I really love those phasmid feet

I've written about by pet Macleay's Spectre stick-insects (Extatosoma tiaratum) before, including moulting and images of cast skins, but had never taken a really close look. Having watched even large, heavy adult females clinging to the near-vertical smooth sides of one of my vivaria, I was interested in looking at the structure of their tarsi ('feet'). So, when a shed skin was left intact (the final moult of a female), rather than being eaten as they often are, it was rescued and taken to the microscope.

Tarsal segments - each has a pad or 'euplantula' (indicated by green lines), the final segment having a larger pad ('arolium') indicated by the red arrow, plus a pair of claws. The spiny segment top-right is the end of the tibia.

If you are familiar with these insects, you'll know how sharp their claws are and how good they are at hooking onto any tiny irregularity in whatever surface they are on. A closer look shows one reason why.

The final tarsal segment showing the claws and arolium

Side view of tarsal claw showing the hardened tip

Hardened and narrowed tip of a tarsal claw

 These photos show not only the hooked shape of the claws, but also that the hardened tip narrows to an even sharper point that can poke into the tiniest of crevices for grip, and undoubtedly pierce soft materials just enough to hold on. These structures also bear small bristles and so also have a sensory function - useful when moving about at night and feeling around in foliage. However, I doubt that the claws are the only gripping devices at work here. It is now well known that geckos' feet have a nano-structure that increases surface area hugely and allows them to adhere to smooth surfaces such as glass through van der Waals interactions with the substrate (Autumn et al. 2002). Given that the arolium can be seen to have an irregular, wrinkled surface at even the low magnification above (x40), it is worth zooming in a little more.

Two pairs of adhesive tarsal pads ('euplantulae') showing the wrinkled surface.

Close-up of one euplantula - the wrinkled surface is more clearly visible.

Upper surface of the arolium - slightly granular but not wrinkled, and bearing numerous bristles.

Lower surface of the arolium - finely wrinkled like the euplantulae, and bearing very few very small bristles.

These images suggest a sensory function for the upper surface of the arolium - the lower surface and the euplantulae are more-or-less bristle-free, and their wrinkled surfaces imply a structure that aids grip through increased surface area. Although I do not have access to a higher-powered (i.e. electron) microscope, others do and some species have been investigated further, with Bußhardt et al. (2012) describing, comparing and imaging the euplantae of Cuniculina impigra and Carausius morosus. Essentially they found that "smooth pads are specialized for rather smooth substrates, whereas nubby pads are better adapted to generate stronger forces on a broader range of surfaces". I will investigate further, but as far as I can tell, no-one has done equivalent work for E. tiaratum and I hypothesise that (at smaller scales than seen here) the pads are neither smooth nor nubby (small-bumpy), but instead are wrinkled, mostly longitudinally, which should generate adhesive forces on rough and smooth substrates, but not equally in all directions. Whether I will get to test this hypothesis is another matter!

References

Autumn, K., Sitti, M., Liang, Y.A., Peattie, A.M., Hansen, W.R., Sponberg, S., Kenny, T.W., Fearing, R., Israelachvili, J.N. & Full, R.J. (2002). Evidence for van der Waals adhesion in gecko setae. Proceedings of the National Academy of Sciences of the United States of America  99(19): 12252-12256.

Bußhard, P., Wolf, H. & Gorb, S.N. (2012). Adhesive and frictional properties of tarsal attachment pads in two species of stick insects (Phasmatodea) with smooth and nubby euplantulae. Zoology 115(3): 135-141.