Something I was working on in the past involved a species of sentient insectoids. The idea was the two sexes usually live in two very different environments and rarely come into contact with the other one, except for when they meet in a sort of 'middle ground' to mate.
Consequently, they look to outsiders (including humans) like two completely different species (we'll assume the embryos, or whatever equivalent they would have, are left to hatch in the aforementioned neutral ground, and once born those of each sex find their 'home turf' by instinct).
The males of this species would be capable of flight (probably even hovering) with limbs that while highly dexterous are also somewhat fragile. The females on the other hand are flightless, with segmented bodies that are stronger and tougher at the expense of dexterity (any arms are not much more articulate, if at all, than pincers).
In what kind of environment/ climate would these insects develop these very different physical traits? What kind environment would be suitable for them to come into contact long enough to reproduce?