filled with small papillae arranged rows with various tip lengths and shapes. The jaw has a dark coloration and is marked by serrations that follow an arc-shaped pattern. E. petersi tadpoles also have a vent tube that is attached to the ventral fin of the body. Their tails are approximately 60% of their entire body length with a narrowly rounded tip. A distinguishing characteristic of E. petersi larvae compared to other Engystomops species is the presence of prominent elliptical paravertebral glands. The coloration of E. petersi larva has only been reported after preservation. In preservative, its coloration is mainly dark to light brown. Aspotty mesh pattern with speckles around certain regions of the body is also visible. The tail is light brown with speckles all over and the fins are translucent with threadlike markings. Habitat and Distribution Speciation Engystomops petersi is a common species that inhabits primary and secondary forests and forest edges. They are terrestrial frogs often found in leaf-litter. They are nocturnal, and prefer humid environments. While they are terrestrial, they breed near water bodies, specifically on the edges of lakes, ponds, and pools. The males sing and send mating calls while floating on their backs in the water. The genus Engystomops is a lineage of frogs located in the Andean foothills of Ecuador, Peru, and Bolivia, extending to the Amazon basin in Brazil. Speciation events of the genus are well studied and typically defined using the study of mitochondrial DNA, mating calls, and cytogenetic variation among different frogs in the area. Engystomops petersi makes up the northwestern clade of the genus Engystomops and was found to be its own species located in northern Peru and Ecuador. Puyo, Ecuador is a hub for Engystomops petersi frogs and has been used for comparative karyotypic analysis with other frog species in the area. Its karyotype is unique compared to other Engystomops species in the area and specifically diverges from the southwestern clade known as E. freibergi located in southern Peru and Brazil. Genetic Population Structure Sexual selection has been thought to play a role in evolutionary speciation of Engystomops. Female E. petersi discriminate against certain mating calls from foreign populations and will recognize calls from specific locations. Such behavioral isolation of the species likely led to speciation. The genetic variation between different Engystomops species is also characterized by variations within karyotypes with different banding patterns as well as clade divergence.