Drain flies in jacuzzi bathtub

We've lived in this house for about 8 years and have used the jacuzzi tub...maybe 3 times when we first moved in. Once we realized what a pain it was to fill and that the hot water ran out before the thing got full, we stopped using it. Now all it does is collect dust and drain flies. It used to be very occasional, but we're being besieged with them right now. I removed the drain a few months ago when we had trouble with the tub draining slowly (have to wash out that dust every now and then) and discovered that the pipes looked like they were put in yesterday: completely white and clean (plastic). So that made me realize that it was probably our shower which is right next to the tub that had the drain problem and it was somehow affecting the tub. Put Drano in the shower--since taking the metal plate off the shower drain still didn't allow me much access as far as cleaning. I now have a rather fine hair catcher on the shower drain, while the tub drain is fully closed and I put saran wrap over the tub spigot. The drain flies are still all over the tub, so we've concluded they're coming out of the holes for the jet spray. HOW IN THE WORLD do we find where they're breeding? We have no way to get to the insides of the jacuzzi tub. And while I'm on the subject, could anyone tell me how expensive/difficult it would be to replace this tub with an old-fashioned, usable bathtub? The space seems too wide for a normal tub. I'd install a bigger shower if we didn't have a window over the tub. I'll see if I can attach a photo of the setup. Attachment 37397 (http://www.doityourself.com/forum/attachments/indoor-outdoor-insect-pest-control/37397-drain-flies-jacuzzi-bathtub-bathtub.jpg)