Geotaxis in the Ciliated Protozoon Loxodes

Geotaxis is demonstrated in the ciliated protozoon Loxodes. This behaviour is mediated by a mechanoreceptor which is probably the Müller body, an organelle characteristic of loxodid ciliates. The geotactic response is sensitive to dissolved oxygen tension: in anoxia or at very low O2 tensions the ciliates tend to swim up and at higher O2 tensions they tend to swim down. This behaviour, in conjunction with a kinetic response allows the ciliates to orientate themselves in vertical O2 gradients and to congregate in their optimum environment. In two appendices, models of the behaviour predicting vertical distribution patterns and considerations of the minimum size of a functional statocyst are offered.

THE GENETICS OF GEOTAXIS IN DROSOPHILA MELANOGASTER

The use of a behavioral character, the geotactic response of Drosophila melanogaster, permitted selection in large replicated populations. Each population was derived from the same base population but after the initial selection, it was maintained independently and treated identically. Diallel cross analysis revealed differences between the additive genetic variances of the strains treated in this way. These differences were largest after about eight generations of selection and decreased by the 16th generation.The analysis of diallel tables also showed that the geotactic response in D. melanogaster was controlled by polygenic factors. On the average, the genes that increase the expression of positive geotaxis were partially dominant while factors for negative geotaxis were recessive. Dominance was not, however, always unidirectional.

On Water-finding and Oviposition by captive Mosquitoes

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.