• Evolution

  To avoid detection by predators, several species of butterfly larvae defecate by firing waste pellets at high speeds away from their homes.
  
  The mechanism behind this extraordinary behavioural adaptation is elegantly simple. They can hold a waste pellet (known as frass) in their anus until a rapid change of blood pressure occurs, expanding a segment of the abdomen and firing the pellet like a pea

  from a peashooter. This behaviour - informally known by some as "scatapulting" - can fire frass a distance of nearly 40 times the larva's body length and at a speed of 1.3m (4.2 feet) a second. To put this in perspective, that's like a 6-foot man being able to defecate 73m (240 feet) away from him.
  
  But this behaviour has not evolved for hygiene - it keeps these larvae hidden from predators such as wasps, who would otherwise locate them by these odour cues. This finding came from an experiment almost a decade ago, when Martha Weiss (Georgetown University) conducted an experiment using two groups of skipper butterfly larvae. The experimental group had fresh waste pellets added to their shelters, while the control group had odourless glass beads. Paper wasps (a natural predator of skipper caterpillars) were more likely to visit the shelters containing frass.
  
  Further support came when paper wasps were offered a choice between two shelters, one containing frass and the other glass beads. Out of 17 trials, wasps visited the frass-containing shelter 14 times. Chemical cues from frass attracts predators, and this great selection pressure has meant similar waste-expulsion systems have formed independently in distantly related caterpillar species.
  
  Photo credit: John Alcock.
  
  http://news.nationalgeographic.co.uk/news/2003/05/0516_030516_caterpillars_2.html
  
  http://www9.georgetown.edu/faculty/emc26/marthaweiss/weissm/mweisspix/ecollett.pdf
  
  Alcock, J. (2009). Animal Behaviour (9th edition). Sinaur Associates: Massachusetts.