Alpine tree weta (Hemideina maori). Photo credit: Keith King.

Alpine tree weta (Hemideina maori). Photo credit: Keith King.

It can be hard to get your head around just how big some weta are if you're used to working on the Drosophila scale, but giant weta routinely tip the scales at up to 70 g and even the more diminutive tree weta tower over other more familiar insects. While some offshoots of the family make their homes in low-lying areas, others have pushed the boundaries into the mountains, where they are exposed to high winds, low oxygen, the searing sun and dry air. ‘Together, these environmental challenges…mean montane insects are at a significantly higher risk of desiccation’, say Keith King from the University of Otago, New Zealand, and Brent Sinclair from the University of Western Ontario, Canada. So how do these high-altitude insects cope with these conditions? Suspecting that they might reduce their water-loss rates, King and Sinclair set about measuring water-loss across the insect's cuticle in weta ranging from sea-level species (the Wellington tree weta, the tokoriro and the Canterbury tree weta) to the mountain dwellers (the alpine tree weta and the Banks Peninsula tree weta).

The duo was impressed to see that the low-level dwellers’ cuticular water loss rate was 0.444 μmol–1 g–1 h–1 while the mountain species’ cuticular water loss rate was 45% lower at 0.245 μmol–1 g–1 h–1. They also found that the mountain dwellers had cut their respiratory water loss by 55%. And when they compared how much water the species lost relative to their metabolic rate, the mountain species were much more conservative than their low-altitude cousins, losing 64% less water.

King and Sinclair then explain that the amount of melanin pigment in the cuticle of Drosophila is known to affect insect desiccation rates, with lighter flies losing water across the cuticle faster than darker species. Knowing that there are two forms of the alpine tree weta – one with a melanin-tinted cuticle and a lighter version – the duo decided to test whether the degree of melanisation affected the insects’ cuticular water loss rates. Through a direct comparison of cuticular water loss rates King and Sinclair could see that the darker form lost 46% less water than the lighter form. And when they stripped the waterproofing hydrocarbons from the surface of the cuticles of both insects and measured their cuticular water loss rates, the duo found that although the insects’ water loss rates rocketed, the darker weta still lost water 57% more slowly than the lighter insects. The high melanin content of the cuticle in the darker weta offers more protection from desiccation than the cuticle of the lighter weta.

So, mountain-dwelling weta have developed various strategies to protect themselves from desiccation in their harsh mountain homes and some have probably benefited from the waterproofing effects of melanin as an unintended consequence when they became darker for camouflage.

K. J.
B. J.
Water loss in tree weta (Hemideina): adaptation to the montane environment and a test of the melanisation–desiccation resistance hypothesis
J. Exp. Biol.