Chaco sanjuanina is a species of mygalomorph spiders of Argentina, named after its type locality: San Agustin, Valle Fertil, San Juan, Argentina.[1] Males can be distinguished from those of its cogenerates by the more conical bulb; females differ from C. tecka in having fewer spines on the metatarsus IV, and from C. patagonica in having labial cuspules arranged in a transverse row and the more numerous maxillary ones.
The male has a total length of 7.95mm, a cephalothorax length of 3.34mm and width of 2.66mm, a cephalic region length of 1.92mm and width of 1.55mm; an ocular quadrangle (OQ) length of 0.34mm and width of 0.69mm; a labium length of 0.25mm and width of 0.59mm; and a sternum length of 1.86mm and width of 1.37mm. The labium possesses seven cuspules. The serrula is non-visible. The sternal sigilla is small, oval, and shallow, while its sternum is not rebordered. Its colour is faded, a uniform yellow, its caput being darker; the abdomen has a light chevron.
The female has a total length of 17.41mm; a cephalothorax length of 3.23mm and width of 2.47mm; a cephalic region length of 1.98mm and width of 1.95mm; a fovea width of 0.37mm; an OQ length of 0.39mm and width of 0.77mm; a labium length of 0.3mm and width of 0.67mm; and a sternum length of 1.98mm and width of 1.4mm. Its cephalic region is strongly convex, the fovea is sinuous and straight. Its labium has eight cuspules. The sternal sigilla is small and oval; the sternum is quite convex, not rebordered. The colour is the same as in the male.