Praying mantids are important models for studying a wide range of chromosome behaviors, yet few species of mantids have been characterized chromosomally. Here we show that the praying mantid Hierodula membranacea has a chromosome number of 2n = 27, and X1X1X2X2 (female): X1X2Y (male) sex determination. In male meiosis I, the X1, X2, and Y chromosomes of H. membranacea form a sex trivalent, with the Y chromosome associating with one spindle pole and the X1 and X2 chromosomes facing the opposite spindle pole. While it is possible that such a sex trivalent could experience different spindle forces on each side of the trivalent, in H. membranacea the sex trivalent aligns at the spindle equator with all of the autosomes, and then the sex chromosomes separate in anaphase I simultaneously with the autosomes. With this observation, H. membranacea can be used as a model system to study the balance of forces acting on a trivalent during meiosis I and analyze the functional importance of chromosome alignment in metaphase as a preparatory step for subsequent correct chromosome segregation.