Ptyleus sp. (Hemiptera: Homoptera: Cercopoidea: Aphrophoridae)
This week’s image is of a species of Ptyleus, a genus in the family Aphrophoridae (spittlebugs or froghoppers). It was captured in Suam Forest, Mt. Elgon, at 2324 m above sea level. Aphrophoridae is a large family of true bugs with over 160 genera and about 1000 described species. Like all true bugs they have piercing and sucking mouthparts, the sharp, paired mandibles slicing into and through the outer layers of the plant on which it will feed. Paired maxillae follow, forming a tube which is inserted into xylem vessels carrying nutrient-poor liquid. The bug begins pumping the liquid through the maxillae and into its digestive tract from which the small quantities of beneficial compounds are extracted from the liquid. The remaining liquid is excreted out the rear end of the bug, forming a bubbly froth that surrounds the bug. Among true bugs, feeding on xylem vessels rather than phloem vessels is unique to the 5 families of the superfamily Cercopoidea, of which Aphrophoridae is one. The many other families of bugs imbibe the much more nutritious, carbohydrate-rich liquid carried in the phloem. Why do the Cercopoidea families forgo the phloem vessels to feed on the less nutritious contents in xylem?
First, a digression.
Imagine that you are taking a leisurely walk through a meadow in the warmth of the early summer sun, a real pleasure until, that is, you notice what appears to be spittle on some of the plants. This gross intrusion into the day’s ambience overwhelms the otherwise idyllic sensations of sweet-smelling nectar and beautiful flowers. There are those of us who might seek the sterile safety of tarmac. Others might be curious enough to poke around the spittle with a twig. They will be rewarded. Inside the “spittle” (which isn’t in fact, spittle) and covered in a coat of tiny bubbles is usually one, but sometimes multiple larvae of an aphrophorid species, and that holds the answer to the puzzling choice of diet made by these bugs. Because xylem, compared to phloem, transports a dilute, nutrition-poor food, individuals of the cercopoid families must pump in copious amounts of it to procure enough nourishment for a larva to eventually reach adulthood. The excess liquid passes out of the bug and contributes to maintaining its foamy, spittle-like abode. What benefits accrue for a larva in such a microenvironment? At least three: Developing larvae in their spittle covering are not visible to potential predators, viruses are not transmittable through the “spittle” (however, bacteria are), and the spittle helps a larva to avoid desiccation.
The means by which a larva creates and maintains the “spittle” environment have been unravelled through the lens of biophysics. The numbers that follow are from work done in Canada at the University of British Columbia. Within the head of these bugs is a cibarial pump that is responsible for the rapid movement of xylem liquid from the plant host into the larva. The scientists estimated that that the pump is so powerful that it produces suction pressures reaching 1.6 megapascals, 80 times the pressure generated by an elephant when drinking, and the equivalent of a human drinking liquid through a straw that is 100 meters long! In addition, they determined that a spittle bug excretes between 150-280 times its own body weight every day. If you find these numbers impressive, consider that we have not even mentioned the reasoning behind the other common name for the family. Suffice it to say that adults (which do not live in the spittle) are capable of jumping 100 times their body length. No wonder the Aphrophoridae are also known as froghoppers!
Credits: Dr Robert Copeland