You may well have seen a robber fly (also known as an assassin fly) and not recognized it as such. Big eyes, pointed mouthparts, a hairy face and a fondness for insects are robber fly characteristics to look for.
Robber flies are among our largest flies and are predatorial ambushers. They tend to perch for minutes at a time on the tips of leaves, or other sunny viewpoints, where they keep a lookout for unsuspecting prey. Once they spot an insect they lose no time in darting after it. With their beak-like mouthparts they spear the insect and inject a mixture of nerve poisons and enzymes that liquefy the tissues of their victim. They then drink the innards of their prey.
Note that there is a prominent “beard” or tuft of bristles (mystax) in the front of the pictured robber fly’s face. All robber flies have this tuft which serves to protect the face and eyes of the predator from the struggling prey as it dines.
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As readers are aware, I spend a significant portion of my time outdoors examining scat, and in so doing, have discovered a family of hairy flies, Heleomyzidae, that are often present on the freshest specimens, especially in early spring and late fall. This spring I have found them mating on both coyote and bear scat. The ardor of males is impressive – without exception the males pounce upon each and every fly of their own species they see. Repeated rejections do not appear to slow them down – just the opposite. In fact, moments after the pictured pair left the surface of a coyote scat and landed on a nearby branch, they were joined by another fly and the threesome tumbled to the ground in an attempt at a menage a trois (see inset).
Different species of flies in this family feed on different food sources. Look for them in and on carcasses, scat, compost, fungi, caves, and bird nests.
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The robber fly family, Asilidae, is one of the largest families of flies. All robber flies are predaceous and are recognized by their long bodies, forward-facing beaks and a tuft of hairs above the beak. You find them on the ground or on leaf tips and other sunny spots where they survey the area for flying insects. Once a robber fly has spotted a suitably-sized prey, it darts out and impales it with its stout beak. It then inserts its needlelike “tongue” into the prey’s neck, eye or other weak spot, immobilizing the insect and liquefying its innards with an injection of saliva that contains nerve poisons and enzymes that break down proteins. Finally, it drinks its meal. Pictured is a species of robber fly in the genus Diogmites, whose members are known for dangling by a foreleg while dining.
Adult hover flies, often referred to as syrphid (family Syrphidae) or flower flies, feed on pollen and nectar, and are often seen hovering at or crawling on flowers. Many have black and yellow bands on their abdomen, and are frequently mistaken for bees. There are certain species of hover flies that mimic stinging wasps, including yellow jackets and bald-faced hornets (see photo). Predators such as birds, ambush bugs, and spiders might think twice about eating an insect that can sting, and hover flies take advantage of this. The process through which this occurs is called Batesian mimicry, and refers to when a harmless species evolves to imitate a harmful species that has the some of the same predators.
Milkweed leaves make excellent platforms for all kinds of insects, particularly those such as dragonflies and robber flies, which sit, wait and watch, surveying the landscape for prey to ambush. This robber fly could quite easily be mistaken for a bumblebee. However, the short, straight antennae and the presence of only two wings (instead of four) tell you it’s in the order Diptera (true flies). The pointed, stout proboscis, bearded face, fleshy feet and long, tapering abdomen narrow it down to a species of robber fly. Robber flies in the genus Laphria resemble bumblebees – they are typically quite hairy, with black bodies and yellow stripes on their abdomens. Like other species of robber flies, they hunt by perching and snagging prey such as other robber flies, bees, wasps or beetles. They often return to their perch, inject the prey with enzymes that dissolve its innards, and then have a long drink. Why mimic a bumblebee? To deceive unsuspecting honeybees, wasps and other insects that would make a good meal.
Abnormal plant growths called galls come in all sizes and shapes, are found on leaves, buds and stems, and are caused by a number of agents, including insects. A majority of insect galls are caused by the eggs and developing larvae of flies, wasps and midges. Jewelweed, or Touch-Me-Not (Impatiens capensis), has a very distinctive looking aborted bud gall that is produced by a midge (Schizomyia impatientis). While some galls provide shelter and food for a lone resident, the Jewelweed Gall Midge is colonial, and several orange larvae can be found residing in separate cavities within the gall. These midge larvae are now emerging and will overwinter as adults.