Imitation, flattery, and body-plans

If imitation is the highest form of flattery, then ants might be among the most celebrated creatures of the insect world. A delightful essay in Current Biology describes some of the more uncanny cases of ant-mimicry among arthropods, among them the treehopper Cyphonia clavata.

Ant-mimicking treehopper Cyphonia clavata. Via Flickr.

The 'ant' that's apparently riding on the back of the treehopper is actually an extension of its headshield. The green coloration of the rest of its body blends in with its leafy background, so on first glance you only see the black 'ant'. The mimic is seated in reverse: if you look carefully at the 'abdomen' of the 'ant', you'll see the green eye of the treehopper staring right back at you. This makes sense because in their defensive posture, ants move backwards.

Creatures that mimic ants are called myrmecomorphs ('ant-shaped', from Greek). There are certainly a lot of ants out there to be imitated (Antweb has high-resolution scans of ant specimens from around the world). One statement in this essay got me thinking:

"... there are about 2,000 species that mimic ants. Not surprisingly, these are nearly all insects or spiders, as a certain degree of body plan resemblance to ants is probably a prerequisite to becoming a myrmecomorph. ... ant mimicry must have evolved many times independently."

Treehoppers are true bugs (Hemiptera) in the family Membracidae. They are known for their headshields, also called helmets, which are responsible for the great diversity of form in this family. Some of them look like they may have inspired the fascinators worn by some of the women at the recent Royal wedding.

Diversity of treehopper headshield forms - Cyphonia clavata is at bottom right. Via Nature

Developmental biologists have now found that the helmet is actually a body plan innovation (original paper abstract), something that's incredibly rare in evolution. New structures are typically formed by modifying or reducing existing parts of anatomy, but innovations such as new appendages or new body segments are much rarer.

Insects typically have a pair of legs on each of the three thoracic segments (T1 to T3), and wings on the 2nd and 3rd (T2 and T3). The helmet arises from the 1st thoracic segment (T1), and anatomical observations have suggested in the past that they might be homologues of wings. By looking at gene expression in a developing treehopper, biologists found that wing-specific transcription factors (which control the expression of other genes), especially Nubbin, are expressed in the developing helmet, which indicates that it's developmentally homologous to wing appendages. They suggest that the Hox genes responsible for suppressing wing formation in T1 have been suppressed in treehoppers, allowing the evolution of the helmet. Because they aren't needed for flight, they aren't faced with the same physical constraints and so have been able to develop into a wild variety of shapes and sizes.

So coming back to the statement about body plan resemblances quoted above, perhaps the idea is more beguiling than it actually should be. An ant mimic for sure will have to be bilaterally symmetrical, of the right ant size, but beyond that I don't think the prerequisites for mimicry can be quantified. For conceptual proof of this, watch this famous octopus species mimic in turn a brittlestar, lionfish, and sea snake.

Highlights of the Week

Some interesting news and articles from around the web for this week:

Cicadas are emerging en masse in the Midwestern United States after yet another 13-year cycle.
Cicadas are insects which are known for their loud and distinctive sounds produced by a mechanism called 'tymbalization' in their abdomens. Several species in the genus Magicicada in the US are called 'periodical cicadas' because they transition into the adult phase of their life cycle in a synchronized manner in cycles of 13 or 17 years. This summer the time has come for the emergence for the so-called brood XIX. These emergences happen in such numbers that early European settlers thought that the cicadas were the pestilential "locusts" of the Bible, and their carcasses litter forests in a deep crunchy layer. Find out more about cicadas at this website from the University of Michigan.

Mathematics and biology have a deep and subtle relationship. 
Viruses, for example, have self-assembling coats made up of protein subunits, which tile together in specific geometric forms. Disrupt these geometries, and one might be able to render a virus harmless.... Other fields of mathematics, such as chaos theory, can help in modeling natural phenomena such as plankton dynamics in the ocean. (I wish this essay was illustrated, though.)

