[1][2] Spider venoms work on one of two fundamental principles; they are either neurotoxic (impairing the nervous system) or necrotic (dissolving tissues surrounding the bite).
Spiders in this family possess a known dermonecrotic agent sphingomyelinase D,[8][9] which is otherwise found only in a few pathogenic bacteria.
[10][11] Bites by spiders in this family can produce symptoms ranging from minor localized effects, to severe dermonecrotic lesions, up to and including severe systemic reactions including renal failure, and in some cases, death.
[12] Even in the absence of systemic effects, serious bites from sicariid spiders may form a necrotising ulcer that destroys soft tissue and may take months and very rarely years to heal, leaving deep scars.
Initially there may be no pain from a bite, but over time the wound may grow to 10 inches (25 cm) in extreme cases.
[13] Systemic effects are unusual but include mild nausea, vomiting, fever, rashes, and muscle and joint pain.
Rarely, more severe symptoms occur including red blood cell destruction (hemolysis), low platelets (thrombocytopenia), and loss of clotting factors (disseminated intravascular coagulation).
[19][20] The skin manifestations of recluse venom are thought to arise from Sphingomyelinase D. The enzyme acts on cell membranes.
Skin infections, in particular the widespread methicillin-resistant Staphylococcal aureus, remain swollen and red.
Spider venom toxicity can be evaluated in experimental animals, or reported from accidental bites.
Small children are considered an exception because the amount of venom dispersed throughout the body is many times the concentration in an adult.
There is at least one recorded case of a small child dying within 15 minutes of a bite from a Sydney funnel-web spider; that death occurred before the development of an antivenom.
** Several other kinds of tarantulas in the pet trade are regarded as giving non-trivial bites.
The effects of a full envenomation are probably unknown for many species of tarantulas, so due caution is advisable.