Geared to complement UML and to the specification of large software systems by non-mathematicians, spider diagrams are a visual language that generalizes the popular and intuitive Venn diagrams and Euler circles. The language design emphasized scalability and expressiveness taining intuitiveness. In this extended abstract we describe spider diagrams from a mathematical standpoint and show how their formal semantics in terms of logical expressions can be made. We also claim that all spider diagrams are self-consistent. Keywords Visual formalisms, software specification, formal methods.