Many spiders are known to ‘walk on water‘ (including dock spiders): spiders are small enough that many species and life stages can be held by the meniscus of water.  Spiders also have eight legs but they often lose one or more of their legs (in the scientific jargon, this is ‘leg autotomy‘).   In general, this is often part of defensive behaviour, and is common in many animals.  Sacrificing an appendage is a better idea than being eaten by a predator.

So… let’s link these thoughts together- spiders run on land as well as water, and they are often missing a leg.

A wolf spider (Pardoa mackenziana). In this photo, a leg that was previously lost has been ‘re-grown’ (4th leg on right side). The cost of this spider’s lost leg must have been minimal, since it survived and moulted again!

So, next comes the research question:   what is the ‘cost’ of leg autotomy and does this cost vary depending on whether the spider is traveling on the land or on water.  This is an interesting question, and one that was addressed directly by Christopher Brown and Daniel Formanowicz Jr in a recent research paper in the Journal of Arachnology.   These authors used the wolf spider Pardosa valens as their model species, and conducted ‘speed trials’ for male and female spiders on a terrestrial track as well as an aquatic track (i.e., these were constructed in a laboratory setting).  After doing the trials with intact spiders, the authors ‘induced autotomy’ (yes, this sounds somewhat horrific, but autotomy is very common with wolf spiders, and although they lose a little of their hemolymph, they heal quickly) and ran the trials again, with the same spiders (sans legs).

Results?  Well, perhaps not surprisingly, in the first year of their study, the species ran more slowly when they were missing a leg, but in the second year of the study, this affect varied by sex (males were slower, and autotomy only affected the females).   They report some rather complicated results when comparing terrestrial to aquatic trials, but in general, the spiders tended to run more slowly on the aquatic tracks when they were missing legs.  Again, this is perhaps not a surprising result, since having seven instead of eight legs will certainly change the biomechanics when considering how the spiders interacts with the meniscus of water.

Clearly, there are some costs associated with missing legs, but it is important to note that even without legs, these wolf spiders were able to run effectively on land and water, and even if their speed was slower than when they had all eight legs, they can still move an impressive speeds.  The range of speeds in some of the trials was between  20 cm/s and 50 cm/s – this translates to  running speeds between 0.72 and 1.8 km per hour!

Leg autotomy in wolf spiders in natural habitats range from between 8% and 32% as reported by Brueseke et al. in 2001 and by Apontes & Brown in 2005.  In the present study, the authors state that natural populations of P. valens exhibit between 25% and 45% autotomy.    These numbers are in line what what I have observed, as well.  This is pretty amazing – wolf spiders exhibit leg autotomy at a very high frequency, and in some cases, half the spiders in a population are missing a leg.  What can we infer from this?   Although there are some costs associated with leg autotomy (as reported by Brown and Formanowicz), they must not be that high – otherwise, natural selection certainly wouldn’t have favoured autotomy as a means to escape predation.  Brueseke et al., research supports this as they found very few costs associated with autotomy in Pardosa milvina.  In their work, Brueseke et al. studied locomotory behaviour as well as prey capture, and found overall support for the ‘spare leg hypothesis’ (i.e., look at all of my legs!  I can manage without one!).

So, here are the take-home messages:

Wolf spiders can run quite quickly, some species can run across water and land, and they can do so with missing legs.  Although they may be a little slower without their full complement of legs, the costs must be relatively minor given the frequency of leg autotomy in wolf spiders. 

This gives you more reasons to watch spiders – count some legs and see how many individuals are without their full complement of legs.

References:

Apontes, P., & Brown, C.A. (2005). Between-set variation in running speed and a potential cost of leg autotomy in the wolf spider Pirata sedentarius. American Midland Naturalist, 154, 115-125 DOI: 10.1674/0003-0031(2005)154[0115:BVIRSA]2.0.CO;2

Brown, C.M., & Formanowicz Jr, D.R. (2012). The effect of leg autotomy on terrestrial and aquatic locomation in the wolf spider Pardosa valens (Araneae: Lycosidae). Journal of Arachnology, 40, 234-239 DOI: 10.1636/Hill-59.1

Brueseke, M.A., Rypstra, A.L., Walker, S.E., & Persons M.H. (2001). Leg autotomy in the wolf spider Pardosa milvina: a common phenomenon with few apparent costs. America Midland Naturalist, 146, 153-160 DOI: 10.1674/0003-0031(2001)146[0153:LAITWS]2.0.CO;2