Black Bugs on Lettuce
I am fairly certain those are aphids, although I can't zoom in on the photo quite as much as I'd like to. Because of the darker color, and the fact that they're on lettuce, they might be bean aphids (which do attack lettuce as well as beans). I can't tell if they have white and black legs, like bean aphids do. Can you see if these bugs have two little spikes on their back ends?
Here are some resources on aphids:
Aphids article from U. of Wisconsin-Madison Vegetable Crop Entomology
Pests of Lettuce from NC State Extension
Aphids – Vegetables from U. of Maryland Extension
The last resource from the U. of Maryland says the following about management:
- Do not plant infested transplants.
- Aphids are mainly a problem May through June but have many natural enemies (e.g. ladybird beetles, lacewings, syrphid fly, parasitic wasps) that keep numbers controlled. Look for brown, swollen parasitized bodies.
- Encourage predators with attractant plants and avoid toxic pesticides.
- Control low aphid infestations with a robust spray of water.
- If damage is obvious and predators and parasitoids few, use insecticidal soap. Check product label for directions before spraying. And avoid spraying when temperatures are above 85 degrees F."