A few items on Republic off the top of my head (and for that reason some specifics might be off a tad)
RC formed in 1978 by merger between North Central and Southern. Hughes Airwest merged into the RC fold in 1980.
Very far-flung system. Here are approximate sizes and ranks of the many RC hubs and focus cities shortly after Hughes Airwest was merged in:
Flights per day & approximate rank:
DTW 80 (1)
MSP 70 (2)
ATL 60 (3)
MKE 60 (1)
MEM 60 (1)
PHX 55 (1)
ORD 50 (5)
LAS 30 (2)
They also flew a great deal of scattered routes left over from the growth spurts of the three individual airlines, such as Salt Lake-Orange County, Cincinnati-Philadelphia, and Denver-Tucson.
In an attempt to stop massive losses, the airline in the early 80's added routes from MSP, PHX, DTW, and MEM and made them more hub-like. Then in about 1983/84, they decided to put all their eggs in MSP, DTW and MEM. In a quick stroke, they dumped *every* route which did not serve MSP, DTW or MEM except GRB-ORD, BNA-ORD and MKE-LGA. And they finally made money.
The old Herman the Goose logo (original to the earliest days of Wiscosin Central, the start of North Central) was replaced with a very-80's livery using white, gray, navy and burgundy.
By the time of the NW purchase, ORD-BNA and then ORD-GRB were also gone, so the only domestic route not serving MSP, MEM or DTW was MKE-LGA (also served nonstop by NW). The RC name disappeared in October of 1986 (if I recall correctly)
To most everyone's surprise, NW *retained* the Convair 580's for a few years after the merger even though they had abundant code-share lift at their disposal. Those disappeared in late 88 or 89 if I remember correctly.