Help understanding the control of an experiment?

Question

I understand the general idea of the control but I'm a bit hesitant on whether my answers to the following situations are correct. Please tell me if my definition of the control in each of these situations are correct. If not, a definition of the control and a brief exp. is appreciated. I'm not looking for someone to do my homework, I just want a full and complete understanding of a control. Thanks a lot! ;)

1. Hypothesis: Female roaches are too large to fly. To test this, scientists select 50 adult male and female roaches with a range of sizes (body length and weight) and then record the ability to fly.

Would the control be male roaches?

2. Frogs captured from polluted ponds. The investigator records the number of limb deformities.

I guessed the control is frogs captured from non-polluted ponds.

3. Cactus plants with different densities of cactus spines are observed for the number of holes made by herbivores.

I guessed cactus with no spines.

Answer 1

You are on the right track. Many experiments have both positive and negative controls. Positive controls assure that the experiment can work; in the first case, male cockroaches are the positive control because they prove that cockroaches can fly. Yes, #2 the control is a frog from a non-polluted pond; if it is deformed as often as the polluted pond, you have no variation and the hypothesis is unsupported; the deformities are coming from something else.

#3, cactus with no spines is a negative control for spine effect.  

Answer 2

It seems like you understand the concept of a control.

A control is an experiment performed as a comparison, in which the factor being tested is not applied/present, allowing the hypothesis to be proved positive or negative.

I can say that you have guessed the correct control for the 1st experiment and by the looks of it the 2nd and 3rd although its not possible to say 100% for those two since they lack a hypotheses.

If the hypothesis for 2. is that frogs from polluted ponds will have more limb deformities then the control you have suggested is correct.

If the hypothesis for 3 is that cactus plants with a high density of spines will have fewer/more holes made by herbivores then the control suggested is correct.

Good luck with you studies. <3