How do flatworms regenerate their missing body parts?
The planarians (flatworms) are favorite classroom examples for regeneration because of their freakish ability to regenerate a complete worm when cut into multiple pieces. New research shows that cells in the worms called neoblasts, which can be thought of as analogous to stem cells in other animals, are pluripotent, meaning that they can develop into any cell type in the body. Researchers have also found some of the factors that determine whether a newly divided cell in a regenerating animal will develop into part of the head or the tail.

Fire ants link up into waterproof rafts

Many insects, such as the aptly-named water-striders, are able to exploit surface tension and literally walk on water. However, the majority are only moderately hydrophobic, and because they are denser than water, have to struggle all the harder to remain afloat the larger they are.

Fire ants live in places which are frequently inundated by water. Individual ants placed in water can float, but not very well. How then do they cope with flooding? It turns out that they assemble themselves into ant 'rafts', cooperating to improve their buoyancy and waterproofing.

Insect cuticle itself is already water-repellent to some extent, but alone it is not enough to explain how a clump of ants, gripping onto each other mandible-to-leg, is more buoyant per unit mass than an individual ant. The answer lies in their ability to trap a layer of air (called a 'plastron') around their bodies. By clumping together, they increase the size of these plastrons, and can entrap larger air bubbles within the clump. These lower the effective density of the ant mass, and also allow them to breathe (insects breathe through pores in their cuticle) even when submerged.

Quotable quotes from the paper:

"Ants were scooped with spoons into 100-mL beakers rimmed with talc powder and weighed to count their numbers. Using the natural adhesion of the ants, a few swirls of the beaker was sufficient to roll the ants into balls."
"Not surprisingly, ant spheres that are placed on solid surfaces quickly disintegrate as the ants flee in all directions."
"By harnessing two live ants with an elastic band, we found that the maximum tensile force between them is F = 620 ± 100 dyn (N = 11), or more than 400 times body weight."

Reference:
Mlot, Tovey, & Hu. "Fire ants self-assemble into waterproof rafts to survive floods." Proceedings of the National Academy of Sciences, USA (10 May 2011) vol. 108 no. 19 pp. 7669-7673.

Entomophagy for the Environment?

The Guardian reports on a new Food and Agriculture Organization (FAO) report that suggests insect-eating as a way to mitigate the human demand for meat, especially in countries where insects are already consumed as part of local cuisines: "the FAO's priority will be to boost the eating of insects where this is already accepted but has been in decline due to western cultural influence."

It's easy to dismiss the idea as cranky or silly, but there is a valid point in there - people in industrialized nations already eat shrimp, which are quite similar. And it's not all about fried locust, either. Insects could be useful as feed for more conventional livestock, or turned into some kind of homogeneous patty, the better to disguise their origins.

My main bugbear about the article is that it talks about insects in the title but the picture shows scorpions on sticks. To be fair, the text of the article does say "insects and other creepy-crawlies," so it's not entirely wrong.

How Caterpillars Crawl

How do caterpillars crawl? We might imagine them as a gooey tube inching forward in waves, but a research group at Tufts has found that their bodies behave in a more complex way, which they term 'visceral-locomotory pistoning', where the elastic gut of the caterpillar stretches and moves independently of the body wall. In order to peer inside the caterpillar's body, they used X-ray synchotron radiation, but later found a much easier way - using newly-hatched caterpillars, which are small and translucent to visible light. The biomechanics of soft-bodied animals is an inherently challenging subject. Modeling the physics is difficult because there are no rigid elements (skeletons, shells, and the like); it is also a challenge to find suitable reference points to track during motion.

Check out the fascinating video abstract freely available on the Current Biology website.

(Via New York Times)

Delusory Parasitosis

"I knew I’d been crazy during that period, but I didn’t know I’d been crazy with a condition that had a name, and that although it wasn’t common, it was well known in psychiatric literature. Not just psychiatric literature. Delusory parasitosis is where psychiatry meets academic entomology and pest control."

