Keeping insects as pets
By Astrid Kruitwagen
Astrid Kruitwagen is an evolutionary ecologist and has a special interest in insects. She is a PhD candidate at the RUG where she investigates whether and how natural enemies can adapt to new invasive pest species and how they can be used for biological control.
When I was young, I thought my favourite animals were birds. I liked to count all the different species in our garden, to wake up at 6 AM to go bird watching and to learn to identify birds by song. What I did not notice, however, was that there were other incredible creatures. I passed by them, every day, but was unaware of their existence.
This changed on a trip to one of the islands in the north of the Netherlands during a ‘nature summer camp’ when I was about 12 years old. Convinced by my new friends, I signed up for an excursion to observe and catch the, for me, – yet – unknown organisms. Before I knew it, I held a little bug between my fingers and I was instantly hooked. When I came back, my parents were fully surprised: my favourite animal had changed from birds to insects!
Now, more than ten years later, my love for insects has grown even more and I have even made it my profession. I have been doing research with lady beetles, ticks, honey bees, fruit flies and parasitic wasps. Recently, I added another species to this list when I invited insects into my apartment, walking leaves. The longer I lived with them however, the more I wanted to understand them. How could this leaf insect be sitting there, unnoticed, observing the world quietly, living without others being aware?
Some of my leaf insects (Phyllium philippinicus): an adult female.
Photo: Astrid Kruitwagen
While looking at the green leaf-like insects underneath the leaf in the terrarium on my kitchen table, I was amazed by their silence. Their rather immobile life style, however, does not seem to come in handy when they need to take care of their offspring. They often just ‘drop’ their eggs to the ground. At first, this sounds rather harsh. However, mothers do actually care for their offspring, but indirectly. According to a group of researchers from the United States and Germany, a key innovation in their evolution is the hard-outer egg shell that protects them when falling on the ground (Robertson and colleagues (2018)). This helps the egg to remain viable for long periods of time and withstand harsh conditions (sometimes even more than one year).
The eggs also look incredibly similar to plant seeds and can, just like seeds, be dispersed by water and animals like birds and ants. A popular hypothesis is that this is yet another example of plant mimicry. But how could such a strategy evolve? Would “playing for seed” not make it easier to be found by seed eating predators? Their evolution is actually still a mystery for us. Australian researchers propose that the similarities in morphology and mode of dispersal between plants and leaf insects might be a remarkable example of convergent evolution, where both species groups have evolved similar traits while living in the same environments (O’ Hanlon et al. 2020).
Some of my leaf insects (Phyllium philippinicus): a ‘baby’ leaf (juvenile stage).
Photo: Astrid Kruitwagen
Leaf insects are amazing in being invisible by imitating the leaves, twigs and barks of plants. This enables them to escape from the spiders, birds, lizards and bats trying to eat them. ‘Their survival strategy is evident as researchers recently estimated that leaf and stick insects living today evolved about 125-90 million years ago (Simon et al. 2019). In Messel, Germany, the first fossil record found of a leaf imitating insect is 47 million-years old (Wedmann et al. 2007). The great surprise was that even after all this time, their morphology has changed very little, something called evolutionary stasis. Hiding does seem to pay off for these creatures!
Not only do they look like their environment, they also behave similarly. While holding a baby leaf insect on my hand, I discovered that they can also move like a leaf blowing in the wind. This motion camouflage seems to be another mode of plant mimicry. Apart from that, they are very quiet and seem to be the master of “stillness”. They can sit for a long time, motionless, to strengthen their appearance as a leaf. Actually, they remain most of their life hiding between the leaves of the trees as most of the leaf insect species are not able to fly (although males of P. philippinicus are able to fly over short distances, and I sometimes have to search my whole apartment when one of them escapes!). Their quietness may seem boring, but it can also be very relaxing to observe them as it reminds me of the words of the Chinese philosopher Lao Tzu: “by letting go, it all gets done”.
Eggs of different stick and leaf insects. Image from Robertson et al. 2018
It is interesting that the ability of leaf insects to become part of their surroundings is totally different from our own human tendency to dominate, exploit and change the world around us. Yet, their strategy seems to be very successful. They exist already for millions of years and thus masters of survival on this plant. Although our drive to control the world has brought the wealth and innovations we are enjoying right now, it is highly questionable whether this will be the best strategy for future generations to survive. Maybe we should try to be more like leaf insects. See ourselves as part of nature, our environment. Not to avoid detection by other animals, but to protect us from ourselves.
References in this article:
O’Hanlon et al. 2020, The dynamic eggs of the Phasmatodea and their apparent convergence with plants
Robertson et al. 2019, Evolution of Oviposition Techniques in Stick and Leaf Insects (Phasmatodea)
Simon et al. 2019, Old World and New World Phasmatodea: Phylogenomics Resolve the Evolutionary History of Stick and Leaf Insects
Wedman et al., 2007, The first fossil leaf insect: 47 million years of specialized cryptic morphology and behavior