John Russell Malloch became a world-renowned taxonomist in entomology, specializing in Diptera (flies). His career spent studying insects developed after leaving his native Glasgow for North America and after an important period with the Illinois Natural History Survey eventually settled in Washington, dc. There he worked for the US Federal Bureau of Entomology whose remit was essentially agricultural. However, Malloch worked on many groups, not all of which were directly of economic importance, and made important contributions on their taxonomy and phylogeny. This account describes the relevance of field-based observation and collecting for building knowledge. Malloch provides a paradigm for the importance of deducing ideas and conclusions from basic natural history practice. The transition of biological science from amateur to professional status is discussed in the context of his work. In doing so some issues on the growth of knowledge in entomology combining the efforts of naturalists working in the field with the growing professionalization of biology based in the laboratory are addressed.