Can you mix surface attributes and decals?
I believe the trick there would be to lower the percentage of the decal to allow some of the underlying surface attributes to show through. For instance, I assume the decal is set to Color (as opposed to Specular, Diffuse, etc.). Setting that to 66% would allow most of the decal to appear while allowing some of the underlying surface through. This could be surface attributes or other decals layered on top of each other. A specular map for instance would allow you to determine what parts of the model are shiny and what are not. An ambiance intensity setting (even if only 5%) would allow you to brighten some areas while leaving the other parts of the surface alone. Layering is definitely a great way to get those surfaces where you want them.
Here's a hastily put together example... not of praying mantis surfacing(!)... but just to show the effect of combining decals with surface attributes. In this case I just grabbed two random leopard skins from the internet and started layering. In the attached image I've used the same leopard image multiple times with different setups (displacement, color, etc.) but with none at 100% which allows. Ideally each of these images would be different to gain maximum advantage of that decal type.
Way back in the day there use to be a nice tutorial on layering of decals but a quick search didn't turn it up.
Aside: Most A:M Users tend to leave the roughness setting of a surface alone but I love to use a setting of 1 or 2 with either really small or huge scale to add noise to a texture. The roughness setting isn't like much because of the unrealistic patterns it produces at higher/default settings. My suggestion for using roughness? Don't use the default setting!
In the case of a praying mantis I'd say we'd be looking for some translucency and other effects to give the appearance of healthy green leaves. (i.e. definitely NOT leopard spots!)