To tell the difference between male and female black soldier flies, we can start by looking at the lower body part. The male have a more round lower tail whereas the female have a more pointy tail.
Let's see how we can tell the differences below:
The tail of BSF female is split up like a fork or scissor shape. Whereas the tail of the male is more round.
If we take a closer look at the male BSF, we can see a more oval tail shape:
If we look more closely at the female fly, you can see the two-pronged fork structure at the tail:
Although we call it tail, a more accurate way to call these parts is the genitals. And this is exactly what we'll explore next:
The genital of the female looks like a two-pronged fork. The male reproductive organ, as you can see, is more of a claw-like structure.
When they mate, the claw-like part of the male will grab & suck the pointy fork of the female in. Then he moves some juices from the chest down to make the fertilization happen. This is one clear way we can tell the differences between the BSF.
Also, we can take a look at:
From one researcher's sharing, we can also tell the difference by looking at their stomach or abdomen. The upper abdomen of the female is slightly more reddish. While the top part of the male abdomen is slightly more bronze.
If you scroll up to look again at the picture above, you'll see the wings of the male fly more round. Whereas the wings of the female are more pointy.
But in a crowded love cage, how can we know which is which? Let's see that next:
In The Love Cage
In the breeding area, when we see the flies hanging around the more brightly lit area, then chances are those are the male flies. They need the light source to warm up their bodies for mating & also to see the signal from the female flies that she's ready.
When the female flies are in the mood to mate, she'll stretch her tails out. More bright areas help the male see this signal clearly & get to action. So they buzz around these spots more often. As you can see the flies on the red piece where it's brighter, they are more likely male flies.
If you randomly catch one of those guys in that area, you can check to see if their tails have claw-like structures. Or see if their stomach is a bit more bronze. Then, we can be more sure that these are the males.
Underneath the red piece is a shaded area where the pregnant female flies are laying eggs. Female flies in general prefer a more cool or shaded area. If you catch one of these flies in the more shaded place, check if they have a two-pronged fork tail. If they do, then it's a girl.
How to Tell if a Black Soldier Fly Is Carrying Eggs?
You can also tell the difference by the movement of the flies. A black soldier fly, when it lands on something, usually stays very still or does not move around at all. When it lands on a netting or a piece of wood but keeps moving around here & there, you'll know that it's a female fly surveying the area and looking for a good place to lay her eggs.
From my humble experience, I have also noticed some flies fly away quickly as they sense people approaching them. Whereas others stay very still and calm with us human getting near them.
From this observation, I make a guess. Possibly the flies that buzz off quickly are the female flies carrying the eggs inside her. As they sense something coming here, she might be afraid someone may take away the eggs from her and fly off.
For the very still flies, I was lucky to be able to get near and catch one. I can verify it was a male BSF because of the tail and bronze abdomen. So for the other half of this guess, possibly the calm flies might be the male ones.
So now when accidentally catching the flies mating, you'll know more clearly which is which. It was a bit confusing for us at first, but here you can see the picture below. The male is on the left & the female is on the right. The male is usually the bigger one.
Can We Determine the BSF Sex Even Before Their Pupation?
It's not scientifically proven yet or anything. But there's some info that a grower shares from her own experience. It's sort of like the temperature-dependent sex determination in some reptiles, insects or fish.
The grower observes that if we leave the darkened pupae in the shade for about 10 days before placing it in the breeding cage, it will tend to pupate out more female flies.
This addresses one major concern of many growers, that is, they don't have enough female in their population to lay eggs. Although this might be totally wrong, it's worth giving it one or two trials. Maybe you'll find something that works better.
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