1. The behaviour of Anopheles labranchiae atroparvus, Culex pipiens molestus and Aëdes aegypti has been studied before and during oviposition.

2. Anopheles and Culex when ready to lay make repeated descents to the ground and fly about with a skimming-hopping movement, while Aëdes flies about generally, at all heights.

3. On encountering water Anopheles breaks into a hovering flight, laying its eggs during this “oviposition dance” or while settled on the water. Culex stops “dead” on the water and lays its eggs settled there, as does Aëdes but with some fidgeting about. After encountering water mosquitoes may leave it again, but fly about the floor more steadily and persistently than before.

4. Among the factors which stimulate mosquitoes after they have arrived directly over a water surface, contact with the surface is essential for actual oviposition by Culex and Aëdes, and almost so for Anopheles. Good pre-ovipository responses may be induced by water vapour alone and by the reflection from the surface, although how this latter stimulus acts is not clear. A dark appearance, especially if the surroundings are relatively light, facilitates these responses.

5. Direct attraction of the mosquitoes over the short range available in a cage was only induced by a contrastingly dark appearance, but Anopheles and Culex are aided in water finding by diurnal changes, correlated with light and activity changes, in the sense of their geotactic response.

Glossina longipennis is a species of tsetse-fly of which very little is known beyond its general distribution. In Kenya Colony, it inhabits extensive areas of the sparsely populated districts in the north and east. It extends across the coastal hinterland and penetrates into Tanganyika, where it is found in the Pare-Usambara belt (Potts, 1937). The species also occurs in a somewhat isolated belt in the vicinity of the lower Southern Uaso Nyiro in the Masai Reserve and continues into the Sonjo-Natron fly-belt of Tanganyika Territory.

Larvae of a flea-beetle, Chaetocnema arida, were found mining in the stems of young barley shoots in June 1939.

Chlorops pumilionis larvae infested barley most heavily in fields where Agriotes larvae were doing most damage. The Chlorops adults showed marked sexual dimorphism in both size and colour. The size range for this species is correlated with a proportionate size range in its parasites Coelinius niger and Stenomalus micans. Perforations 1·3 mm. in diameter in the top of a cage were sufficient for the escape of Stenomalus but not of Chlorops or Coelinius.

Lasiosina cinctipes larvae were commonly synoecious with Chlorops in barley shoots, but in some cases (5–10 per cent.) they were living without Chlorops. In these shoots the nodes were not swollen as in Chlorops infestation. Lasiosina was not attacked by the above parasites of Chlorops.

The larvae of Elachiptera cornuta infested shoots already badly damaged by other species and were accompanied by many Lasiosina larvae. Wet weather in July favours these two species. Their puparia differ in the form of the posterior spiracles. Elachiptera also occurred in maize and wheat.

Meromyza saltatrix var. nigriventris was present in stunted barley shoots in small numbers, producing swelling of the nodes very like that due to Chlorops. The summer pigmentation in both these species in Hampshire differed from that reported by Mesnil in France. Meromyza was also bred from wheat. Meromyza puparia, unlike those of Chlorops, were attached to barley sheathing-leaves by silk.

Oscinella frit larvae, infesting young sweet maize shoots at Gosden, Surrey, were either accompanied or followed by those of Aphanotrigonum trilineatum and Elachiptera. Infesting oat shoots at Long Sutton, Hampshire, Oscinella frit larvae were accompanied by larvae of Lasiosina and Camarota curvinervis. Adults of the latter emerged from stunted barley shoots in September. They showed some resemblance to flea beetles, possibly protective. Oscinella frit was also bred in small numbers from stunted wheat and barley shoots. The Chalcid, Callitula pyrrhogaster, was reared from pupae of this species found in oat shoots.

Larvae of a Muscid, Hylemyia (Phorbia) genitalis, infested a winter wheat crop in large numbers in May 1940. Tropidoscinis albipalpis, Oscinella? gracilior and Agromyza nigripes, were bred in small numbers from potted plants of this crop kept in a cage in the laboratory during the summer. Hylemyia genitalis and Agromyza (Domomyza) ambigua pupated in the soil surrounding the roots of these plants. The adults (together with a Braconxd parasite, Phaenocarpa pullata, Hal.) emerged from the soil in the following spring.

