Literature

The hawkmoth, Manduca sexta, used CO2 sensors to assess how much nectar it is likely to get from the flowers of Datura wrightii. Honeybees use CO2 sensors to assess the quality of air in their hives: if too high, they show a fanning response. It is not clear why Drosophila flies need CO2 sensors, but they certainly have them and new research has uncovered the architecture of the sensors.
Two genetic receptors are involved: Gr21a and Gr63a. Both are needed for the sensor to work, and they are only sensitive to CO2. “The simplest scenario is that the two receptors form a complex that binds to CO2. It is possible, however, that other molecules are also required.” In a News & Views essay, Rachel Wilson writes: “Considered as tiny chemical sensors, these neurons are wonders of natural engineering.”
There is an important distinction to be made between engineering that is found in the natural world and engineering that is the result of natural processes (law and chance). To have two complex neurons to be present to bind with CO2 and then to have the rest of the circuitry to communicate their signal to the brain of the insect bears all the signs of complex specified information (which in other contexts is associated with intelligent design).
Ths research does appear to have great potential for tackling malaria, because mosquitos are known to use CO2 from animal breath as an arousing stimulus. “If this molecular insight permits the design of novel mosquito deterrents, it could have a major impact on global health.” It is curious that scientific publication allows “design” to be invoked for knocking out one or both of the relevant neurons, but not for the origin of the neurons in the first place!

Two chemosensory receptors together mediate carbon dioxide detection in Drosophila
Walton D. Jones, Pelin Cayirlioglu, Ilona Grunwald Kadow and Leslie B. Vosshall
Nature 445, 86-90 (4 January 2007) | doi:10.1038/nature05466

Blood-feeding insects, including the malaria mosquito Anopheles gambiae, use highly specialized and sensitive olfactory systems to locate their hosts. This is accomplished by detecting and following plumes of volatile host emissions, which include carbon dioxide (CO2)1. CO2 is sensed by a population of olfactory sensory neurons in the maxillary palps of mosquitoes2, 3 and in the antennae of the more genetically tractable fruitfly, Drosophila melanogaster4. The molecular identity of the chemosensory CO2 receptor, however, remains unknown. Here we report that CO2-responsive neurons in Drosophila co-express a pair of chemosensory receptors, Gr21a and Gr63a, at both larval and adult life stages. We identify mosquito homologues of Gr21a and Gr63a, GPRGR22 and GPRGR24, and show that these are co-expressed in A. gambiae maxillary palps. We show that Gr21a and Gr63a together are sufficient for olfactory CO2-chemosensation in Drosophila. Ectopic expression of Gr21a and Gr63a together confers CO2 sensitivity on CO2-insensitive olfactory neurons, but neither gustatory receptor alone has this function. Mutant flies lacking Gr63a lose both electrophysiological and behavioural responses to CO2. Knowledge of the molecular identity of the insect olfactory CO2 receptors may spur the development of novel mosquito control strategies designed to take advantage of this unique and critical olfactory pathway. This in turn could bolster the worldwide fight against malaria and other insect-borne diseases.

See also: Wilson, R.I., Scent secrets of insects, Nature 445, 30-31 (4 January 2007) | doi:10.1038/445030a