Several families of beetles cause toxic reactions on exposed human skin. Cantharidin provokes nearly asymptomatic vesicles and blisters, while pederin leads to itching and burning erythema with vesicles and small pustules, later crusts. Paederi are attracted by fluorescent light especially after rain showers and cause outbreaks in regions with moderate climate. Clinical findings and patient history lead to the diagnosis: dermatitis linearis.