Mealworms beetles are nocturnal throughout their life cycle, so you will need to make behavioral observations at night.
People in the United States and other Western countries commonly use mealworms as fishing bait or food for insect-eating pets, including lizards and hedgehogs, or in science experiments for school-aged children. Mealworms are actually not worms at all, but the larval stage in the life cycle of Tenebrio molitor, or mealworm beetle. According to the University of Arizona's Center for Insect Education, determining the sex of mealworms is difficult without specialized equipment and knowledge. However, there are some behavioral and anatomical characteristics that can help distinguish between male and female adult beetles.
Determining Sex in Larval Mealworms
Identify the mealworm's head and tail. Look for the three pairs of legs that protrude from the body. The end of the worm closest to the legs is the head, the end farthest from them is the tail.
Identify the mealworm's back (dorsal side) and belly (ventral side). Once again, the mealworm's legs will help to orient you. If the legs point downward and away from you, you are looking at the back of the worm; if they point toward you, you're looking at its belly.
Look for the mealworm's genital swelling. The body of the mealworm is a little more than an inch long and divided into bands called segments or sternites. Turn the mealworm so that you are looking at its belly (ventral side). Using a hand magnifying glass or low-powered microscope and starting at the head, count seven segments down. According to Dr. Désirée Vanderwel of the University of Winnipeg, female mealworms have a large, dark swelling with two nipple-like protrusions called papillae. If you do not see a large swelling and papillae in this area, the larvae is most likely male.
Determining Sex in Adult Mealworm Beetles
Examine the posterior sternites. Turn the adult beetle so the belly (ventral side) is facing you. Using a hand magnifying glass or low-powered microscope, counting from head to tail, examine the separation between the third, fourth, and fifth sternites (segments). In females, the separation between these is minimal and the fifth sternite is pointed; males have markedly separated sternites and the fifth is rounded.
Observe courting. Male beetles court females by chasing them, and females avoid the males until they are exhausted. If you observe a chase, the pursuer is male and the one being pursued is female.
Observe mating. Males mount females to deposit semen in the genital tract. If you observe mounting, the topmost beetle is the male, the bottommost is the female.
- gender simbol image by Nataliya Galkina from Fotolia.com