In my previous post I mentioned that one of the most common questions I got was whether orchid bees are some sort of fly. Indeed there are many flies that have metallic colors, but the resemblance usually stops there. The best example are bottle flies, members of family Calliphoridae, which occupy a niche different from that of orchid bees and do not share any similar behaviors with them. It does not mean, however, that Euglossinae-mimicking flies do not exist. In the tropics, some hoverflies (family Syrphidae) have evolved to look like orchid bees. Several species of the genus Copestylum resemble Euglossa species and they are often found foraging near active orchid bees. Even more interesting is Genus Ornidia, which bears a strong visual resemblance to some Euglossa species, and even copies some of the bees behavior.
The genus Ornidia contains five species, all have shiny metallic colors and body structure that resemble those of orchid bees. Their legs in particular are thick and robust to look more like bee-legs than the typical skinny legs of hoverflies. Ornidia are distributed mainly in the tropical regions of Latin America, however one species, Ornidia obesa, reaches the southern United States and has also spread into the Afrotropical, Oriental regions and Oceania, probably due to human activity. Ornidia flies are quite common and they are frequently found close to human habitations.
These beautiful flies can be observed safely even from a close distance. They are not very skittish, and usually when disturbed they quickly take off, hover in the area for a few seconds, and return to the same perch. It is especially rewarding to watch them warming up during the morning hours, when they hover in a single spot for a while, trying to catch some sun rays penetrating through the canopy. The loud buzzing sound produced by their wings during flight is very similar to that of Euglossa species. During flight, the fly also displays a behavior that appears to mimic orchid bee behavior: it crosses its legs several times, similarly to a male Euglossa transferring fragrant compounds to the hind tibiae, or alternatively to a female Euglossa transferring resin to the hind legs.
The adult flies feed mainly on liquid food such as nectar and animal feces, but can also take small-sized particles like pollen and fragments of decomposing organic matter. Ornidia larvae are generalist feeders and seem to exploit various food sources to complete their development. Firstly, they can be found in rotting fruits, leaf litter and compost piles. Several interesting papers report the larvae to feed even on vertebrate corpses, suggesting the potential use of these maggots for forensic work. Lastly, Ornidia larvae were also found to cause intestinal myiasis in humans, after being ingested with infested food. Nevertheless, these flies pose no threat to us; Myiasis caused by Ornidia larvae is rare relatively to other fly species, the flies have plenty of abundant food in their habitat and there are no records of Ornidia flies completing their development inside a human host.
How would a fly benefit from looking like an orchid bee? As I mentioned in my post about Euglossinae, these bees are not very aggressive due to their solitary lifestyle. However, the flies may still benefit from this mimicry because the bees are dominant in the rainforest habitat. The female orchid bees have a stinger and can deliver a painful sting, this alone can deter a predator. In addition, the highly territorial male orchid bees are usually left alone by other flying insects. The mimicking flies take advantage of the fact that orchid bees are common and recognized by other animals, including predators.