Male Violin Mantids will have thicker longer antenna as adults.
Determining Gender
The gender of a mantis will determine many things about them as they grow and mature. The overall size and appearance can vary greatly in a process called sexual dimorphism. Females as adults are typically larger and more robust while males can have thicker and longer antennae. Some species the difference in size can be quite drastic, such as yellow and white orchid mantids males who are around 1/3rd the size of the adult females.
Determining sex of nymphs can be difficult as both genders look nearly identical in early instars, only gaining different traits or sizes as they reach their later instars. When first hatched all will be the same size, color, and shape.
Some species like spiny flower mantis are nearly identical all through out their life and even into adult with only a slight size difference between males and females.
So how do you tell the difference?
Males and females of nearly every instar have a key difference in the number of sternites (the number of segments) visible on the underside of the abdomen. Males will have 8 segments while females will have only 6. This can be hard to see on smaller species until they reach later instars. A good quality magnifying glass or head piece can be the only way to determine sex of early instars.
The females will also have a inverted V shape at the tip of their last sternite that will become more pronounced as they grow. As adults male will develop a inverted "cup" shape to their final sternite.
Females will have 6 sternites on the underside of their abdomen
Males will have 8 sternites on the underside of their abdomen