The silk then hardens into a thread. Respiratory system Spiders have 2 kinds of breathing organs-tracheae and book lungs . Tracheae, found in almost all kinds of true spiders, are small tubes which carry air to the body tissues. A spiracles is an opening in front of the spinnerets in most true spiders. Book lungs are in cavities in the spider's abdomen.
Air enters the cavities through a tiny slit on each side and near the front of the abdomen.
Each lung consist of 15 or more thin, flat folds of tissue arranged like the blood vessels. As air circulates between the sheets, oxygen passes into the blood. Tarantulas have two pairs of book lungs. Most true spiders have 1 pair. The blood of spiders contains many pale blood cells and is slightly bluish in color.