I first saw a flower wasp (Scolia soror) many years ago, on my walk home from school. Their black bodies and iridescent blue wings made them stand out, in stark contrast to the ubiquitous orange honey bee, or the overtly menacing European wasp. ‘How pretty!’ I thought, ‘I didn’t know we had shiny bugs in Melbourne!’
I’ve continued to spot them for years, here and there, waddling around on the ground. Recently however, I was walking my dog through the park and I came across a dense swarm of them. They were congregating on a particular flowering shrub, its small white blooms attracting them by the hundreds. Thankfully I’m not afraid of bees, as they were completely blocking the path! Seeing so many of them together piqued my curiosity. So, I went digging for information.
Pleasantly, I discovered that flower wasps are native to Australia and are found in Victoria, New South Wales and parts of Queensland. They’re 2.5-3cm in length, with relatively short antennae and bristly hairs covering their stout bodies. Adult flower wasps eat nectar and are solitary, meaning they don’t have a hive to defend and are therefore unlikely to sting.
Another interesting thing about these wasps is that they’re part of the most species-rich group of animals on the planet. While it has long been thought that the order Coleoptera, (which contains all the beetles in the world), held the title for most species, recent research suggests that actually, Hymentoptera is the real winner. Hymenoptera contains the wasps and it is actually the parasitoid wasps, which have been somewhat neglected in academic literature, that make Hymenoptera so speciose. This is perhaps not great news for other invertebrate species, as the larval stage of the parasitoid wasp life-cycle requires the nutrient-rich diet provided by the body of another animal. Adult parasitoid wasps lay their eggs in or on another insect or grub and upon hatching the larva will eat it alive.
The flower wasp is a member of this grisly group and its target of choice is a beetle. This is why I normally come across them on the ground; the mother wasp is wandering around, searching for a suitable host for her eggs. Despite being quite off-putting, parasitoid wasps have been used by humans as early as the 1920s as a pest control measure and make up an important part of the earth’s biodiversity. Every animal has its place in the ecosystem after all, even ones which might make us squirm.
But what of the beautiful blue iridescence which attracted me to the flower wasp in the first place? The colours we see on the fur, feathers, skin or scales of an animal come from two different sources. These are pigments or structural components in the surface of the animal’s body. Pigments work by absorbing specific wavelengths of light and reflecting others. To an observer looking at the animal, it will appear the colour that is reflected. This is how colour works in most everyday objects as well, however, iridescence is not produced in this way. Instead, in the case of insects, it comes from the way light interacts with the structure of the animal’s exoskeleton.
The surface of the exoskeleton is made up of microscopic imperfections such as bumps and layers which divert the path of incoming light. This leads to interference between incident light rays, a property that is due to the wave nature of light, which eliminates certain wavelengths. Some light will remain, and will exit the exoskeleton, to be sensed by us, similarly to how pigments work. However, interference is dependent on the angle at which the light hits the surface, which is why a butterfly’s wing looks different if you view it from varying positions. Interference is a very interesting, simple, concept in physics; I love when a clear understanding of biological phenomena requires delving into the realm of physical principles and iridescence is a great example of this.
While I thought them pretty before, researching the flower wasp has given me a new appreciation for their appearance and behaviour; even the smallest creature can be fascinating. Unfortunately, there haven’t been any detailed studies of their behaviour and lifecycle. Perhaps in future, a budding researcher will find them as worthy of interest as I do, and we may know more about these alluring creatures.