Because the fly is an arthropod in invertebrates, its evolution is relatively low, and its brain is just a slightly developed ganglion, which can't completely control the metabolism of the whole organism, but it still has certain regulatory ability. When its head is cut off, the ganglion in the lower part of the brain can continue to control the most basic life activities of the organism, so it won't die for the time being. Someone did an experiment to cut off the head of Pieris rapae, and it lived for more than a week (finally starved to death). In fact, not only flies do this, but arthropods can basically survive for a while after losing their heads.

The nerve center of flies, in addition to the brain located in the head, also has the ganglia on the ventral nerve cord. These ganglia also have functions similar to those of the brain, which can perform the functions of movement and sensation. Therefore, removing the chest and abdomen of the fly can also perform motor and sensory functions. This is related to the fact that the nervous system of flies is not highly concentrated in the brain. Moreover, removing the head of the fly will not cause massive bleeding of the fly (because the fly is an animal with open tube circulation), so removing the head will not cause the immediate death of the fly.

Although the fly has lost its head, it can move as long as its internal organs are not damaged, that is, the spinal cord is intact. The nerve center of flies, in addition to the brain located in the head, also has the ganglia on the ventral nerve cord. These ganglia also have functions similar to those of the brain, which can perform the functions of movement and sensation. Therefore, removing the chest and abdomen of the fly can also perform motor and sensory functions. This is related to the fact that the nervous system of flies is not highly concentrated in the brain. And removing the head of the fly will not cause massive bleeding of the fly (because the fly is an animal with open tube circulation), so removing the head will not cause the immediate death of the fly.