Note on the Prevalence of Anopheles

Extract

In a previous paper a record of the distribution of Anopheles in Great Britain was given, the localities mentioned including Cambridge and the surrounding country. Since 1901, Anopheles imagines have been searched for in cellars and houses during the winter, in places where they were known to congregate. During the summer a yearly search for larvae was made in various collections of water in the vicinity of Cambridge with a view to securing material for purposes of study and instruction. It appears worthy of note that there has been a notable decrease in the number of imagines caught in cellars, etc. in the winter months. The insects were fairly numerous in my own house during 1901–1902, as many as 60–100 being found hibernating in the cellar. In 1903–1904 there were fewer, only about 6 being caught. Not a single imagine has been caught in the house since that winter. Whereas Anopheles were frequently caught in houses during 1901–1903 in the warmer months, very few were encountered in 1904, and I have not found a single specimen this year. The common Anopheles in this region is A. maculipennis, as I have already recorded, A. bifurcatus being less frequently encountered. Apparently corresponding results have to be recorded with regard to Anopheles' larvae. These were certainly very numerous during the period 1900–1903, and they have become scarcer since, being apparently quite absent in certain waters where they formerly abounded. That the Anopheles are not extinguished is certain, for I have found a few larvae (A. maculipennis) in the river Cam this summer, and my friend Mr Pearce has brought me an imago from Grantchester (close to Cambridge), in addition to larvae of A. bifurcatus which must have hibernated there. Hibernating larvae of this species have twice been found by me since I first recorded their presence about Cambridge.