Membracids, particularly the nymphs, are rather sedentary creatures. They spend long periods simply clasped to plant stems with their beaks burst through the cambium to feed on sap. A by-product of feeding on so much sap is 'honeydew', translucent excrement. Because this liquid is still sugary and of nutritional value, you will often see ants attending membracids for a feed. In return, the ants swarming around the foliage fiercely protect the bugs from predators and parasites. This is a nice example of mutualism. To capitalize on the advantage of living with ants, some membracid species have adapted to live as groups, thereby increasing the number of ants in attendance. Yet other species are solitary, and so they are rarely attended by ants because they are harder to find and can offer only the occasional droplet of honeydew.
Now the reason for the anal whip can be explained. While ants like honeydew, so do sooty molds. If the honeydew is left on the foliage for any length of time it will become moldy. Also, if a membracid happens to get honeydew on itself, it often becomes very agitated and tries to kick or clean it off. They need to stay clean to avoid becoming diseased with mold. It is imperative for any sap-feeding bug that produces honeydew to have an efficient method of honeydew disposal or dispersal. Fulgorid bug nymphs produce copious wax, which may repel the sugary liquid and keep them clean. Membracids evolved the anal whip (or tube) for this purpose. Membracids probably first evolved as solitary species, therefore, the anal tube was used to extend and place the droplets of honeydew away from the body. The tube can move from side to side, allowing droplets to be placed as a semicircle around the nymph. The deposited droplets sitting on the foliage could then be found and consumed by ants, or the droplets could be left to simply evaporate and dry into a harmless state. Support for this theory is that solitary (therefore rarely ant attended) species tend to have longer anal whips than gregarious (ant attended) species. The anal whip of ant attended species can be shorter because they are less reliant on self dispersal of honeydew. The species Sextius virescens shown on this page is ant attended, and so the nymph pictured has a relatively short anal whip.
Observations on membracid nymphs being attacked by predators have also shown that the anal whip is rarely flashed when attacked, and if so is ineffectual. The reaction of protruding some of the anal whip when gripped seems more similar to a 'nervous' reaction than a serious attempt to ward off enemies.
Another interesting facet of membracid biology is the way they care for their young, which they often extend to other broods as well. Membracids usually cut small slits into the bark of stems to lay their eggs. However, the eggs (and nymphs that later emerge) can be predated or heavily parasitized by wasps and flies. Therefore, the adult females of many species sit over the broods to protect them. For gregarious species, other females in the vicinity will also help protect other broods not their own. The females will fan their wings and buzz at any egg parasite that approaches. Some females will even sit over another's brood of eggs or nymphs if needed. When the eggs in a cluster hatch, adult females will shepherd the nymphs into a group on the stem where they are easier to guard by the several females. Otherwise, the nymphs would wander over the foliage one by one and get picked off by predators. The adaptive advantage of a female looking after young beyond her own is that more in the group will survive, which includes her own offspring. In other words, the membracids have evolved a sensible working relationship that benefits all. Of course, if you are a human seeking the moral high ground that might justify why you send so many membracid species to extinction, you would look for a selfish slant to this brood behavior, and thereby claim 'objectivity'.
(Posted June 2001)