Mobility of Spiders

On this page the probable mechanisms for walking and other activities of the limbs of spiders are examined.

Almost all spiders have walking legs composed of seven segments and the joints between most of these are hinge-like with a soft membrane covering a fluid space on the flexion side of the joint. This means the leg can bend in only one direction and pressurizing the haemolymph in the leg will extend it in the same way that inflating a long cylindrical balloon will straighten it. When this pressure is reduced the elastic elements of the joint might then draw the leg back into a flexed posture, which is what is normally seen when a spider dies suddenly or decides to 'play dead' in order to deceive a predator.

The mechanism for hydraulic extension of spider legs during walking was presumed to involve brief compression of the cephalothorax by its own internal muscles, perhaps even the same ones that operate the sucking stomach. On most species it is possible to see the muscle insertion points for at least some of these muscles, including the fovea on the centreline of the carapace and the six (normally) pits called sigilla which form an oval on the sternum.

But does this hydraulic theory adequately account for the known leg movements of a typical spider? In support of this explanation is the fact that extending legs as long and thin as those of a pholcid (daddy-long-legs) or tetragnathid spider by use of tendons appears likely to be much less efficient than simply straightening them by hydraulic action. In addition, it has been shown that there are changes in the haemolymph pressures within the cephalothorax whenever a spider becomes active. On the other hand, this increase in pressure reduces the ability of the spider heart to perfuse the cephalothorax with oxygenated haemolymph at a time when it might actually need more oxygen there to ensure the nervous and sensory systems are adequately supplied.

But is there evidence that spider leg movements are NOT controlled simply by a hydraulic system? Yes, there certainly is. Some researchers have claimed that they have managed to make electrophysiological recordings from nerves linking a spider's nervous system to muscles in its legs during movements. In addition, the following observations support the belief that the movement of a spider's limbs must involve at least some muscle activity:

(1) Many spiders rest for long periods of time with all legs extended. If leg extension requires pressurization of the leg haemolymph this seems to be very energy-intensive, although it is possible the extended leg can be held in place by the claws or hairs at the ends of the legs, these gripping onto any crevices on the surface on which the spider is resting. But then of course the spider needs a mechanism for releasing these anchorages when it decides to move its legs. Are there minute valves in the coxae of the legs that allow the pressure to be maintained in the leg without continuous effort by the haemolymph-pressurizing muscles and does the nervous system regulate these valves? No one seems to be sure of the answer to this question.

(2) It also seems likely that more than just a fall in limb fluid pressure is required to cause the sustained leg flexion needed in order to be able to hold onto a twig during strong winds or to prevent the loss of a relatively large insect that is struggling to escape.

(3) Perhaps the strongest evidence in support of the suggestion that the leg movements of spiders are at least partly regulated by neuromuscular mechanisms is the fact that not all legs are extended at the same time when a spider is walking.

On the contrary, careful observation of running spiders and the use of high speed movie cameras have established that the equivalent legs on the two sides of a spider normally move in an alternate fashion. Thus, when Leg 4 on the left side is flexing Leg 4 on the right side is extending, then shortly afterwards the reverse is true. It is also known that on one side of the body Legs 1 and 3 will generally be flexing at almost the same time whereas Legs 2 and 4 on that side will be extending. Conversely, on the other side of the body Legs 1 and 3 will be extending and Legs 2 and 4 will be flexing. This means a spider's legs work like two sets of legs on a horse arranged in tandem but with the equivalent legs operating in the opposite phase at any moment. On the Oricom Technologies website it is stated that at least for the Central American ctenid species, Cupiennius salei, the legs on either side of the body move in the order 4-2-3-1 most of the time but on some occasions change to either 4-1-3-2 or 4-3-1-2.

For some spiders the legs appear to rotate forwards in the extended or extending state along the spider's midline axis then flex as the body moves forward past the point where they are in contact with the surface on which the spider is walking. If this is the case then it can be argued that there must be muscles that swivel each leg forward and that simultaneous flexing of some other legs lifts the body enough to allow this leg rotation and the flexed legs are then straightened by hydraulic means to lever the body further forward. Another important role of the first two pairs of leg may be to lift the cephalothorax so the third and fourth pairs of legs can more readily push the spider forward.

But could these leg rotations be achieved without the need for leg muscle involvement? The diagrams in the following graphic are an attempt to show how this might be possible. Spiders have claws and sometimes claw tufts or scopulae (brushes) on the ends of their legs and these allow them to briefly grip the surface the spider is walking on. If when a spider is running only some of the legs are driving at a particular instant these will move the spider's body past the anchorage points of those legs that are not driving, causing the latter legs to flex and to rotate in a fore-to-aft manner. Subsequent repressurising of the flexed, rotated legs may then allow them to contribute forwards force in turn.

Presumably, this mechanism will mostly be slightly different for Leg 1 and perhaps Leg 3 (at least compared with Leg 4) since these must extend forward, anchor onto the surface and then flex as the spider's body catches up. In other words, the first two pairs of legs extend forward then drag the body along by flexing while the last pairs of legs flex forward then push the body forward by extending. In any case, if the phasing of such leg rotations is not the same for all eight legs at any moment the spider might be able to walk or run in a straight line, though perhaps with a slightly jerky action, using only cyclic changes in haemolymph pressure in its legs.

These suggested mechanisms are probably somewhat modified but even more pronounced in sparassid and thomisid spiders that have laterigrade (legs rotated sideways in a crab-like fashion) limbs and there may be a number of other spiders that display rather different walking patterns. For example, some spiders will rock back on Legs 3 and 4 and use the first two pairs of legs in a sensory fashion somewhat like the way people use their arms to feel their way around when in a dark room. While these movements may be accounted for entirely by a hydraulic mechanism the nervous system must surely still play a role in the sequencing of these tactile movements. Indeed, Ernst-August Seyfarth (see reference below) even claims to have carried out experiments on Cupiennius salei that showed that "stimulation of tactile hairs evokes reflex activity in several leg muscles. Coordinated contraction of these muscles raises the body - as in doing push-ups." As shown in the following graphic, the author of this Find-a-spider website also had no difficulty in finding muscle bundles in the larger leg segments of the Australian tarantula, Selenocosmia stirlingi.



Thus in summary, there is still uncertainty as to how spiders move their legs but the presently available data indicates that