One-way traffic

Published 07/03/17

DURING our visit to the Little Desert, I remembered that on occasion, I witnessed an unusual phenomenon on my trips to the semi-arid mallee regions of south western NSW. This was the strange insect-driven conga-line of the processionary caterpillars, making their way across the sandy roads of the outback. For anyone who has witnessed this remarkable behaviour, it is a sight to behold. Apparently, these species cohabitate in a silken bag-like nest during the night and then travel in the procession lines throughout the day in search for food. A single line of these little hairy critters can be up to several metres in length, where each individual has the role of following the individual in front and reporting when the one behind has become detached. 

Out of curiosity, I experimented by using a twig to separate the last caterpillar from the main train. Almost immediately, the next caterpillar at the end of the train seemed to sense that the one behind had lost its way and came to a halt, sending a jolt-like signal gradually up the line to the foremost caterpillar. This caused the whole procession to efficiently come to a halt. Once the little lost individual from behind caught up and reattached to the back of the procession, then another message was sent up to the foremost caterpillar to cause it to begin its journey again. I wonder what the benefit is for these species to form long lines. Perhaps it because there is greater safety in numbers and/or the snake like formation makes them appear more dangerous to other animals. 

I decided to investigate some research on the processionary caterpillars and found out some interesting information. The early French naturalist “Jean Henri Fabre” experimented by causing the front caterpillar to career around a large round object and to join the last caterpillar. This caused the procession to continue going in circles for several days until individuals started dropping dead from exhaustion or starvation. 

In another experiment, researchers clipped the hairs on the back of several caterpillars in the procession and found that the individual behind was unable to remain attached, revealing that they maintain the process line by touch stimuli through their hair. 

On a darker note, there is evidence that the hair follicles left behind from these caterpillars can become ingested by grazing animals and cause equine amnionitis (inflammation of the placental membrane) in mares and the subsequent death of unborn foals. This has become a serious issue in parts of northern Australia and elsewhere in the world where this family of caterpillar occur in large numbers. Despite this, the processionary caterpillars are a remarkable creature and definitely worth looking at if you are lucky enough to witness them on the march. 

– Joab Wilson