How can you tell if a stick insect is a female or male?

Some male and female Spiders can be identified by color. Those that cannot be identified by their color can be determined by checking the reproductive organs which are located in the underbelly of adult spiders.

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That would seem to need a good deal of knowledge and a powerful lens! Another, simpler method for many species, is that it's the female spider who spins the web. The males of such species find their food more by hunting.