Robber Flies as a Group
Robber flies (Asilidae) are a large group (~7000+ species) within the true flies (in the order Diptera; they only have two wings). All robber flies have a distinctive look once you get to know them, with extraordinarily large eyes dominating small heads. The depression in the middle of the head in which sit their ocelli (simple camera-type eyes you can see as a little triplet of lenses). They are often long, with extreme examples beginning to look somewhat like damselflies (Zygoptera). Some robber flies can get to extraordniarily large sizes, such as those in the genera Microstylum or Laphria.
Other characteristics that identify robber flies include: their long (especially the hind pair) bristled legs that are generally thick and muscular for grappling with prey, their proboscis (pointy face-straw that may feature barbs and spines to harpoon prey), and in some cases a mystax (moustache of bristly hair shielding the eyes from the thrashing of any victim harpooned onto the proboscis) though this is not always present.
Hunting Strategies
Robber flies have a few different methods of attacking prey, with different groups being specialists in each. These can broadly be separated into three different categories: Sallying, Dog-fighting, and Gleaning. Each of the categories is discussed below, but please bare in mind these are artificial partitions of a diverse array of behaviour. Behaviour is rarely categorical nor invariable by its very nature, thus these are meant as a guide, not a definitive statement of separation. It's important to remember that hunting behaviour is species specific, and each species will have carved out their own niche and tactics.
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SallyingSallying involves using a perch as a home-base from which to conduct forays out to catch prey, which is then brought back to the perch to be consumed.
Sallying animals generally attack prey from below, silhouetting the target against the sky. This makes the prey more salient and easily tracked, while the predator is harder to spot against the ground below. Sallying robber flies are generally small, slightly built and hunt targets much smaller than themselves. Their size advantage means that they generally have motor superiority over their targets, allowing for their vertical flight and greater flight control. However, it does mean that they will hunt many targets over the course of the day. |
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StalkingStalking is possibly the most common method of hunting in robber flies. Stalking robber flies roam widely, and whilst they attack targets from perched position, they will rarely return to the same perch after each flight.
Flights are much less formulaic than in sallying, and can be horizontal, vertical, and may involve multiple stages or passes at the target. Some species may spend time shadowing targets in the air, plausibly to study them before attack. Stalking robberflies will likely attack targets as large if not larger than themselves, relying on venom to subdue the target. This means that the target is often fast and strong, leading to an aerial dogfight for supremacy, and the predator dropping to the ground with the heavy prey rather than carrying it off. |
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GleaningGleaning is a behaviour in which the predator snatches a target from a surface whilst remaining airborne. Whilst this has the guidance benefit that the target is near-stationary, it has the drawback that the prey has something to hold on to and may require prying-off.
Many insects have an escape response to release their feet and drop off a plant when threatened. If a gleaner is after them, this could well seal their fate. Within the robber flies, the members of the sub-family Leptogastrinae most clearly demonstrate the behaviour. These flies have a distinctive long body, though it is not yet clear how this might be related to their hunting style. |
Holcocephala: Our Goggle-Eyed Fly
In particular, my research has focused on the exploits of Holcocephala fusca, a miniature (7 mm from head to tail) species of robber fly from North America. Holcocephala is a genus of about 40 species, all of whom are sit-and-wait perchers. They wait at the tips of plants for smaller insects (generally < 2 mm) to fly overhead, at which point the fly will take-off and fly to a future meeting point with the target. They will often use the same perch for many hours and can occur in very high population densities, with 20+ in a square metre of terrain.
Holcocephala fusca with prey (left) and head-on without (right).
Holcocephala occurs only where patches of the sky are visible, and so rarely deep within woodland. They are particularly common in sheltered clearings and along the borders of forests. Unusually for robber flies, they appear not much enamoured with open ground and direct sunlight. In the forest clearings in which I've researched Holcocephala, many of the suitable perches will be abandoned during the middle of the day when direct sunlight falls into the clearing.
This could be due to more than the direct visual and temperature related effects of sunlight. During the hottest parts of the day, male Holcocephala take to the air in order to find females to mate with. Males will fly slowly between likely perches and spend time studying each one, suggesting that they find females visually. Once a female is found, the male will hover below the perch before surging up and forward, grappling with the female and attempting to couple. Males do not seem adept at distinguishing sex and will often attempt to couple with other males. The response on the part of the grapplee is generally to kick the long hind legs, attempting to dislodge the assailant. The wings, which are a dark black, are often flicked open, possibly as a visual signal. Kicking and flicking the wings is a common reaction to a looming object, and has been reported (Scarborough 1982) to be elicited when wind is blown across the animal from a small pipe, suggesting they may detect the airflow from wingbeats. The midday hiatus in hunting behaviour could be a mixture of males leaving to find mates and females being so harassed by amorous males that they are chased from their perches.
Once coupled, the male will hang upside down from the rear end of the female and remain coupled for tens of minutes. The female is able but reluctant to fly with the male attached, and her flight capabilities are vastly reduced.
This could be due to more than the direct visual and temperature related effects of sunlight. During the hottest parts of the day, male Holcocephala take to the air in order to find females to mate with. Males will fly slowly between likely perches and spend time studying each one, suggesting that they find females visually. Once a female is found, the male will hover below the perch before surging up and forward, grappling with the female and attempting to couple. Males do not seem adept at distinguishing sex and will often attempt to couple with other males. The response on the part of the grapplee is generally to kick the long hind legs, attempting to dislodge the assailant. The wings, which are a dark black, are often flicked open, possibly as a visual signal. Kicking and flicking the wings is a common reaction to a looming object, and has been reported (Scarborough 1982) to be elicited when wind is blown across the animal from a small pipe, suggesting they may detect the airflow from wingbeats. The midday hiatus in hunting behaviour could be a mixture of males leaving to find mates and females being so harassed by amorous males that they are chased from their perches.
Once coupled, the male will hang upside down from the rear end of the female and remain coupled for tens of minutes. The female is able but reluctant to fly with the male attached, and her flight capabilities are vastly reduced.
(Left) Holcocephala kicks its leg in a characteristic manner. (Right) a male hangs upside down; coupled to the larger female.
Holcocephala fusca is almost indistinguishable from its sister species Holcocephala abdominalis, both in reports of behaviour or from photographs, thus there is likely a high level of conflagration of reports on these species. The fusca was chosen as a species name due to its darker colouration than abdominalis, however there is considerable intra-specific variation which makes this a difficult diagnostic.
Both species co-occur with the much more separable species Holcocephala calva which is larger, has a grey complexion and a different posture. calva hunkers down on its perch, and is likely to be found fully against the stem rather than at the very tips of plants. Fusca and Abdominalis are generally much more abundant than calva in those habitats where they co-occur.
Both species co-occur with the much more separable species Holcocephala calva which is larger, has a grey complexion and a different posture. calva hunkers down on its perch, and is likely to be found fully against the stem rather than at the very tips of plants. Fusca and Abdominalis are generally much more abundant than calva in those habitats where they co-occur.
