I've got some recent data on Drosophila montana (a rapid re-mater) populations suggesting that incompatibilities at he post-copulatory level can act as a strong barrier to gene flow in interpopulation crosses. The article has been accepted for publication pending minor revision. Are there better insects for this stuff than Drosophila? :) Maybe not...

I work with fly Euxesta bilimeki, this species has a behavior in which the female ejects a drop of ejaculated after mating. This behavior can bias the amount of sperm that the female retains in their sperm storage structures. Maybe the probability of retention of semen varies between copulations inter and intra populations.

Another interesting aspect is that the male has testes that occupy about 70% of its abdomen. Maybe it's a trait developed by expulsion or spermatic competition, but other than that I like to do other things, so I'm looking for any recommendations

Do you have multiple populations? Why not set up a series of intra vs. interpopulation matings and get data on how long it takes for female to expel the previous males ejaculate? See if she has preferential dumping... somewhat like conspecific sperm precedence. I don't work with Tephritidae, so I'm not sure how easy they are to observe in the lab...

maybe I can collect individuals from different populations. I can see in this laboratory. In addition to the expulsion time I can measure some other behaviors, like I did in my previous experiment.

one insect group that is particularly interesting for these studies is probably Tribolium beetles (or any other beetle from family Tenebrionidae). They are easy to breed, observe, and manipulate, and there is ample pre-copulatory and copulatory courtship behaviors, And as far as I know, the function of none of these behaviors is clear.