July 28, 2009

Crickets Use Blood And Guts Defense To Thwart Predators

Armored crickets have a strange yet remarkable way of defending themselves from being attacked, squirting out toxic blood from tiny gaps in their body and then throwing up to make themselves unpalatable to predators.

While a few other insect species, such as beetles and katydids, actively bleed when attacked, the benefits of such extreme measures were not clear. New research shows the strategy does indeed work in deterring predators such as lizards. 

Armored ground crickets (Acanthoplus discoidalis) are plump, flightless insects that dwell within the African bush across Namibia, Botswana and South Africa. They can grow up to 2 inches long, have sharp spines and legs in addition to a pair of strong biting jaws.  The male crickets are also able to make a loud noise by rubbing body parts together in something called stridulation.

Each of these characteristics contributes to its ability to defend itself against predators.

However, the crickets also use two highly bizarre tactics to thwart attacks: regurgitation of the food they have just eaten, and squirting blood from gaps in their exoskeleton on their backs and legs.

Some other insects, including blister beetles, stonefly larvae and bushhoppers, exhibit similar behavior when attacked.  Indeed, one particular katydid is actually known as 'blutspritzer', which in Germany means 'blood squirter'.

Although this strange behavior is well known, its effectiveness has never before been tested.

"When I was moving them to larger quarters I was thinking about how they would grip a branch and when I pulled I would inevitably feel the squirt of the blood jetting out from under their legs," said entomologist Bill Bateman of the University of Pretoria in South Africa during an interview with BBC News.

"This is a recognized defense mechanism and has been mentioned in other invertebrates, but no one had published on what exactly makes them do it or whether it actually is effective against predators," he said.

So Bateman and his colleague Trish Fleming of Murdoch University in Western Australia decided to study the defensive capacity of the armored ground crickets.  They began by grabbing the insects from the side or above with tweezers to simulate attacks by predators.  They found that the crickets responded differently depending on the type of attack.

When attacked from the side, the crickets stridulated and attempted to bite their predator.  Roughly two-thirds of the time, the crickets also squirted out acrid-smelling blood, known in insects as haemolymph, from seams in the connective tissue of their legs and from behind the head.

When attacked from above in a direction in which they could not bite, the crickets squirted toxic blood nearly 90 percent of the time, reaching distances of up to 2.5 inches, Bateman said.

"The blood is pale green and rather acrid smelling. I couldn't bring myself to actually taste it fresh but it leaves an acidy, tobacco-like taste on your fingers if you do not wash it off," he said.

Bateman then further examined how the insects responded to both bearded dragon lizards (Pogona vitticeps) and striped skinks (Trachylepis punctatissima).

When he put the male cricket in a cage with four bearded dragons, one lizard immediately tried to eat the insect.  It immediately autohemorrhaged, forcing the lizard to back off and wipe clean its jaws.   The same happened with the second lizard, while a third lizard approached the cricket but did not attack it.

Bateman also tested the effectiveness of the haemolymph and regurgitated food by painting another smaller species of cricket with either substance, then waiting to see if the striped skinks would attack it.   While the lizards ate all of 24 clean crickets, they often refused to eat those covered in haemolymph, and occasionally refused to consume those insects covered in regurgitate.

"What impressed me is that they control the release depending on how they are grabbed," said Bateman.

"If it's from above the blood wells out and coats your hand. If grabbed by forceps from the side, by a leg, they lean towards it and crouch down, then there is a slight cracking sound and the blood jets right along the line of attack."

"Any predator would get a faceful, and our experiments indicated that lizards do not like it all."

Bateman was surprised by the complexity and sophistication of the crickets' defenses.  The male crickets would aim their blood towards the angle of attack, while females, which do not stridulate, rely on squirting, biting and regurgitating.

However, these defensive mechanisms have one drawback.  Ground crickets often swarm in search of new sources of protein and salt.  But since one of the best sources of these is other crickets, the insects can become cannibals, feasting on each other.

"When the swarms in the African bush meet a road, lots get squashed and the others gather for a feast, so more get squashed until there can be a thick, acrid pancake of dead and moribund crickets on the roadside, bleeding and attracting more cannibals," Bateman said.

In other words, crickets with haemolymph on their bodies attract the attention of other cannibalistic crickets that believe it is injured.

"Crickets that I induced to squirt blood would assiduously clean any droplets off their limbs when put back in the colony, presumably to avoid cannibalism," Bateman said.

"I saw other crickets approach a bloody one and begin nipping at it. If intact, the bloody one usually runs off."

The study was reported in the Journal of Zoology.  An abstract can be viewed at http://www3.interscience.wiley.com/journal/122368498/abstract.


On The Net:

University of Pretoria

Journal of Zoology