The second and third cysteine are absent and present together (after alignment),
so they form the bridge.
ASDFGCHIKLMCNPQRSCTVW YSDYGCNIKLFCQPQRSCTWW ATDYPVQIKLMCNPQKSCSMW YTDFGCHVKLLVQPNRSVTVW -TDFGVHVKLMCNPQKSCSFW |
Cysteine is a dangerous residue to have unbridged because of its reactivity and
metal binding skills. So, if during evolution one Cys of a bridge accidentally
mutates, then the whole species only staands a chance to survive if the other
one accidentally mutates quickly too.
The cysteines at position 6 probably have some function or the other. But given that two
of the species don't even have this Cys, that function is not overly crucial.