Yamaha 150 TLRW 1998 part diagram

I can't seem to locate the portion of the engine between the bottom cowling/apron and the lower casing in the parts schematic. I had borer bees (bumble bees that bore holes in timber) enter through the exhaust ports in the apron and travel down, getting caught in the vented area of the lower casing. A dozen of them! I had to terminate them and crush them through the vented area using a paint stick. I'm concerned some of their bodies may still be between where they entered and where I found them and that they might clog a critical function of the engine. Is there nothing but exhaust/dead space travelling between the two ports in the apron and the vented area on the upper rear of the lower casing?