Chemical control of the sign and precision of heliotropic response

In 1906 Loeb reported that Gammarus left alone cling to solid bodies anywhere in the aquarium. If disturbed, by being moved or by stirring the water, they become negatively heliotropic, at least for a time. Adding certain chemicals such as esters made them at once intensely positively heliotropic.³ He could not obtain this alkali effect in Gammarus.⁴

In Cyclops, adding HCl, another not too weak acid, or carbon dioxide made the animals all gather in a narrow region at the window side of the vessel.⁴ Loeb summed up the Cyclops result by saying that acid, especially carbon dioxide, made negative Cyclops positive. It also strengthened the positive response of animals that were already positive.⁴ In 1912 he wrote that Daphnia usually need the water cooled as well. Cooling plus carbon dioxide made light-indifferent Daphnia strikingly positive.⁵ He also stated that marine copepods can be made positive by lowering the temperature alone, or by a sudden rise in the concentration of the sea water.⁵ In 1918 he restated the finding for Californian Daphnia, copepods and Gammarus, using weak acid, especially carbon dioxide.⁶

Other authors, as summarised by Washburn in 1908, studied Daphnia's response to light without chemicals. Von Frisch found that a blue screen made Daphnias avoid light even though it lowered the intensity.⁷ Ewald reached conclusions about colour blindness in the side regions of the eye. Washburn called this a heavy weight of inference on slender evidence.⁸ Loeb and Moore reported that ultra-violet rays instantly made Daphnia avoid the light containing them. Lubbock found that Daphnia sought such light in preference to darkness.⁸ These reports suggest that the sign of the response depends on more than intensity, though the evidence is mixed. Washburn doubts Ewald's inference and notes that Von Frisch is determined to find colour vision in invertebrates. Lubbock's finding also contradicts that of Loeb and Moore.⁷

In 1916 Loeb wrote that the results of his Fundulus experiments are of general application. Osterhout had shown that plants of soil or fresh water are readily killed by pure NaCl at a certain concentration. They survive the same concentration if some CaCl2 is added.⁹ Ostwald had shown the same for a species of Daphnia.⁹ These results concern toxicity, not orientation to light.