If you suffer food poisoning or otherwise fall ill because of the insect in your food, then you may be able to sue the restaurant.  However, you must show that your illness was caused by the insect in your food in order to recover damages for food poisoning.  Illness or injury arising from the mere presence of an insect in one's food is rare and extremely difficult to prove.

What if I Suffer Emotional Distress from Seeing an Insect in My Food?

You may not recover damages for emotional distress without any evidence of physical injury.  Moreover, many lawyers are hesitant to take a case based solely on emotional distress because the amount of damage can be relatively small.

Do I Need a Personal Injury Lawyer?

Generally finding an insect in your food does not give you a basis for bringing a lawsuit.
However, if you have suffered food poisoning as a result of an insect or bug in your food, seek immediate medical attention and then consult an experienced products liability attorney.  An experienced attorney can help you recover your costs associated with the poisoning, such as lost wages and medical expenses.