Muscle spasms or stiffness can result from a
toxic reaction to the venom of an insect or spider.
This can be caused by a poisonous insect or spider or by multiple bites or
stings from an insect or spider that is less toxic. A toxic reaction is most
common with a
black widow spider 
bite.
A muscle spasm, such as a charley horse, can occur for a variety of
reasons unrelated to insect or spider bites. Most of the reasons are not
serious.
