  In this work we describe upgrades to the Spider balloon-borne telescope in
preparation for its second flight, currently planned for December 2021. The
Spider instrument is optimized to search for a primordial B-mode polarization
signature in the cosmic microwave background at degree angular scales. During
its first flight in 2015, Spider mapped ~10% of the sky at 95 and 150 GHz. The
payload for the second Antarctic flight will incorporate three new 280 GHz
receivers alongside three refurbished 95- and 150 GHz receivers from Spider's
first flight. In this work we discuss the design and characterization of these
new receivers, which employ over 1500 feedhorn-coupled transition-edge sensors.
We describe pre-flight laboratory measurements of detector properties, and the
optical performance of completed receivers. These receivers will map a wide
area of the sky at 280 GHz, providing new information on polarized Galactic
dust emission that will help to separate it from the cosmological signal.