Author Jenny Diski writes about delusory parasitosis, also called Ekbom Syndrome, a psychiatric syndrome where a person imagines that he or she is being invaded by tiny invisible bugs. As she tells it, it's something that the unsuspecting entomologist or pest control specialist will be unnerved to encounter for the first time!

The entomologist whom she cites, Nancy Hinkle, has written a review of Ekbom Syndrome for Annual Review of Entomology (2010, vol 55: 77-94, doi:10.1146/annurev.ento.54.110807.090514).

(Scratching cat photo: Pavel Ševela / Wikimedia Commons)

Parasite makes ant mimic fruit

Parasites frequently modify the behavior of their hosts to encourage the infection of new hosts. For example, see this video (taken from the Planet Earth documentary) of the fungus Cordyceps that makes insects climb to the top of grass stems, and then erupts its fruiting body from the host's body, and disperses its spores over more hapless hosts from this elevated position. Yanoviak et al. (Am Nat 2008. Vol. 171, pp. 536–544; DOI: 10.1086/528968) describe a case of parasite-induced mimicry in the ant Cephalotes atratus. A nematode infection causes the gasters (rear portion of the abdomen) to become bright red and swollen, resembling a berry fruit, where normally it is black and inconspicuous. The infected gasters are also full of parasite eggs. Birds that feed on berries would then pop off these packets of parasite propagules, and pass out the eggs in their faeces. Ants congregate around bird faeces, which represent food resources to them, and collect them to feed to their brood, completing the cycle.

Here's the lesson from all this, kids: don't eat dung.

Synchotron Radiation Tomography Illuminates Hidden Bugs

http://news.bbc.co.uk/2/hi/science/nature/7324564.stm

We've all seen pictures of ancient insects trapped in the golden, honey-like transparency of amber. Amber is fossilized tree resin, that when it was formed, trapped and preserved the form of insects and other small animals that it flowed over. But much amber is cloudy, and short of breaking it open, there's not been anyway to look inside to see what fossils might be found within. Now, scientists at the European Synchotron Radiation Facility in Grenoble, France have used high-intensity X-ray radiation to peek inside the amber and through computerized tomography (the same method as CT scans used in medicine) reconstructed 3-D images of fossils found in the amber. This was previously not possible with conventional X-ray sources. What's even neater - they use a method called 3D printing to produce a plastic resin scaled up model of the fossils in the amber, so palaeontologists have something tangible to manipulate and observe, rather than just pictures on a screen. Really amazing!

Giant Sea Scorpion!

We usually think of invertebrates as small animals, and arthropods in particular as being limited by the structural engineering of an exoskeleton, which is less capable of supporting large body sizes than an endoskeleton. A new fossil discovery however should creep out anyone who thinks that crabs and lobsters are already bigger than a decent invertebrate should be.

From a 43 cm long claw of the fossil eurypterid (sea scorpion) species Jaekelopterus rhenaniae found in Germany, researchers extrapolated the length of the animal's body to be between 233 to 259 cm, using claw size to body length ratios from other sea scorpinons. Eurypterids are members of the extinct subclass Eurypterida within the class Merostomata of the subphylum Chelicerata, i.e. they were chelicerates (like spiders and scorpions) most closely related to the horseshoe crabs.

Eurypterids were aquatic and the buoyancy conferred by water may help explain structurally their large size, but what about the problem of gaseous diffusion to tissues? They presumably had an open circulatory system like other arthropods which is less efficient than the closed circulation of vertebrates. The authors hypothesise that the higher oxygen levels in the atmosphere in the past could have helped them attain their large size, or that it was driven by an evolutionary arms race with their prey.

Some questions to think about:

• Why is extrapolation using data from other sea scorpions a valid means of predicting the body length of the animal from only its claw?
  
• Among the extant (still living) chelicerates, how do the methods of gas exchange differ between the aquatic and terrestrial groups?
  
• What can we infer about its mode of feeding and possible prey?
  
• How can we explain why such giant arthropods are no longer extant today?