Reactions to Temperature1. The reactions of Culex fatigans to temperature were studied by means of a new type of temperature gradient apparatus based on the same principle as the humidity alternative chamber.2. Females at different stages were exposed to a range of 5°C. at different parts of the temperature scale, and it was found that the sensitivity was very much greater at high temperatures than at low ones.3. The most striking feature of behaviour at all stages was the strong avoidance of high temperatures. This was strongest in the hungry females, less strong in the blood-fed females and those with mature ovaries, and least strong in the newly emerged females.4. Newly emerged females showed avoidance of high temperature below 30°C., but not below 25°C. They also showed a weak avoidance of low temperature. At 29°C. they were sensitive to a difference of 1°C. or a gradient of 0·05°C. per cm.5. Hungry females showed a strong avoidance of high temperature below 25°C., the reaction still taking place below 15°C. There was no avoidance of low temperatures.6. Blood-fed females and those with mature ovaries showed a strong avoidance of high temperatures below 25°C., but below 20°C. they were unaffected by temperature differences. Blood-fed females were sensitive to a difference of 1°C. or a gradient of 0·05°C. per cm. at 23°C.7. Except in the case of newly emerged females there was quite a close relation between the reactions to temperature and the effects of temperature.8. Reasons are given for regarding the temperature reactions of Culex as of first importance in determining the behaviour of the mosquitoes when seeking a resting place.Reactions to Humidity9. The reactions of Culex fatigans to humidity were studied by means of the alternative chamber, in a dark constant temperature room at 25°C.10. At all stages the strongest humidity reaction was an avoidance of high humidities above 95 per cent. R.H.11. This reaction was strongest in the blood-fed females and those with mature ovaries, less strong in the newly emerged females, and weakest in the hungry females.12. The avoidance of high humidity was strongest when there was a difference of 20 per cent. R.H., such as a 78–98 per cent. R.H. gradient. Near saturation point all stages except hungry females were sensitive to a difference of 1 per cent. R.H. or a gradient of ·05 per cent. R.H. per cm. Hungry females at this point were not sensitive to a difference of less than 3 per cent. R.H.13. All stages showed a slight but regular avoidance of low humidities, provided a sufficiently large humidity range, not less than 40 per cent. R.H., was present.14. Hungry females, despite the onset of mortality due to desiccation, showed no sharp avoidance of low humidities which were rapidly fatal to them.15. Between 30 and 85 per cent. R.H. all stages were unaffected by humidity differences of as much as 40 per cent. R.H.16. The avoidance of high humidities was equally strong in blood-fed females at 20, 25, and 30°C. At 35°C., greatly increased activity eliminated the reaction.17. The reaction was much less intense in daylight and disappeared altogether after sundown.18. Reasons are given for considering that the measure of humidity which determines the behaviour of the mosquito is relative humidity and not saturation deficiency, even though the latter is the important one in the water relations of insects.

Aims: To characterize bacteria associated with turbot larvae feeding on Artemia and identify pathogens causing mortalities in larvae. Methods and Results: To identify bacteria associated with mortalities in larval turbot rearing, bacteria were isolated from homogenates of Artemia or from several batches of well-performing or poorly performing turbot larvae. Samples were plated onto marine agar and were characterized using biochemical tests and BIOLOG GN plates. Total culturable aerobic bacteria ranged from 1AE9 · 10 5 to 1AE8 · 10 6 CFU per larva and >96% of bacteria identified were vibrios. Almost all bacteria were haemolytic and clustered into two phenons represented by Vibrio alginolyticus and Vibrio splendidus. The bacterial flora of Artemia was almost entirely V. alginolyticus, whereas V. splendidus biotype 1 dominated the larval turbot gut flora (69/115 isolates in seven experiments) and formed four different groups based on BIOLOG GN reactions. Of 16 isolates tested for virulence towards turbot larvae, four of the 11 V. splendidus biotype 1 isolates were lethal and all belonged to the same group of V. splendidus biotype 1 isolates. Conclusions: In a commercial turbot hatchery, the microbial flora of the larval gut was dominated by V. splendidus biotype 1. Four of the 11 V. splendidus biotype 1 isolates caused mortalities in larval turbot and all belonged to one group of the biotype 1 strains identified. Significance and Impact of the Study: Identification of four isolates of V. splendidus that are pathogenic for turbot larvae from three separate batches of larval turbot will allow these to be compared with avirulent isolates to define how V. splendidus causes mortalities in larval turbot.

