Sunday, April 26, 2009

Predatory caterpillars

Recently, I came across some literature about predatory caterpillars. I guess lepidopteran larvae are not strictly herbivorous as I assumed. Several ambush predator caterpillars have been found in Hawaii including the green grappler Eupithecia. A YouTube video demonstrates how this kind of caterpillar modifies the strike reflex for ambushing a pasing termite. They have modified thoracic legs and A6 prolegs for grabing the prey and manuveuring the mostly airborn body.

More recently, a paper in Science reported a case-bearing caterpillar named "Hyposmocoma molluscivora" which feeds exclusivly on snails. This kind of caterpillar spins silk over a resting snail in a spiderlike fashion in order to anchor the prey. Then it would start digging into the shell opening all the way to consume the snail, even if that means leaving its silk case behind.

Another class of caterpillar carnivorousity is consipecific cannibalisim when the population density is too high. Semlitsch and West described the relationship between body size and caterpilalr cannibalism in the journal Oecologia. It was found that smaller caterpillars are more likely to become the victims simply because they are not capable of killing the bigger conspecifics.

Sunday, April 19, 2009

Reconsidering "caterpillar locomotion"

Want to guess the highest speed by which a caterpillar can move?
Answer: ~15" per second!!!
No way... what species and how?

I recently reviewed a few papers about rolling locomotion in nature, and Mother-of-Pearl moth caterpillar (Pleurotya ruralis) is one of the two active rollers ever discovered. Check out a YouTube clip by BBC Aniamls and you will believe what I claimed. In this movie, the caterpillar was rolling downhill, but the rolling action is actually initiated by rapid muscle contraction. According to Dr. John Brackenbury reported in his paper "Fast locomotion in caterpillars", this active rolling could hit a top speed of 39cm/s.

Here are a couple of references for those who want to read into this subject.
J. Brackenbury. Fast locomotion in caterpillars. Journal of Insect Physiology. 45:525-533 (1999)
J. Brackenbury. Caterpillar Kinematics. Nature. 390:453 (1997)

Tuesday, April 14, 2009

Three forms of caterpillar and their locomotion

Manduca caterpillar only represents a class of Lepidoptera with four abdominal prolegs. Many other species have reduced the size or number of prolegs. Geometrids (loopers, inchworms, and spanworms), for example, retain only the A6 abdominal prolegs which work in conjunction with the terminal (anal) prolegs to facilitate the rear end attachment during a looping action. Lepidoptera larvae in the limacodid-group have very short legs at the belly that their modes of locomotion resemble slug creeping in apparence. For these so called "slug caterpillars", there can be abdominal prolegs from A2 to A7 with a combination of crochets and suckers. After starring at the "typical" 4-Abd prolegs caterpillars for three years, I start to wonder how the differences among these locomotor modes are associated with the evolution of these three forms of caterpillars. First thing that came to my mind was the mass scaling issue. According to my experiences with live caterpillars in the wild, 4 Abd body plan is common in species that can get really big and juicy. Inchworms are rarely bigger than 5cm. Slug caterpillars seems even smaller.

Larger caterpillars --- 4 Abd prolegs (use as many body anchors as the locomotion allows)
Loopers/Inchworm --- 1~3 Abd prolegs (reduce unneccessary attachements for efficiency)
Slug caterpillars --- short prolegs close to the ventrum (increase contact surface for fluid/silk adhesion mechanism)

This line of reasoning is very consistent with my current biomechanical finding: prolegs=attachments. (results from the caterpillar substrate reaction force analysis) Since soft legs cannot provide effective leverage, they are just acting as anchors for caterpillars. And it follows that the evolutionary modifications of prolegs morphologies should be determined by the maximum body size and mechanisms of attachement.

Saturday, April 11, 2009

Body pressure controled by the smart integuement...

Integument of soft-bodied animals is often a very interesting biomaterial. Besides unfolding and stretching to accommodate growth, it may function as a body pressure monitor in some instances. More specifically, the viscoelastic effect of the Manduca body wall prevent stress from building up as the body volume increases. When the caterpillar lost body volume, the body wall will automatically shrink to maintain the body pressure. The above effect might sound like a typical barometric reflex in mammals, except that this is done via the passive properties of the cuticular integument. Indeed, a simple reflex may be substituted by a smart material! The following graph shows the "adaptation" response found in the body wall, muscles, and the stretch receptor organ lining along the body axis. More detailed experimentation on caterpillar body pressure is ongoing.

Tuesday, April 7, 2009

Manduca goes aquatic!!

Caterpillar locomotion is slow and quasi-static. However, it is very robust over a wide range of environmental conditions. In addition to maneuvering in highly branched vegetation, caterpillars can burrow underground and even crawl underwater. In my experiment, water submersion converts gravity into buoyancy and increases the external pressure by a factor of two. Nevertheless, a Manduca 5th instar caterpillar can continue to function up to ~3min before the hypoxia kicks in.

InchBot-III comes alive!

InchBot is a biomimetic soft-bodied inchworm robot actuated by shape memory alloy springs, controlled by a pair of coupled neural oscillators, and powered by a Li-Po battery. It is completely autonomous with the ability to conform to the substrate as a soft body. The exploitation of this compliance allows the robot to operate without feedback over a range of terrain variations, much like the real animal. InchBot-III can crawl up to 5mm/s on a tabletop and carry a payload backpack up to 15g (bottom photo). The whole robot can be rolled up and compressed into one palm (top picture).

Welcome to Huai-Ti's research blog!!

Hi there!

In the field of animal locomotion, fast ballistic motions are always eye-catching. However, on the other end of the spectrum, slow soft body movements can be equally amazing. My research focuses on force transmission in soft-bodied animal locomotion. In particular, we use Manduca sexta caterpillar as the model system.

In response to the feedbacks and requests from recent encounters at professional conferences, I decided to start this blog as a window for people to check on my research progress. I will do my best to update new cool graphics and info about my research. Please note that this site is not intended for formal publication. I cannot post any unpublished data or engineering details. I apologize if the materials cannot satisfy your curiosity, but I promise to publish my research as fast as I can. Thanks for the interest.