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Flies

Bumble Bee-Mimicking Robber Flies

7-10-19 robber fly 0U1A0212Robber flies are a special group of predatory flies that possess stout, spiny legs, a dense moustache of bristles on the front of the head (mystax) that protects the robber fly’s head when it encounters struggling or stinging prey, and three simple eyes in a depression between two large compound eyes.

Certain robber flies resemble bumble bees. The evolutionary advantages of a harmless organism mimicking a harmful organism (Batesian mimicry) include deterring predators that are fearful of stinging bees. In addition, it also makes it more likely that a potential prey insect will come closer to the bee mimic, thinking the “bee” is looking for nectar or pollen, not a meaty meal.

A quick way to distinguish a robber fly from a bumble bee is to look at the antennae and wings. Flies usually have short, stubby antennae; bees, including bumble bees, have “elbowed” antennae (an obvious joint). Flies have two wings, while bumble bees (and most other insects) have four. The pointed, stout proboscis, bearded face, fleshy feet and long, tapering abdomen of robber flies are also identifying characteristics. (Photo: Laphria thoracica piercing a click beetle between the beetle’s hardened outer wings (elytra) in order to inject enzymes which will eventually allow the robber fly to drink its prey.)

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Robber Flies Active

8-3-18 robber fly with butterfly2_U1A3399

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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March Flies Emerging

5-30-18 march fly_U1A4814If you’ve been spending time in fields and meadows recently you may have been witness to the mass emergence of a species of fly known as the March Fly (Bibio albipennis). Their common name is a misnomer, for they are usually seen in April and May in the Northeast. The hatching of March Fly eggs in the soil produces larvae that feed mostly on decaying organic matter. After the larvae pupate and emerge as adults, you find dozens of flies clinging to grass and other vegetation.

Another species of fly in the same genus is known as “Lovebugs,” due to the habit of the males remaining “plugged into” the females during long copulations. Entomologist Stephen Marshall has this to say about their presence in southeastern U.S.: Even though adult Bibionidae are innocuous non-biting insects, the sheer number of fornicating flies fouling car windshields, pitting paint jobs and clogging up radiators renders Lovebugs a well-known Bible Belt nuisance.

The male and female March Fly are sexually dimorphic, differing in appearance. Like many other flies that form male swarms, the males have large heads with massive eyes. The females’ eyes are much smaller. Both are common on flowers and can be significant pollinators of fruit trees.

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Flies Mating

4-13-17 mating flies 042

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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Snow Scorpionflies Active

2-15-17-snow-scorpionfly-039On mild winter days you may wish to look closely at all the dark, little specks on the surface of the snow. Most of them will be bits of lichen, seeds or pieces of bark, but with luck you may find one or more of them moving. Active winter insects aren’t plentiful, but they do exist.

Scorpionflies are fairly common and can be recognized by their long beaks ending in visible mouthparts. (They get their name from one family of scorpionflies that possesses a scorpion-like tail.)  Snow Scorpionflies belong to a family of small, flightless insects, of which there are two species in the Northeast. They only measure about 2/10’s of an inch, so careful scrutiny is necessary to spot one. Their dark color and an anti-freeze substance in their blood allows them to remain active to 21°F., during which time they feed on mosses. When startled, Snow Scorpionflies often jump up in the air and land with their legs crumpled up (see insert) looking even more like an inedible speck of dirt. It’s fairly easy to tell their gender, as female Snow Scorpionflies (photo) lack wings, and males have bristly wings adapted for grasping females.

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Lovebugs Mating

11-18-16-lovebugs-mh_20091020_004606_2If you see a swarm of small black insects bobbing up and down in the air, you may well be observing the mating ritual of March Flies, also known as Lovebugs. These flies belong to the family Bibionidae, and the majority of species emerge in the spring, hence their common name. (In the Northeast, many appear in April and May.) However, there are certain species which emerge and mate in the fall and they are active now.

March Flies are dimorphic. Males (fly to the left in photo) are easily discernible because of their large eyes, essential for finding mates and chasing competing males. Males comprise most of a swarm, performing an aerial ballet as they bounce up and down, courting females as they emerge from the ground en masse. Once mating has taken place, the male and female of certain species remain attached to one another, even in flight. This is where the name “Lovebug” came from.

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Hover Fly Mimics Bald-faced Hornet

8-5-15 bald-faced hornet and hover flyAdult 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.

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