Although spiders have well-defi ned blood vessels, they lack capillaries
and have few veins; thus, their circulatory system is basically
open. The heart is suspended in the pericardial sinus, and blood
enters the heart through paired slits called ostia, which open when
the heart is at rest. The heart primarily pumps hemolymph from the
abdomen forward through the aorta into the cephalothorax, supplying
oxygen to the central nervous system and the skeletal muscles. Once
depleted of oxygen, the fl uid passes into two sinuses, which lead to
the base of the abdomen where the fl uid is reoxygenated by the book
lungs and (if present) by tubular tracheae before pressure pulls it
through pulmonary veins back into the pericardial sinus.
Unlike most insects, spider hemolymph has an oxygen-carrying
pigment, hemocyanin. Hemocyanin is structurally similar to hemoglobin,
but instead of iron it uses copper as its oxygen-binding metal,
which can make spider hemolymph appear bluish green. Compared
to hemoglobin, hemocyanin is less effi cient ( 5%) in oxygen transportation
and is not concentrated in specialized cells.