Onion seed head - maggots

Help neded please

I have just cut down my onion seed heads to collect the seed and found little white maggots at the base of the head.

Are they onion fly? Do they eat the seeds?  I have squished as many as I can see but will there be further eggs in there to look for,  I have rather a large bag of seed heads and do not want to lose them after looking after the plants for nearly two years.