ARACHNIDS

The class Arachnida contains such animals as spiders, ticks and mites. These animals all have similar morphologic characteristics and can be readily differentiated from their close relatives, the insects, because of these characteristics (below).

In Aracahnids:

The body is divided into one or two notable parts.
Adults have 4 pairs of legs (larvae have three).
They lack wings and antennae.
The males are smaller in size than the females.

There are several venomous spiders that occur in most areas of the United States.

VENOMOUS SPIDERS

BLACK WIDOW - Latrodectus mactans (below)

This black spider can be easily recognized by the red hour-glass shaped marking on the ventral abdomen. It is a secretive spider that builds a poor web. Encounters with people or animals are usually accidental and occur around the home or garage. Pain, resulting from the bite, is felt almost immediately and increases in intensity, reaching a maximum in 1 to 3 hours and continuing for up to 48 hours. Rigidity and spasms of most large muscle groups, particularly those of the abdomen, are common symptoms. There is usually a slight rise in body temperature, increased blood pressure, a leucocytosis, and an increase in spinal fluid pressure. The patient often becomes dizzy, weak, thirsty, nauseated and shows symptoms of shock. Local treatment is generally ineffective and incision and suction are of little value. Methocarbamol (1 gm in 10 ml saline), 10 ml I.V. followed by 10 ml in I.V. drip of 5% dextrose is often used to control muscle spasms and pain. Intravenous administration of 10 ml of 10% calcium gluconate is also used. Cortisone has also been reported to provide some relief. Mortalities are generally low.

FIDDLE-BACKED - Loxosceles reclusa(below, left)

This is another secretive spider that usually is encountered accidentally in common locations including the home, barn or garage.. It can be recognized by the dark brown fiddle-shaped marking on the dorsal cephalothorax. The bite of this spider is localized and produces considerable necrosis which may lead to an unsightly scar. The bite site usualy apears as a raised area with necrosis of tissue at the immediate site of the bite (above, right). The necrotic area turns black and dry and may slough in a few days, leaving a granular area. Healing usually take place slowly and secondary infection is common. Painful edema and erythema may develop within an hour of the initial bite. Systemic involvement may also occur, being characterized by anemia, hematuria, fever, cyanosis and occasionally convulsions. Early administration of hydrocortisone (200 to 400 mg I.V.) followed by oral prednisone (40 to 60 mg) for 4 to 7 days is effective in reducing clinical signs. Recovery generally provides a solid immunity.

There are several other common spiders that are often confused with those that are venomous. Although their bites may be painful, they are not considered venomous. These include the wolf spider (below, left) and the tarantula (below, right).