If a firefly is male its light will come from the last two segments of its abdomen. If a firefly is female its light will come from the second to last segment of its abdomen.
The females are slightly larger than the males, and have a much larger space between their redcompound eyes.
The word "fly" is a common noun. In English nouns, common, non-gender specific nouns are neutral. They take neither gender.
the gender is a guy you can tell
Your parents
No. The gender is not related to the relationship of the parents.
If that gender turns you on
If you know your rodents, you should be able to tell the gender of baby rodents at birth (I can).
You can tell by their size and color.
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magic.
You feel it
yes you can?
Look underneath it. It should be easy to tell.