Like humans, dogs and cats get bitten and stung by insects; especially in the warmer months. This may yield little effect, or could be an emergency situation, depending on the venom, the pet's allergic reaction, and the location of the bite (swelling in the neck and throat region could cause severe difficulty breathing). Dogs most commonly have swelling of the face and muzzle from snapping at insects with their mouth, cats more often suffer from swollen paws from batting at insects.
After the initial pain from the sting or bite, the allergic swelling that results isn't usually too painful. The swelling is usually sudden and dramatic and may itch. Conversely, infections (i.e. bacterial) typically are painful. The swelling needs to be monitored closely to make sure the animal can breathe and eat properly and that the skin isn't too "tight", eventually suffering cell death (necrosis) and secondary infection. If spiders are a concern in your location, please check with local health officials to learn what the signs and symptoms could be for spiders in your location.
Benadryl® is a great emergency drug for allergic reactions related to insect bites and stings. Relatively safe, it can be used in most pets, children and adults to calm the allergic reaction and possibly advert an emergency situation due to extreme swelling. Caution advised for pregnant and nursing animals. The most commonly seen side effect is drowsiness.
While a fairly safe drug, does have potential adverse effects and is not for every pet. Benadryl is to be used with extreme caution in patients with glaucoma, prostatic disease, cardiovascular disease, and hyperthyroid, among other conditions. It may also interact with drugs that your pet is taking, so be sure to discuss use of ANY drugs for your pet with your vet first. (Sidenote: some human over-the-counter drugs are toxic or fatal to pets; always discuss with your vet before "self-medicating" for any condition.
Speak to your veterinarian if Benadryl® is a drug that would be appropriate for your pet should the need arise (i.e. while camping). If so, your veterinarian should be able to give you an idea of safe dosage range based on the weight, age, condition of your pet.
Reference 1Diphenhydramine HCl/Benadryl® reference: Veterinary Drug Handbook, 3rd. Edition, Donald C. Plumb, pp 248-250