The hygroreaction of the larvae of the Oriental rat flea Xenopsylla cheopis Rothsch. (Siphonaptera: Pulicidae)

The humidity reaction of the larvae of Xenopsylla cheopis Rothsch. was studied in an air-chamber with a 0–100% relative humidity gradient.Highly significant differences (P < 0·001) between the reactions in the various R.H. ranges was found. The most preferred relative humidity along this gradient was 100%. Relatively few larvae were collected at each R.H. in the range between 0 and 90% R.H.The reaction of the larvae under the experimental conditions was seen to be of orthokinetic type.

Humidity Reactions of Three Aquatic Amphipods, Gammarus Duebeni, G. Oceanicus and Pontoporeia Affinis in the Air

1. Specimens of the Baltic population of the aquatic amphipods Gammarus duebeni and G. oceanicus are able to avoid the lower of two relative humidities of the air when out of water. 2. This humidity reaction is based on the orthokinetic and the klinokinetic types of orientation. 3. G. duebeni is an inhabitant of seashore rock pools and has been reported to move on land between the pools. 4. Although G. oceanicus is seldom exposed to air in nature in the practically tideless Baltic area, it lives between tide-marks in other parts of its distributional range. 5. The third amphipod studied, Pontoporeia affinis, does not discriminate between high and low humidities of the air in an alternative chamber. It is always confined in nature to the sea bottom below the low tide level. 6. Of the three studied species, individuals of G. duebeni move rapidly on dry land and survive a markedly longer time in humid air than the amphipods belonging to the two other species.

The seasonal and diurnal activities of individual sheep ticks (Ixodes ricinus L.)

1. Activity in Ixodes ricinus was studied by laying down newly emerged ticks in natural clumps of vegetation from which all hosts (with the possible exception of ‘mice’) were excluded. After a period of quiescence the ticks climbed to the ‘active’ position at the vegetation tips. Activity was assessed either by simple observation (in adults which were marked individually) or by ‘brushing’ the vegetation with the hands (nymphs and larvae). 52 % of the adults and 44 % of the nymphs were recovered.2. The seasonal activity behaviour was followed in three series of ticks set out during May, July and October 1945. The results were used in interpreting the time relations of seasonal activity in ‘wild’ populations. The timing of the activity cycle appears to be largely determined by the availability of unfed ticks rather than by the prevailing meteorological conditions.3. The diurnal pattern of behaviour in active ticks was closely examined during a 24 hr. period. Many remained at the tips without moving. The others began or ended a phase of activity by night or by day; but among these, the proportion ending a phase at night was significantly greater. This is the main reason for the previously reported diurnal fluctuation of activity in a tick population.4. Two hundred and seven adult ticks spent, on an average, a total of 9 days at the vegetation tips (individual limits 1 and 54 days). This activity was spread over an ‘active period’ (interval between first and last appearances) averaging 30 days. Very active ticks rarely remained continuously at the tips. The mean number of visits to the tips was 4 (limits 1 and 19), each lasting for an average of 2·5 days. Where long spells of activity alternate with short periods of quiescence, the behaviour is probably regulated by the humidity reaction, the quiescent phase providing an opportunity for restoring the depleted water balance.5. Newly moulted adult ticks possess reserves of fat adequate for many months of quiescence. During activity these reserves are exhausted in a few weeks or even days. Duration of survival mainly depended on whether the onset of activity was immediate or delayed. Some adults remained quiescent for 1 year before becoming active for the first time.6. Several types of orientations were observed in the field. In attaining the position at the tips favourable for encountering a host, the gravity response (upward-turning near the tip) is of major importance. The tick avoids wind and direct sunlight by sheltering behind its supporting stem. In sensing the approach of a ‘host’ (the observer's finger) the perception of eddies of warm air is particularly significant. The response (questing, then orientation) is elicited much less readily if the stimulus is applied from the leeward.

The Humidity Behaviour of the Mealworm Beetle, Tenebrio Molitor L

1. Removal of both antennae of the mealworm beetle, Tenebrio molitor, completely abolishes the animal's reaction to humidity. Removal of both maxillary palps has no effect on the reaction. 2. The quantitative distribution of the five types of sensillae present along the eleven segments of the antenna is described. Two of these types occur also on the maxillary palps. 3. Progressive symmetrical amputation of antennal segments leads to a gradual reduction of the humidity reaction to zero; there is a reaction with four segments remaining on each side and no reaction when only three segments remain. 4. Asymmetrical amputation of the more basal segments shows that humidity receptors are nevertheless present on these and that a threshold number of sensillae must be left in action before a reaction occurs. 5. The experimental work, taken in combination with observations in the distribution of sensillae, shows that the pit peg organs are hygro-receptive. Either the peg organs or the bristles or both are hygro-receptive as well. It is, however, unlikely that the bristles are hygro-receptive. 6. Experimental work on the locomotory activity of this beetle shows that the conclusions are not invalidated by any general ill effects of amputation of antennae. 7. It is suggested that the hygro-receptors function hygroscopically.

The Temperature And Humidity Relations Of The Cockroach

1. In a diffusion gradient of humidity at uniform temperature, some cockroaches (Blatta orientalis, L.) show a tendency to spend more time in the drier region. Other individuals appear to be indifferent to the stimulus of air humidity. 2. On desiccation, there is a tendency for cockroaches to become hygro-positive. 3. In a temperature gradient, those individuals which react to humidity have a slightly but significantly higher preferred temperature in somewhat moist air than they have in dry air. 4. It seems, then, that the observed preferred temperature represents a kind of balance between a pure temperature reaction and a humidity reaction. The change in humidity reaction resulting from desiccation is qualitatively satisfactory to explain the fall in preferred temperature which occurs at the same time.

The Reactions of Mosquitoes to Temperature and Humidity

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.