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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 X 1 X 1 X 2 X 2 (female): X 1 X 2 Y (male) sex determination. In male meiosis I, the X 1 , X 2 , and Y chromosomes of H . membranacea form a sex trivalent, with the Y chromosome associating with one spindle pole and the X 1 and X 2 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.