Cândido de Mello Leitão (1886-1948) started his career as a pediatrician. Being extremely dedicated to the study of spiders, he became one of the greatest arachnologists of his time, and earned wide recognition from his peers. The present article deals with some aspects of his career, while trying to address the question of how one could become a biologist in Brazil in the first half of the 20th century, when there were no graduation courses in Biology and the profession held no official recognition. Research conducted on books, newspapers and Mello Leitão's personal documents has shown how this specialization came to being in the crossroads of practical experiences in various scientific institutions, national and international collaboration and the changing social-historic conditions. A spider expert, Mello Leitão had great knowledge on webs that allowed him to become a biologist. Amid these personal and institutional relationships, he built and spread knowledge, and carved a role for himself, his colleagues, the institutions he worked in and the biology he practiced.
History of Biology; Aracnology; Cândido de Mello Leitão; Scientific cooperation; National Museum of Rio de Janeiro; Brazilian Academy of Sciences