The law does not cover experiments on invertebrates, so insects and worms are not given legal protection. Some of the most important recent advances in biology relevant to medicine have come from studies of fruit flies and microscopic worms. The genes that are important in the normal development of flies from eggs and larvae are similar to genes in humans that cause diseases such as cancer. Scientists can produce transgenic flies in a fraction of the time and at a fraction of the cost of making transgenic mice. Consequently, they will be increasingly used in medical research.
People come to very different conclusions if the suffering is caused in order to help humans.
The answer depends on how we view animals and humans. There are three conflicting contemporary views. By that argument, animal experiments are just as offensive as racism or sexism. It is purely cruel treatment driven by prejudice. The opposing view is that humans own animals, which are intrinsically inferior.