It is not just female black widows consuming their male mates after copulation, but young black widow siblings seem programmed to try to kill each other. Researchers have discovered that black widow mothers produce egg sacs that are all the same size, and black widow spiderlings all grow at the same rate, making it impossible for the spiderling to be larger in size then its brother spiderling. This size similarity may be due to an evolutionary development aimed at preventing cannibalism by preventing one black widow sibling from becoming larger, and therefore predatory, towards the other siblings.
Researchers placed two different sized black widows siblings together in order to observe their behavior. Of course, the larger black widow consumed its smaller sibling every time. For young black widows procuring their own food can be difficult since they are still too small to catch a fly, so they will often feed on spare food leftover by the mother or they will be forced to eat their sibling.
Do you think black widows would still eat each other if they were placed together in an environment where food sources were plentiful?