The brackish water form of A. gambiae on the East African coast—and probably in Mauritius—is not the same as A. melas of West Africa.In salt-water gambiae a variable proportion of the females have an additional dark band on the palps, resembling 4-banded melas, but the remainder are indistinguishable from typical gambiae.Eggs and larvae of salt-water gambiae show no morphological differences from those of fresh-water gambiae, thereby differing from A. melas of West Africa.Larvae of the two forms show a clear-cut difference in reaction to sudden changes in salinity, and a simple test has been worked out whereby wild-caught females can be accurately identified by the reactions of their progeny.This physiological test has formed the basis of all work in comparing the incidence, habits, and infectivity of salt and fresh-water gambiae in Dar-es-Salaam.Exposed to equal chances of infection in the same village during 1947 and 1948, fresh-water gambiae had a sporozoite rate of 9·4 per cent. while that of salt-water gambiae was 0·8 per cent.About 4 per cent, of both forms were infected with filaria larvae, but monthly figures showed that infection rates in salt-water gambiae may rise to 22 per cent.Fresh-water gambiae show little tendency to leave African houses at dawn after feeding, whereas in salt-water gambiae over one-third of freshly blood-fed females leave the house at dawn.In fresh-water gambiae many half-gravid females leave the shelter of the house at dusk on the night after the blood feed. There is no marked difference in infectivity between those which leave the hut and those which remain indoors at this stage.Blood-fed and gravid females of fresh-water gambiae, funestus, and salt-water gambiae have been found in outdoor resting places, gravid females predominating in the case of the first two.Although larvae of salt-water gambiae can complete their development in pure sea water, in nature increasing salinity becomes a limiting factor before it reaches that of sea water, continuous breeding being no longer possible at salinities over 83 per cent. sea water.Salinity as a limiting factor explains the rather restricted breeding of salt-water gambiae on the coast, and suggests that certain coastal fresh-water swamps at Dar-es-Salaam could be cleared of all Anopheline breeding by salinifying with sea water.

The differences between the eggs of A. gambiae and A. melas form an absolutely reliable diagnostic character for distinguishing the two species in the Lagos area.Identification of eggs laid by wild-caught females has formed the basis of work on the seasonal incidence of gambiae and melas adults in houses in different localities in and around Lagos.In the area as a whole both melas and gambiae are abundant in most localities, but the greater part of Lagos town on Lagos Island is an almost pure gambiae area.The sporozoite rate of gambiae—10·0 per cent.—is consistently much higher than that of melas—3·5 per cent.—in all catching stations. Fluctuations in sporozoite rate in relation to population density have been studied in both species.The composition of the mosquito population in houses, and in outside resting places has been studied in relation to the movements of mosquitos into and out of houses. Melas makes use of outside resting places to about ten times the extent of gambiae.In African village houses about 90 per cent. of the blood feeding by these two species takes place after midnight, the peak of activity being the hour or two before dawn.An improved design of experimental hut and window trap is described. It has been used in studying the exodus of mosquitos from houses after feeding, and on experiments on host selection by Anopheles.Swarms of male “gambiae” in nature have been described. In captivity both melas and gambiae have been persuaded to mate by using artificial light.By cross fertilisation between melas and gambiae a generation of healthy hybrid adults can be obtained. Of these the females are evidently normal sexually, while most if not all of the males are sterile.The breeding places of melas and gambiae in and around Lagos have been studied in relation to control. In this lagoon area, where conditions are very different from those of Freetown Estuary, melas breeds under a variety of conditions, and Avicennia mangrove is only of limited value in indicating major breeding grounds.

The eggs of A. gambiae var. melas are distinctly different from those of typical gambiae, and it is now regarded as a distinct species, A. melas. Other workers have found that the larvae also differ, physiologically and structurally. All adults with an extra dark band on the palps—4-banded forms— are known to be melas, but those adults with normal 3-banded palps can so far only be distinguished from typical gambiae by egg characters.A. melas is now known to be an important vector of malaria in coastal districts in West Africa. In some estuarine and mangrove swamp areas it may be even more important than typical gambiae. In melas caught in Freetown estuary (mostly from Wellington village), 42 out of 1,000 glands dissected were positive, giving a sporozoite rate of 4·2 per cent. for all months of the year. The oocyst rate was 4·7 per cent., and the total infection rate was 7·8 per cent.In Freetown estuary melas is rare in Freetown itself, and in the adjoining village of Kissy, but in all other parts of the estuary is at least as important as typical gambiae. In many places it is the dominant vector. In Wellington to the east of Kissy, and on the Bullom shore which forms the north shore of the estuary, melas forms about 90 per cent. of Anophelines caught in houses.