The spermatheca become darkened when the female is ready to breed. Sure someone should be able to get some pics or else google could get some pics maybe.
The chances of losing a male are probably less when the female is smaller and has less of a chance to chomp on the male.

pobably an ignorant question but would the size of the female affect the size of the egg sac? and would breeding a smaller female affect her breeding in the future as opposed to a ''mature'' female as such. love this thread and topic guys

Well talking about size, I have just seen one of my H incei females has got an egg sack, she is only at a leg span of 4.5cm ^+/-, the egg sack is almost bigger than her.
She is in a communal of 4 H incei,the time I counted them there was 1 male and 3 females that was 2 weeks ago. and all the females are still small.

As far as I am aware, the spermathecae have to sclerotise (harden around the edges). This process will cause the spermathecae to darken to an Amber color. If the spermathecae are still translucent, the female is immature and thus not ready to breed. The above is just a description of what I as a layman know and look for. It is possible spiders might breed as juveniles, but for the purposes of my breeding, I don't mate spiders that have translucent spermathecae.

These images might indicate the Amber edge I am referring to. I don't have a picture of juvenile spermathecae.

panda wrote: pobably an ignorant question but would the size of the female affect the size of the egg sac? and would breeding a smaller female affect her breeding in the future as opposed to a ''mature'' female as such. love this thread and topic guys

The smaller females will tend to drop smaller egg cases.
Don't think breeding them earlier will affect future breeding, but haven't really done any experiments like that myself. Not sure if anyone has either.

Thanks Scarab those pictures are perfect to explain, looking at the B emilia picture you can see the spermathecae has started sclerotise (harden around the edges), which means this spider was almost ready, but now that she has moulted her new spermathecae that's in her will be fully sclerotise and she will be ready for breeding. There is exceptions where some species females will take more than one cycle to sclerotise though.
I have found younger females are more eager to mate and reproduce where older females, or females that has gone through a couple of reproductive cycles without being mated being much more difficult to mate.
One very important note I want to make here, if you decide to ever mate a young female you should rather than protecting the male look after the female, if she shows any signs that she's not interested remove the male. Young females have much shorter moult cycles and if the female has started her next moult the male will damage the female and this could be fatal. I'm speaking from experience here and have made this mistake ones to often.