[Jakub Liu]

Jakub Liu asked the Naked Scientists:

Hi Dr.Chris,

my name is Jakub, 8.5 years old. I have the following question: Once I killed a fly, then I saw white creatures crawling out of its body, looking like slugs. What were they?

Regards, Jakub

What do you think?

[blakestyger]

Did they crawl out or were they ejected when the fly was squashed - I've seen this happen and they were, in that case, the eggs that the fly would have laid sometime later. I'd really need to have seen it to be sure and I may be wrong and there is another explanation - we'll see what turns up.

[chris]

Do any flies rear their young inside their bodies? Could these have been future progeny?

Female flies certainly carry with them a supply of sperm conserved from their last mating. This is used to fertilise eggs subsequently, or until another mating takes place. The penis of male fruit flies is equipped with a digging tool to enable them to root out from the female any sperm already present to  enable them to "displace" the genes of competitors.

Chris

[blakestyger]

The penis of male fruit flies is equipped with a digging tool

Like a sort of Swiss Army penis - the possibilities are endless.

[rosalind dna]

They might be the eggs of the fly or maggots which look like
transparent earth worms.