The insects appear to be aphids. They have piercing/sucking mouthparts, and they feed on the lettuce. The aphids will secrete a shiny, sticky substance, called honeydew, on the lettuce. When populations increase, you will notice a large number of shed skins stuck in the honeydew or on the foliage. Eventually, sooty mold may grow on the honeydew and cause the lettuce to have a gray-to-black discoloration. Sooty mold is a fungus that lives off the sugars in the honeydew. Sooty mold does not harm the plant, other than potentially reducing its photosynthetic capabilities.