In the previous post I've defined reflex and gave some examples of it in the newborns. I've also said that heritable reactions can be further subdivided into reflexes and instincts. While the concept of reflex is now tractable, the concept of instinct demands further explanation.
Generally, people think of the instincts as the more-complex forms of the heritable behavior. Some scientists claim that one must view instincts as a complex or chain-reaction-like reflex. By that one means a certain kind of connection between the chain of reflexes whereby the response part of one reflex serves as an irritant for the next reflex. Then an insignificant impulse from the outside or some irritant can cause a complex sequence of actions and behaviors, connected among each other in such a way that every activity/action will automatically cause the next one to occur. Not unlike the dominos falling or a trigger wave in bistable or excitable media.
Such view, however, appears to be inadequate. First of all, it suggests that instinct exists in strongly-bounded and exact connection with the elements of the environment. Reflex is a unique, well defined and deterministic connection with the environment. On the contrary, instinct connection is less determined and freer.
Young squirrels, separated from their mothers from birth and always kept in a room (never being outside, never seeing the ground or the trees) and fed always by a human hand, during the Fall season begin to develop an instinct of collecting and storing food for the winter. Squirrel buries the nuts in the rug, sofa, armchair or collects them all in the conner of the room. Such conditions rule out the possibility of learning and negate all those elements of the environment that usually precede development of the instinct. Therefore, one must adopt a more extendable and pliable connection between the instinctive reaction and the environment.
Further, instinctual actions can never be divined and considered, completely, ahead of time; never forming an exact pattern and vary from instance to instance.
Thirdly, the uniqueness of the instinct is contained in the large complexity of the performed actions. While reflex, typically, causes activity of one organ; instinct, typically, causes a large number of cooperating activities of various organs. In other words, it's fair to say that reflex represents a reaction of one organ while instinct represents a reaction (behavior) of the whole organism.
To illustrate the distinction between the instinct and the reflex it's illuminating to look at the mating behavior of the headless flies. Headless flies are capable of mating but only under the condition of one headless fly and one normal fly. All the actions performed before the connection, which can not be considered and predicted ahead of time and that involve participation of various organs, are performed by the normal fly. But the actual act of mating even the headless fly is capable of performing. In this instance, experimental observations differentiate the behavior into reflexorial and instinctual forms. All the actions performed prior to connection must be attributed to the instinctual behavior tied to the workings of the head(nervous) centers. While the actual act of connection ends up being a simple reflex, not requiring the participation of the head(nervous) centers.