Reversal of Phototropism in a Parasitic Water Mite

The water mite Unionicola ypsilophorus var. holdemani (Piers). living between the gills of the fresh water clam Anodonta cataracta Say, shows a positive reaction to light after removal from the clam. 2. An extract of the gills, or water from the mantle cavity of a clam, will cause a reversal to a photonegative state. 3. This reversal may be considered adaptive, for, aided by a positive chemotropism and stereotropism, it enables the mites to enter and remain within the host. 4. The phenomenon due to the parasitic life may be described as a "conditioned reflex"; the positive response to light being primitive and the negative response more recently acquired.