Is the antibiotic cephlaxin good for poison spider / insect bites? if not what is?

Yes and no: A poisonous insect bite that is not superinfected with bacterial infection will not be helped by antibiotic treatment. If there is bacterial superinfection with streptococcus sp. Or methicillin sensitive staph. Aureus it will improve with cephalexin(keflex) treatment. This antibiotic is not active against mrsa.

No.: Cephalexin (and all other antibiotics) are prescribed for bacterial infections. Bug bites do not respond to antibiotics unless there is also an infection present, which does sometimes happen if bacteria gets into the bite. Most bites do well with a mild cortisone cream and antihistamines for the itching, but if pain or significant tenderness develops, see your doctor.