(1) The object of the investigation was to ascertain the condition of the Second Army area of Southern Flanders in regard to Anopheline mosquitoes.

(2) The data of this report have been derived from three sources (1) from the writer's surveys during the summers of 1915 and 1916; (2) from the writer's survey of September 1918; (3) from various collectors the bulk of whose specimens were collected during September 1918.

(3) Circumstances determined that the method of investigation should deal almost exclusively with breeding waters. Except on a very small scale no attempt could be made to rear larvae and pupae or capture adults.

(4) The distribution of a circular and pro-forma resulted in a certain amount of information being obtained from medical officers and officers commanding sanitary sections. Valuable assistance was also rendered by certain non-commissioned officers and men.

(5) The surveyed area of the Second Army zone bounded, roughly, by imaginary lines joining the places Millam, Woesten, Ypres, Kemmel, Armentiéres, Arques and Moulle was about 350 square miles; most of its districts were touched upon, their areas totaling about 120 square miles; eight main districts and 12 isolated districts, representative of the country, were surveyed with varying thoroughness and skill; the main districts were the Advanced Zone, Poperinghe, Hazebrouck, Southern, Rubrouck, Central, Waterlands and Armentiéres.

(6) Out of 40 adult Anophelines, caught wild or reared from larvae and pupae, all proved maculipennis.

(7) Twenty-three A. maculipennis, females, were caught in one afternoon in a dark, damp medical aid-post.

(8) No seasonal preponderance of any one size of larvae was observed during September 1918.

(9) Out of about 1233 records of sites inspected, 178 refer to Anopheline haunts, 5 referring to captures of adults.

(10) Of the 173 Anopheline waters discovered 127 (73·4 %) were pools or lakes, 16 (9·25 %) were holes (shell-holes or pits), 12 (6·9 %) were running waters (9 streams, 3 canals), 8 (4·6 %) were ditches, 6 (3·5 %) were marsh, 2 (1·2 %) were receptacles (1 a tin and 1 a concrete basin); 2 records give no details.

(11) From 106 records the number of waters with few Anopheline larvae (up to about 10) was 67 (63·2 %), with numerous larvae (3 and more per dip) 22 (20·7 %); 17 (16·1 %) records are doubtful. In 2 cases the number of larvae was many thousands.

(12) On 9 occasions Anopheline larvae were found in polluted water.

(13) Anopheline larvae were found almost certainly in all waters where the visible vegetation was grass or algae or predominantly one of these. Association with water-cress was not infrequent. Ponds entirely covered with a dense mat of duckweed never gave Anophehne larvae but, at times, such pools may have open patches among marginal vegetation and in these patches the larvae have been taken. They have also been found in water which showed no visible foodstuffs.

(14) Anopheline larvae were found alone in 65 % of cases, and co-existing with Culicine larvae in 24 %, with water hog-lice (Asellus) in 6·5 % and with fresh-water shrimps (Gammarus) in 1·2 % of cases; they may also co-exist with any two or all three of these, and with fresh-water fish of many kinds.

(15) Water-boatmen (Notonecta) have been observed in an aquarium to prey upon and suck the juices of Anopheline larvae.

(16) All waters are suspect.

(17) It is probable that further summer work would show the number of Anopheline waters and the degree of infestation of the districts to be higher than the results here cited.

(18) Anophelines have been found generally distributed all over the area and in close proximity to billets. In 4 districts out of 20 examined, none was found, but more careful search would probably reveal them.

(19) The results obtained in the Hazebrouck district are taken as representative of the approximate degree of Anopheline infestation to which the Second Army area attained. The findings are (1) 14 % (1 in 7) of waters examined were Anopheline; (2) there were 5 Anopheline waters per square mile; (3) 25 % of Anopheline waters contained numerous larvae and 56 % contained few; of the remainder there is insufficient record.

(20) Generally speaking, and as far as information of the conditions obtaining in other regions in the temperate zone permits comparison, the degree of Anopheline infestation of the whole area appears to be low; compared with sub-tropical regions formerly war zones—Italy, Macedonia, and Palestine—it is very low. Two districts, Moulle and Arques, showed waters highly infested, both being in the neighbourhood of hospitals. The ditches of the Waterlands district showed a low degree of infestation.

(21) The nature and character of the Anopheline waters render them amenable to treatment by dragging of weeds and oiling and, military exigencies permitting, sufficient preventive measures could be instituted to bring Anophelines under control.

(22) Notwithstanding that the area has been denśely populated with a great variety of malarial troops and that a few cases of “primary” malaria have occurred, an epidemic of malaria among troops or civilians is unlikely.

(23) In instituting preventive work under war-conditions cognizance should be taken of conditions in which Anopheline waters are the sole watering places for military and civilian needs; also, sites for hospitals and large permanent camps likely to house malarial subjects should be carefully chosen and kept free from Anophelines.