When it comes to flies, many of us immediately associate them, often wrongly, with annoyance and diseases. However, not all flies fit this negative views as many species act as pollinators for instance. Robber flies, in particular, defy these stereotypes with their expert predatory habits. 𝘗𝘳𝘰𝘮𝘢𝘤𝘩𝘶𝘴 𝘪𝘯𝘥𝘪𝘨𝘦𝘯𝘶𝘴, a native robber fly in Hong Kong, is remarkable for its agilityand predatory behaviour. If you take a walk in a country park and carefully observe leaf tips, logs, or other sun-splashed objects, you may notice not only butterflies and bees perched upon them, but also these impressive, large-sized flies.
Just like other insects in the Diptera order, robber fly (Asilidae family) presents only one pair of fully functional wings (something you may learn in your visit at the HKBM). The hindwings have evolved into small, knob-like structures called halteres, allowing them to perform incredible body rotations during flight. This increases their maneuverability significantly and strengthens their ability to catch animals in the air. To distinguish them from other flies, robber flies have distinctive characteristics, including a robust and elongated body, a pair of prominent compound eyes, and a signature tuft of hairs above the mouthparts. In general, robber flies show great body size variation ranging from 2 millimeters to 6 centimeters in length, and are able to tackle dragonflies or bees. Unlike many other species that mimic the yellow and black pattern found in spider wasps or bumble bees (see our recent posters for many astonishing examples), individuals of 𝘗𝘳𝘰𝘮𝘢𝘤𝘩𝘶𝘴 𝘪𝘯𝘥𝘪𝘨𝘦𝘯𝘶𝘴 are gray in colour and do not have those sharp colourful patterns.
Their name « robber flies » refers to their exceptional predatory habits and excellent flying skills, especially when it comes to catching prey in flight. A feat shared with several other groups of animals, including dragonflies (Odonata order). How are robber flies, however, able to catch prey in mid-air? First, they perch on the tips of leaves or logs in an open area, allowing them to use their large compound eyes to search for insects or spiders. Once they spot the suitable targets, they take off and swiftly seize the prey with their clawed legs. They will then return to a perch, often choosing the same location, to enjoy their well-earned meal.
After catching the prey, they face the challenge of effectively consuming the food, especially when the prey is as large as their own body size. To overcome this, the robber flies adopt an interesting feeding strategy similar to that of spiders, which is to digest the food externally. It begins by piercing the prey with its needle-like hypopharynx (tongue). Usually, they will choose weak spots to punch the body, such as the soft regions between the elytra (for a beetle), eyes, and cervix (neck). They will then immobilize and liquefy the prey through the injection of saliva that contains nerve poisons and proteolytic enzymes. These enzymes can break down and liquefy the tissue of the prey, allowing the robber flies to consume the prey and extract essential nutrients more effectively.
Species of robber flies may hunt different animals due to their variation in size. For the species in the 𝘗𝘳𝘰𝘮𝘢𝘤𝘩𝘶𝘴 genus, which are generally large, bees and wasps are usually on the menu. This explains why they are often observed among the foliage of trees, attentive to potential prey. Unlike the adults, the larvae of 𝘗𝘳𝘰𝘮𝘢𝘤𝘩𝘶𝘴 have a distinct feeding behaviour. They feed on scarab beetle larvae (Scarabaeidae family) within the soil. A study conducted in China showed that the presence of six to eight robber fly larvae per 5 square meters of soil can result in a remarkable 96% reduction in the damage caused by scarab grubs to wheat seedlings. This indicates that 𝘗𝘳𝘰𝘮𝘢𝘤𝘩𝘶𝘴 larvae present an interesting potential to serve as biological agents in controlling scarab grubs, offering a natural solution to mitigate their negative impact on crops.
These expert predatory skills both as adults and larvae, agility and body structure define these fascinating airborne killers. They also play valuable roles in maintaining a balanced ecosystem, yet their diversity remains largely underestimated in Hong Kong with two new species discovered and described in recent years. We hope this story provides you a new perspective on flies, one of the most diverse and abundant organisms on Earth; so be sure to come to the Hong Kong Biodiversity Museum and explore more about them and other fascinating animals and plants!
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