Introduction: This article explores the scientific ideas and alternative conceptions that pre-service teachers have regarding animal classification and spiders in particular. Method: The study involved 40 pre-service teachers of elementary education in Spain and the data was collected by means of questionnaires, descriptions and drawings. The students completed all the tests proposed in this research at the beginning of class sessions. Results: We found several misconceptions, some of them consistent with children's interpretations. The results show that the majority had great difficulty distinguishing between vertebrates and invertebrates, and although all the participants said they were familiar with spiders the results indicate that the majority have scarce knowledge of them. Discussion and conclusions: These prospective teachers may begin teaching at school without really having a good understanding of living things. These results should serve to reformulate the current development of the curriculum on invertebrates in Spain and foster practical instruction about living organisms for future teachers. (Contains 5 tables, 12 figures and 6 online resources.)