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.

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.



What are the major components of spider haemolymph?
Spider 'blood' is just as different from its mammalian equivalent as the circulatory apparatus is. A more correct name for it is haemolymph because it has many features in common with the lymph that diffuses through human tissue spaces before being returned to the circulatory system. It is not red because it does not contain the oxygen-carrying pigment, haemoglobin. Instead, it is a pale blue colour due to the presence in it of haemocyanin, an oxygen-carrying molecule that is blue because it contains copper rather than iron as found in haemoglobin. Both are oxygen-carrying proteins but haemoblobin is packed into cells called erythrocytes whereas haemocyanin is simply dissolved in the haemolymph despite its large molecular size (1,700,000 daltons compared with 66,000 daltons for haemoglobin). In addition, haemoglobin can carry about 17 times as much oxygen as haemocyanin. The haemolymph of a typical spider also contains some cells but it is far less cellular than human blood, which is about 45 percent cells by volume.

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.



What anatomical structures do spiders use to accumulate oxygen?
Mammals obtain oxygen from the surrounding air by placing blood cells in very close proximity to air that has been drawn into and out of the lungs in a cyclic fashion. This provides a very large gas exchange area so oxygen can enter the blood efficiently and carbon dioxide can leave it with equal ease. While spiders have relatively less need for oxygen than warm-blooded mammals, they cannot survive indefinitely without it and are also first anaesthetized and eventually killed by high concentrations of carbon dioxide. In spiders there are no sponge-like lungs, their equivalent being one or two pairs of book lungs which have a close resemblance to the gills of fish.

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.

To what extent are breathing and the circulation of blood under the control of a spider's nervous system?
Comparatively little research appears to have been done on the circulation of blood in a spider and even less on the regulation of gas exchange within its respiratory apparatus. There is no convincing evidence that air flow in either the book lungs or the tracheae is regulated by deliberate neuromuscular activity but some passive air exchange undoubtedly does occur and the spider may be able to vary the amount of haemolymph perfusing them when necessary. On the other hand, there is research data that suggests there is a circulatory centre within the brain, at least in tarantulas. This centre exerts neural control over a cardiac ganglion located in the first segment of the spider heart. Stimulation of the nerves to this ganglion sometimes leads to either an increase or a decrease in heart rate, suggesting the circulatory centre may have both cardioaccerator and cardioinhibitor areas.

Some related sources of information
The pages on spider movements, growth and reproduction, and silk production contain some information that is related to what is covered in the above paragraphs. In addition, the following articles are worth reading:

Perry D.A. and Brown R.H.J. (1959) "The hydraulic mechanism of the spider leg" J. Experimental Biology,36, 654-664

Zentner L., Petkun S. and Blickhan R. (2000) "From the spider leg to a hydraulic device" Technische Mechanik, 20, 21-29

Gonzalez-Fernandez F. and Sherman R.G. (2005) "Cardioregulatory nerves in the spider Eurypelma marxi Simon" J. Experimental Zoology, 231, 27-37

Schmitz A. and Perry S.F. (2001) "Bimodal breathing in jumping spiders: morphometric partitioning of the lungs and tracheae in Salticus scenicus (Arachnida, Araneae, Salticidae)" J. Experimental Biology, 204, 4321-4334.