Dark Banquet - Bill Schutt [63]
As I rotated the canning jar in my hand, I couldn’t help noticing that Lou’s bed bugs were in a frenzy.
“They’re attracted to your body heat and the carbon dioxide in your breath,” Lou said.
Within the crowded glass confines, the bed bugs had responded to stimulatory cues in much the same way as the leeches did when hunting Bogie’s character in The African Queen. In this case, however, the thermoreceptors and chemoreceptors responsible for prey detection were stimulated by increases in temperature and carbon dioxide concentration (rather than touch, from the incoming waves of disturbed water, or vision, from changes in light intensity). At a basic level, though, the wiring of the bed bug and leech nervous systems, and their function, is quite similar. A stimulus is detected that prompts signals (afferent nerve impulses) to be sent from sensory receptors to the body’s data processing center (the brain). After rapidly sorting through incoming information (like direction of prey and distance), a response is generated that takes the form of outgoing (efferent) nerve impulses. These are sent to the muscles of locomotion. Activation of these muscles and their subsequent contraction leads to coordinated movement of the leech or bed bug’s body (either swimming or running toward its respective prey). In both instances, if the initial stimulus had been interpreted by the brain as DANGER, rather than FOOD, the outgoing response would have resulted in defensive behavior–like fleeing.
This is certainly a simplification, but on one level the only difference between the nervous systems of leeches, bed bugs, and humans is that we have many more neurons, packed into specialized regions of the brain (like our wrinkly cerebral hemispheres). This complex and intricately interconnected wiring allows us to do things that the relatively simplified nervous systems of the leech or bed bug cannot achieve—such as deciding whether to respond to a stimulus in the first place or choosing to vary that response. In the previously mentioned blood feeders, fewer neurons lead to limited or even one stereotypical response to each stimulus encountered. For example, bed bugs are thought to release an aggregation pheromone, a chemical that initiates clustering behavior in members of the same species (conspecifics).*94 In this case, think of a trio of bed bugs that have just hopped off someone’s luggage. After scurrying across the floor for a few seconds, one of them encounters a wall and follows it, eventually finding a crack in the molding large enough to slip through. Stimulated by physical contact with the walls of this dark, safe haven (hereafter referred to as a harborage), the bed bug releases a pheromone whose message is interpreted by the other two bed bugs as something very much like SAFEDARK.
Initially released in response to a stimulus, the pheromone itself becomes a stimulus, triggering a highly specific response. Soon enough, the harborage has three bed bugs in it. Killing time while they await a meal, the bugs behave predictably, making more little bed bugs and producing copious piles of bloody feces.*95
The point is that once pheromones are sensed, there’s no choice and little behavioral variation in the response. These chemical messages are also a major key to the seemingly bewildering degree of organization exhibited by social insects like ants, termites, and bees.