Circulation of Blood in Spiders
This page examines the roles and importance of circulating blood and the mechanisms for oxygen acquisition within the body of a spider.
Many people assume that spiders have closed blood circulation and respiratory systems not much different from that of humans except perhaps in size. As the following paragraphs will show, this assumption is largely invalid but there are many parallels between the apparatus used by spiders and the equivalent human cardiopulmonary apparatus. It should also be noted that the circulatory and respiratory systems of only a few spider species have been studied so far so what is stated below may not be entirely true for every species.
What kind of circulatory structures are found in the bodies of spiders?
In mammals the circulatory system is an entirely closed double circuit, blood flowing in an alternating fashion first through the blood vessels of the lungs and then through the larger vascular network that supplies the rest of the body. The mammalian heart is actually a double pump that ensures that blood normally does not accumulate in either the lungs or the rest of the body but perfuses all tissues to the extent necessary to keep them alive and functioning normally at a constant body temperature. Spiders have a very different vascular system. Firstly, it is an open network which means its arteries carry haemolymph, the arthropod equivalent of mammalian blood, out into the tissue spaces where it diffuses past individual cells before being collected back into the heart. There are few, if any, veins in this system and definitely no capillaries.
The spider heart is actually a simple, moderately muscular tube located not in the cephalothorax but just under the upper surface of the
abdomen and running along the midline. It can actually be seen in many species
that have a pale coloured abdominal cuticle. A thin membrane which is the equivalent of the mammalian pericardium encloses it. The heart
pumps much of its haemolymph forward through the pedicel and into the cephalothorax using a large artery that some authors refer to as the
anterior aorta. There is also the equivalent of a posterior aorta that delivers oxygenated haemolymph to those abdominal organs that need it. To
ensure one-way fluid flow a simple valve may be present at the beginning of each of these major arteries. In those spiders that have been
studied so far the heart rate seems to be somewhere in the range 30 - 200 beats per minute, depending on the species involved and on the
extent to which it is active.
Return of haemolymph to the heart mostly is by simple negative pressure as the heart relaxes between beats. However, haemolymph that has been pumped into the cephalothorax is believed to be driven back through special channels in the pedicel when the pressure in the cephalothorax is higher than that in the abdomen and this fluid then collects in spaces called lacunae before perfusing the many lamellae (individual leaves) of the book lungs and being drawn up through the tissue spaces to the heart, which it enters through a number of small holes called ostia. This secondary pumping action is appropriate in that there are muscles in the cephalothorax that compress and expand it during leg movements.
Each compression of the cephalothorax will therefore drive oxygen-depleted haemolymph
backwards through the pedicle and into the gas exchange areas of the book lungs before returning it to the heart. It is a curious fact that
vigorous leg activities tend to cause slowing of the heart rather than an increased heart rate and the proposed explanation for this is that
increases in haemolymph pressures within the cephalothorax cause a greater flow of fluid back into the abdomen and inhibit the flow from
abdomen to cephalothorax via the anterior aorta. The heart then compensates for these changed pressure gradients by slowing down. As is
explained below there is evidence that tarantulas possess a cardioregulatory area in the spider brain that may help regulate
these heart rate changes.
So what role do the cells in haemolymph play if they are not for carrying oxygen around the spider's body? Well, the available evidence
indicates that some of them help minimize bleeding from small injuries such as a lost leg and thereby also promote healing. In this respect
they probably work in a manner similar to that of the platelets in our blood or the thrombocytes of vertebrates other than mammals. It is for
this reason spiders can lose a leg or two without dying, although damage to the more fragile abdomen is almost always rapidly lethal.
There are also some cells that function as phagocytes and perhaps as a primitive but still highly effective immune system.
There is no cyclic movement of air over the individual 'leaves' (lamellae) of the book lungs but there are
many of these in each book lung and they are well perfused by haemolymph so the total area for exchange of oxygen and carbon dioxide is quite
large. In those species that have only one pair of book lungs the second pair are believed to have gradually changed into a system of
fine tubes called tracheae, these having similarities with the trachea and bronchial system of the human lungs although they are kept open by
chitin rather than by cartilage as in mammals. They only have one or two openings called spiracles, these being located on the
underside of the abdomen between the spinnerets and the book lungs. Unlike the lungs of mammals the tracheal system of spiders does not have
cyclic exchanging of the air it contains. There may be no deliberate breathing movements but other movements the spider makes will cause
some incidental replacement of the tracheal air.
What advantages do tracheae have over book lungs? The answer to this question varies with the size, behaviour and habitat of each spider
species. Tracheae allow better direction of oxygenated haemolymph to those structures that need it most. Hence, tracheae that extend forwards
through the pedicel provide an efficient oxygen supply for the spider brain and those species that have them in the cephalothorax
tend to have significantly reduced maximum heart rates. Surprisingly, it appears that not more than ten
percent of a spider's tracheal tubes are in the cephalothorax and they do not penetrate the muscles that allow cyclic compression of it
although in web-monitoring spiders such as the uloborids they do enter the first segments of the legs. There are two other
advantages that tracheoles have over book lungs: they are better for water conservation in spiders such as the salticids that are active
during the daylight hours, and they also can store a small but significant amount of air when the spiracles are closed.
Email Ron Atkinson for more information. Last updated 30 December 2009.