It has been shown that G. morsitans and G. tachinoides annually shift their breeding-ground in March from the edge of a residual forest island to the centre— a distance of some 40 yards. The move is associated with an increase in mortality amongst the last puparia to be found, death usually having occurred late in pupal life.

The object of the investigation has been to discover what prompts the female to move just before pupal mortality increases.

It has been found that there is little difference in the temperature conditions of the two breeding-grounds, but that the evaporation rate is one-third less in the centre of the forest. However, the abandonment of the forest margin is not explained by attributing to the female the ability to detect differences in the evaporative power of the air, but rather by the negative reaction to light developed by the whole community under conditions of universally high temperature. The period of migration coincides with an abrupt increase in the number of hours when the shade temperature is above 90°F. (32°C), the temperature at which Jack & Williams found that the negative reaction to light begins. During March the diurnal temperature is below this figure only immediately after dawn, when under laboratory conditions parturition rarely occurs. Since the temperature remains above 90°F. until well after activity has ceased at dusk, it is probable that at any rate the non-hungry tsetse spends most of the day and all the night in the centre of the forest. In the evenings field observations suggest that the stimulus of hunger may overcome the negative reaction to light, as then flies will undoubtedly attack in the forest edge, but since pregnant females rarely feed for several days before parturition, this counter-stimulus is unlikely to be operative at the time the female extrudes her larva.

By moving into the centre of the forest the adults will gain a reduction in temperature of only 3 or 4°F., but they will be less susceptible to heat, as the much lower evaporation rate will reduce their water loss.

The climatic conditions in the pupal environment of the forest edge at 1 ½ inches depth were found to favour survival, so it was concluded that the factor which inhibited emergence must have been operative before the puparia were formed. Possibly high surface evaporation and unsaturated soil atmosphere affect the formation of the pupal integument at the time of pupation, rendering subsequent emergence impossible.

The meteorological data collected in this investigation are more detailed and localised than anything yet published by the writer, and should give a better picture of the dry season adult and pupal environments in a locality which is very close to the hot, dry limits of the range of both species.

Puparia are found in large numbers in soils whose mean monthly temperatures range from 69°–82°F. (20·6—27·8°C). Field observations suggest that a constant temperature of 72°F. (22°C.) with a R.H. of 80 per cent., or 77°F. (25°C.) at R.H. 100 per cent., are conditions which should provide very healthy puparia if reproduced in the laboratory (see Appendix).

(1) In seepage-filled borrow-pits in South India it was observed that there was a progressive decline in the density of larvae of Anopheles culicifacies, Giles, as the pits became older. The largest numbers of larvae were tound soon after water entered the newly-dug pits.

(2) There was less ovipositing by culicifacies in older pits than in new ones dug late in the irrigation season. Newer pits seemed definitely more attractive in this species than older ones. These newer pits sheltered more culicifacies larvae late in the season than the older pits.

(3) The decline of culicifacies larva density in a borrow-pit seemed to be due mainly to factors internal to the pits. There was no evidence of the influence of external factors, except from October to January, when perhaps meteorological influences supplemented the internal factors. The attractiveness of new borrow-pits to culicifacies appeared to be due mainly to internal factors.

(4) Certain simple factors studied did not seem to have any significance in relation to culicifacies density in the pits. Rainfall, predators, macroscopic vegetation, pH, CO2, dissolved oxygen, bicarbonate alkalinity, ratio of free to bound and half bound CO2, hardness, chlorine, ammoniacal nitrogen, nitrates, nitrites, sulfates and iron, appeared to have no significance in this regard. Albuminoid nitrogen and oxygen absorbed perhaps had some significance, which was not clear.

(5) Among planktonic organisms, the individual groups of organisms, such as green algae, diatoms, rotifers, and copepods, definitely showed no relation to culicifacies breeding. Protozoa as a group appeared to be negatively associated to a slight degree. Blue-green algae also seemed to have a negative association.

(6) Amorphous organic matter and total plankton, however, showed statistically significant negative association with larval density of culicifacies The decline in culicifacies larvae was clearly associated with increase in total plankton and amorphous matter. The attractiveness of new borrow-pits also seemed to be related to their low total plankton content.

(7) The exact manner in which the total organic matter acted as an inhibitory factor against culicifacies breeding was not determined.