Many organisms rely on other species to transport them from one spot to another — particularly in harsh environments where such meagre resources as there are tend to occur in clumps. As Leslie S. Saul-Gershenz and Jocelyn G. Millar report (Proc. Natl Acad. Sci. USA doi:10.1073/pnas.0603901103; 2006), one species of the blister beetle Meloe franciscanus is a particularly innovative passenger: it can hail its taxi ride.

This blister beetle lives in the deserts of the southwestern United States. It feeds and lays its eggs under a plant that also provides nectar for the beetles' host and transporter species, a solitary bee of the species Habropoda pallida. Larvae of the beetle cooperatively form a spherical mass on the plant (right image), and simply hitch a ride when a male bee intent on mating with a female makes contact (left image). When the infested male copulates with a real female, the larvae are transferred and carried to the bee's nest. There they set up camp, and complete their development into adults nourished by the pollen and nectar stores of the nest and by the bee's egg.

Saul-Gershenz and Millar set out to test what lures the bees to make contact with the larvae in the first place. They found that visual models of the larval aggregations held no interest for the bees — but if the models were scented with an organic extract from either the larvae or the female bee's head, the bees found them just as enticing as real larvae.

Comparison of chemical profiles of the larval and bee extracts identified two alkene molecules common to the larvae and female bees that were not present in the males. And this heady blend of alkenes did indeed attract the male bees. It seems the beetle larvae have evolved a way to exploit the bee's sexual communication system as a means of calling a cab.