A lot of spots on the arm may be related to mosquito bites, contact dermatitis, eczema and so on. 1. Mosquito bites: may be due to the arm is exposed outside, by the mosquito bite, its mouthparts carry toxins, so cause spots on the arm. 2. Contact dermatitis: may be in the field activities, bare skin contact with some plants, causing localized inflammation, the appearance of red spots, and in severe cases there will be blisters. 3. Eczema: Inflammation of the superficial dermis and epidermis caused by a variety of internal and external factors, such as pollen and animal hair. Red spots appear, usually accompanied by itching. A lot of spots on the arm can also be seen in other causes, it is recommended to consult a doctor in a timely manner, follow the doctor’s instructions for relevant examinations, with the help of the doctor to clarify the cause, and targeted treatment or treatment.