Monday, 26 June 2017

Tree Weta Encounters - why do they smell? and other questions

I've always felt that wētā, New Zealand's bizarre endemic insects, have an unfair reputation. They seem to bring out the worst in people, who shudder at the thought of touching those feelers and spiky legs. Children are often more curious about insects and bugs and less judgemental. So I happily included a tree wētā in Whose Feet are These? And it's one of the few animals in the book that most children guess right straightaway.
From "Whose Feet are These?" a male weta
Here's some of the things I learned about wētā when I was writing Whose Feet are These? and In the Garden:

  • the spike that you sometimes see at the tail end of a wētā is an ovopositor, this is for laying eggs and shows that the wētā is a female (see giant wētā below)
  • male wētā have two shorter curved spikes at their rear
  • wētā raise their spiky hind legs when they feel threatened
  • wētā are nocturnal
  • tree wētā and giant wētā are mostly vegetarian
  • tree wētā aren't endangered but giant and tusked wētā are at risk of extinction.
I've come across a lot of wētā recently while volunteering on Mana Island. Mana Island is free from introduced mammals  -  mice, rats, stoats, possums, cats etc. The Cook Strait giant wētā has been introduced to this sanctuary and thrives, with the result that the simple act of sitting down in the grass can mean a giant wētā takes a piggy-back. Giant wētā are quite docile unlike their biting, kicking tree wētā cousins.
Female Cook Strait giant weta hitching a ride
on someone's back, Mana Island - photo Elly Gowers
A gecko monitoring project on Mana Island has provided the perfect opportunity for some tree wētā encounters. It seems that tree wētā like to hide in the same places as the ngahere (forest) geckos that we were looking for.
A female tree weta in the gecko hide
In March this year, we found so many wētā that we began to ask ourselves some questions:
  • why do wētā smell of cow dung? (in large numbers they have a very distinctive scent)
  • was that brown liquid really wētā vomit?
  • do only the males bite and will they draw blood?
  • why do wētā produce so much poo?
  • if we remove all the wētā from this gecko hide will they return?
  • how do wētā get to be brown, if they are white when they emerge from their old exoskeleton? 
Some of the answers we found out quite quickly - yes they can draw blood - yes females bite. Or hypothesised about - and yes we think it's all that poo that smells so strong. But other questions required a bit more investigation.
This weta chose to poo on the tally sheet
George Gibbs in "The Demon Grasshopper" says of tree wētā, that the smell or scent in their poo helps them identify their home territory and also helps the males find females. Perhaps that also means that they may be able to find their way back to the same hide? On the mainland, that strong scent could mean wētā become a meal for predators, as rats and other mammals have a good sense of smell and could easily find wētā. This is a good reason to build wētā motels - they protect the wētā and also allow us to see how many there are. 

George Gibbs also says that the males with the largest heads are dominant and gather a harem of females and juveniles together. We certainly found some large groups - up to 20 - although often there was more than one male, so perhaps there was enough room for more than one group under the hides. On the "WetaGeta" website it states that tree wētā only get together in harem groups in summer and autumn, I hope that means that when we monitor geckos this winter we won't encounter so many wētā.
Several large headed males and their female companions 
An internet search showed me that apparently crickets, grasshoppers and wētā can all vomit, they do this to put off potential predators.

Like all insects wētā have an exoskeleton, as they grow they need to shed the old one and then a new larger one will develop. We were lucky enough to see a ghostly white wētā emerge from its exoskeleton while we were spotlighting at night. We think they need daylight to harden and brown but couldn't find any more information about this.
Ghostly white weta emerging from exoskeleton
Now that I'm better informed, I'm curious to see how many wētā are in the gecko hides during winter!

Further reading
"The Demon Grasshopper" by George Gibbs, New Zealand Geographic
Weta Ecology, includes identification tools

For Kids
Building wētā motels
All about wētā

Related blogs
Geckos in the Spotlight: volunteering on Mana Island