I HAVE discussed, in British Spiders (1926) on p. 123 and more fully in The Biology of Spiders (1928) on pp. 185-91, the curious problem offered by the common spiders of the genus Zilla. These two spiders, Zilla x-notata Clerck and Zilla atrica C. L. Koch, are orb-spinners or Epeiridae making a characteristic web in which there is a free radius between two segments devoid of spiral thread. Morphologically these two spiders are very closely allied indeed, but they differ almost invariably in the situations in which they spin their webs. One species, Z. atrica, spins on shrubs and bushes, the other, Z. x-notata, spins on walls, sheds and houses. The problem was to explain this separation of two species otherwise so similar and this explanation I believe I can now give. Various considerations (vide J. Queckett Micros. Club, 15, 269-73, 1927) had made it apparent to me that there must be a much closer relationship between the spider and the factors of its physical environment, such as temperature, humidity, wind-velocity and so on, than is generally realised, and that such conditions must largely determine the exact spot in which any individual spider spins its web. My attention was therefore attracted when, on January 10th, 1929, I noticed two webs occupied by Z. x-notata directly above the water-tank in a room where the normal situation for such webs is in the corners of the window frames. These two spiders were captured and put, successively, into a cardboard box measuring 18 x 2 x 2. At one end of the box was placed a small basin of water, at the other a small dish of granular calcium chloride. There was therefore soon established a continuously falling relative humidity in the atmosphere of the box, varying from the maximum vapour pressure at the wet end to a very small value at the dry end. The box, when closed, was dark so that light tropisms had no influence, and the spider was free to choose its own resting-place. At intervals of two or, three hours the box was opened and the position of the spider noted. All silk threads were then swept away, the calcium chloride renewed if necessary and the experiment repeated. This was continued for several days with the two spiders, the only two available at that time, in the middle of a cold winter. Occasionally, instead of removing all silk threads and disturbing the spider, the lid, on which it always rested, was reversed before replacing it, so that the spider was now over the other end. In all the observations made on these spiders the results were consistent. If the box was in an unwarmed room, whose temperature in those days was