They are the pair of tubes between the bottom bracket and the rear wheel dropouts.
Even for these tubes it is preferred carbon fiber solution more for aesthetic reasons than structural needs. Longer chain stays move the cyclist backwards by making the bike more comfortable, the other way for a bicycle ready to shoot.
Sources:
Fonti:
http://www.cyclinside.it/web/category/componenti/
http://www.albanesi.it/corsa/ciclismo.htm