Phlebotomine sand flies (Diptera, Psychodidae, Phlebotominae) are small diptera that represent a group of approximately 1,000 known species around the world, of which 530 were found in the Americas. The females are hematophagous, a characteristic that makes them capable of participating in the transmission of etiological agents that cause diseases, such as leishmaniasis, bartonellosis, and arboviral diseases. Classical taxonomy requires knowledge of morphological and morphometric patterns for the correct classification and identification of species. In the classification and identification of sand flies it is common incomplete species descriptions, erroneous associations between sexes, as the existence of morphologically close or indistinguishable species, and polymorphic species make a correct diagnosis difficult. Another problem with classical taxonomy in sand flies is the occurrence of anomalies, which are generally observed in paired morphological structures. In September 2017 during surveillance for leishmaniasis in Iñapari Town, Peru, sand flies were captured using light traps for three consecutive nights. During entomological surveillance, 55 specimens were identified, including a female of Bichromomyia olmeca bicolor (Fairchild & Theodor) showing an unusual structure not previously reported in sand flies. Within the median region of the scutum, a spine projection was observed, measuring 39.4 μm. The spine displayed a discrete surface convexity directed towards the anterior region of the thorax. Bilateral and unilateral anomalies have often been described in sand flies, mainly in structures that are under substantial evolutionary pressure, such as reproductive organs of males and females. The anomaly observed in Bi. olmeca bicolor is the first reported in the thorax of sand flies. An observation of similar anomalies from different species and in different countries shows the need for more studies to elucidate the causes for the occurrence of this phenomenon.