Being a mother can be very hard work, and finding enough food to feed your young is risky and time-consuming. But some parasitoid species give their young the best start in life by having an energy-rich meal ready and waiting when they emerge from the egg. The catch is that this very nutritious breakfast is the body of a hapless victim, its muscles paralysed by the mother's venom. However, not all wasp mothers use the same paralytic approach to ensure her brood's future: Ampulex compressa uses a sophisticated alternative mechanism; she catches and manipulates her victims. Instead of disrupting the signals between the victim's motor neurons and muscles to paralyse them, A. compressa's venom alters the victim's behaviour by targeting its stings to specific parts of the cockroach prey's nervous system,leaving it at the wasp's mercy. In a review published in the Journal of Comparative Physiology, Frederic Libersat describes his work on this intriguing system.
When a female A. compressa is ready to provide for her brood, she attacks the cockroach and lands on its back. She then delivers the first of two deadly stings, accurately injecting venom into a part of the nervous system called the pro-thoracic ganglion. This interferes with the synapses connecting to the leg motor neurons and causes temporary paralysis of the cockroach's front legs so that it is unable to fight off the attack. The wasp is now poised to deliver the second sting, injecting the venom directly into the brain and the sub-oesophageal ganglion, causing the cockroach to behave in a strange and unexpected way. Once recovered from the temporary paralysis caused by the first sting, the venom induces the cockroach to groom its antennae very carefully. This continues for about 30 minutes, during which time the cockroach remains where it was stung and does not escape. It is at the mercy of the wasp, who leads the cockroach to a pre-prepared burrow before total paralysis sets in.
The venom is so effective because it works by targeting only certain parts of the nervous system. Firstly, the venom probably activates the neural network that controls grooming because it contains large quantities of the neurotransmitter dopamine; the signalling chemical used in this network. The venom also overrides many other behaviours by stimulating grooming behaviour. Most importantly for the wasp, escape behaviour is stalled. It is almost impossible to provoke the cockroach to run away from danger. The venom probably interferes with the part of the walking network that receives signals from all over the body and makes connections with the motor neurons that control the leg muscles. Specifically, the motor neurons that produce rapid movements are deactivated, so the cockroach can walk but not run to save its life.
The wasp has developed a highly efficient way of controlling its prey with a cocktail of chemicals, ensuring that when its larva emerges after 6 days of incubation it has a feast waiting to be devoured!