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Malaria in the Netherlands

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parts of Guelders, in which over 24,000 larvae were identified *. The result was that the "fresh" breedingplaces (0-0.08 per cent of salt) were found to contain a large majority of longwing larvae, the "salty" breedingplaces (0.25 per cent of salt or more) an even larger majority of shortwing larvae, andthe"brackish" breedingplaces (0.08-0.25 Per cent of salt) were intermediate between the two t:

_ ,. . No. ol larvae per 10 dips

Breedmgplace Shortwing Longwing

fresh, o—0.08 per cent of NaCl 10 35

brackish, 0.08—0.25 per cent of NaCl 58 26

salty, over 0.25 per cent of NaCl 125 8

The two accompanying maps show the same results. One (fig. 14) marks the breedingplaces with a salinity above, or below, 0.16 per cent of salt; the other (fig. 15) whether shortwing or longwing larvae (if at all present) occur in a majority in these breedingplaces. The principal thing these maps bring out is the difference between North- and South-Holland, in respect of the salinity of the water in the breedingplaces and of the composition of the larval fauna. Without regard to details, the difference may be stated as follows: North-Holland is salty, its anopheles are shortwings; South-Holland is fresh, its anopheles are longwings. Taking into account the distribution of malaria, as shown in fig. 4 (p. 25),

* Quart. Buil. Hlth. Set. Lg. o. Nt. Vol. 5, 1936, p. 280-294. Also: G. van der Torren, De geographisehe verspreiding van Anopheles maculipennis atroparvus en messeae. Amsterdam 1935, P.H. Vermeulen, 81 pp.

t To put it more simply: Below a salinity of 0.16 pet. there are 18 shortwing larvae and 39 longwing larvae per 10 dips, a proportion of 1 to 2; at a salinity of 0.16 and over, there are 117 shortwing larvae and 10 longwing larvae per 10 dips, a proportion of 12 to 1.