Parallel colour form distributions in European and North American populations of the spittlebug Philaenus spumarius (L.).

Over most of Europe, with the exception of Lapland and the periphery of the Gulf of Bothnia, the combined frequency of the dark (melanic) Philaenus spumarius dorsal colour forms other than marginellus increases toward the north and cooler temperatures. Cooler sites tend to show higher frequencies of these dark forms independently of latitude. The other dark colour form (marginellus) increases in frequency toward the south. This distribution closely parallels patterns observed in North America, but in Europe the dark forms generally occur at higher frequency. In both North America and the Soviet Union, dark abdominal pleurites increase in frequency towards the north and cooler temperatures. These distribution patterns, common to two large and widely separated areas, strongly suggest that natural selection influences colour form frequencies and provide indirect evidence that thermal melanism plays a role in P. spumarius colour polymorphism.