Stinky sticks

I keep black beauty stick-insects, Peruphasma schultei which hide, not by looking like vegetation but by being black, velvety (hence non-reflective) and nocturnal. However, although usually calm when handled, they do have active defences which I discovered recently when retrieving a large females that had escaped to climb the curtains. I felt something wet on my hand, and when I looked there was a milky, eye-wateringly acrid liquid. I knew that a lot of stick-insects can spray defensive chemicals but hadn't witnessed this one before. Fortunately a quick Google told me that this had been investigated by Dossey et al. (2006), who discovered a new defensive compound that they named 'peruphasmal' after the insect producing it.

Peruphasma schultei

Peruphasmal is an isomer of dolichodial, a compound in the iridoid group.

structure of dolichodial

Iridoids are found in many plants (usualy in the form of glycosides) and may be active ingredients in those used medicinally. They are also likely to have a defensive function, protecting the plant against herbivores. Dolichodial and its isomers are found in plant essential oils, and as in this case, in the defensive sprays and secretions of some insects, possible being sequestered from food plants, or being produced by the insect's metabolism (they are intermediate compounds in the production of alkaloids for example). My P. schultei are fed on privet (Ligustrum sp.) which is in the family Oleaceae, one of those known to produce iridoids, so sequestration is plausible. The toxin in Ligustrum appears to be the glycoside syringin, known originally from lilacs, but now known to be a white, crystalline, bitter toxin in many plants including privet, hence its alternative name 'ligustrin'. So, this is a likely candidate for the source of the insect's defence. In plants, iridoids are usually bound to glucose, and peruphasmal is also sprayed along with glucose. Whatever the source - there is more research, but not much - it is an effective defence; my reaction was to put the insect down and wash off the secretion - a predator would have been sprayed in the face...



References

Dossey, A.T., Walse, S.S., Rocca, J.R. & Edison, A.S. (2006). Single-insect NMR: a new tool to probe chemical biodiversity. ACS Chemical Biology 1(8): 511-514.