The for use are of a yellow colour

Henry Wade, Rod-Fishing in clear waters by Fly, Minnow, and Worm, Londres, Bell & Daldy, 1860, p.104-105.

The Cod-bait, Cad-bait, Cadis or Case-worm, sometimes called Ruffcoats or Straw-worms. These baits are of three different kinds. The first is found under stones that lie loose and hollow in small brooks, shallow rivers, or very fine gravel, in case or husk, and when the best for use are of a yellow colour, rather biger than a gentil, with a black head. The second is a green sort, found in pits,ponds, and ditches, in still water among rushes, water-weeds, straw, &. and hence called the Straw-worm…..