I draw the line at sentience because, as I have argued, sentient beings have interests and the possession of interests is the necessary and sufficient condition for membership in the moral community. Are insects sentient? Are they conscious beings with minds that experience pain and pleasure? I do not know. But the fact that I do not know exactly where to draw the line, or perhaps find drawing the line difficult, does not relieve me of the obligation to draw the line somewhere or allow me to use animals as I please. Although I may not know whether insects are sentient, I do know that cows, pigs, chickens, chimpanzees, horses, deer, dogs, cats, and mice are sentient. Indeed, it is now widely accepted that fish are sentient. So the fact that I do not know on what side of the line to place insects does not relieve me of my moral obligation to the animals whom I do know are sentient.
As a general matter, this question is intended to demonstrate that if we do not know where to draw the line in a matter of morality, or if line drawing is difficult, then we ought not to draw the line anywhere. This form of reasoning is invalid. Consider the following example. There is a great deal of disagreement about the scope and extent of human rights. Some people argue that health care and education are fundamental rights that a civilized government should provide to everyone; some people argue that health care and education are commodities like any other, not the subject of rights, and that people ought to pay for them. But we would, I suspect, all agree that whatever our disagreements about human rights–however unsure we are of where to draw the line–we most certainly agree, for instance, that genocide is morally wrong. We do not say that it is morally acceptable to kill off entire populations because we may disagree over whether humans are entitled to health care. Similarly, our uncertainty or disagreement regarding the sentience of ants is no license to ignore the interests of chimpanzees, cows, pigs, chickens, and other animals whom we do know are sentient.