface).
However, are these differences enough to consider molestus a separate species from pipiens? The short answer is … not yet. Originally described as a different species, across the years molestus has been considered either a subspecies or simply a physiological and ecological variant of pipiens. The latest statement seems to be confirmed by the fact that molestus and pipiens are morphologically indistinguishable, and there has been some evidence of interbreeding between the two. However, considering genetic data recorded over the years and in particular the results from a recent comparative genomics study, it seems that molestus should actually warrant the status of subspecies which may be undergoing incipient speciation from pipiens.
How about natural underground environments? It is well known that caves are used by many mosquitoes as a place to rest and overwinter. Mosquito species living and reproducing exclusively in caves have indeed been described, but very little is know about these true cave-dwelling mosquitoes.
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Harbach, R.E. Culex pipiens: Species Versus Species Complex – Taxonomic History and Perspective. Journal of the American Mosquito Control Association 28(4), 10-23 (2012). https://doi.org/10.2987/8756-971X-28.4.10
Yurchenko, A.A., Masri, R.A., Khrabrova, N.V. et al. Genomic differentiation and intercontinental population structure of mosquito vectors Culex pipiens pipiens and Culex pipiens molestus. Scientific Reports 10, 7504 (2020). https://doi.org/10.1038/s41598-020-63305-z
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