She summarized the results in her doctoral thesis, published in 1928 under the title Über den Farbensinn der Tagfalter (“On the color senses of butterflies”) in the Zeitschrift für vergleichende Physiologie (Journal of Comparative Physiology). Ilse first studied botany and zoology in Berlin and then Göttingen, beginning with field trials in Molkengrund, “a very sheltered valley” that was particularly well populated by the great spangled fritillary (Argynnis aglaja) (Fig. 1).
Through her observations, she noted a preference of these butterflies for blue, crimson, and violet flowers. During her research, Ilse carefully crafted colored-paper “star-shaped artificial flowers” with standardized pigments for scientific classification, and distributed them in the meadow. She watched as the butterflies started to appear: “In the ‘visits’ witnessed here,” she claimed, “Argynnis aglaja touched the pigmented flower, just like the real Ajuga flowers, with its sucker unrolled: the proboscis extension reflex, a clear feeding reaction.” This was not all: the butterflies had a sense of what color was, clearly, but was this innate or something they acquired through experience? Curious to learn more, Ilse bred butterflies of different varieties in her laboratory and performed experiments under controlled conditions. In her laboratory experiments, the paper flowers were placed in colorless glass tubes containing juice from which to feed (Fig. 2). It may seem surprising to us, but her experiments showed that butterflies, like other insects, can be trained: “the animals are fed for a long time on a particular color, so that they form a mental association with a certain color as being food; eventually this color, even when presented in the absence of food, is able to trigger a feeding reaction. The response to the color is thus acquired secondarily.”
Ilse’s lengthy, detailed experiments led to important results: as a consequence, she was able to prove that butterflies have a sense of color, and that most of them discover flowers on their first approach through their color and scent. The young researcher now had a promising scientific career path ahead of her. Upon completing her PhD, she took up a position as a Scientific Assistant at the hereditary biological archive in Bonn, in 1931 moving to the Kaiser Wilhelm Institute for Biology in Berlin Dahlem as a Scientific Guest Researcher. In 1933 she was invited as a Scientific Assistant to the Munich Zoological Institute by the mellitologist—or bee researcher—and later Nobel Prize winner Karl von Frisch. Following Hitler's seizure of power, however, Ilse was no longer permitted to work as a scholar: the discriminatory law “On the Restoration of the Civil Service,” of April 7, 1933, prevented her doing so owing to her Jewish origins. She was now able to work only as a private assistant at the Zoological Institute, alongside which she undertook unpaid work producing educational films.
In 1935 she moved to Britain in exile, where she lived until 1952 and taught biology in schools. She also published articles in Nature and Proceedings of the Royal Philosophical Society of Glasgow, and in 1938–39 gave a series of lectures in America, supplemented by scientific films. On a trip to Germany in 1951, in an attempt to assess the situation in her home country, she sought conversation with old acquaintances. From Munich she wrote to the biologist Georg Melchers, whom she knew from her time in Göttingen, asserting: “More important than my own personal feelings is the desire to help towards such things never happening again.” Ilse resumed her activity as a zoologist between 1952 and 1955, at the University of Pune, India, where she helped to establish the Zoological Institute. Her commitment as a scientist and mentor is further attested to by publications co-authored with Indian colleagues, correspondence from the 1950s, and her 1953 translation of Aus dem Leben der Bienen (The Dancing Bees) by Karl von Frisch. Ilse died in Munich in 1979; the details of the latter part of her career are, as-yet, unknown.