Wednesday, March 9, 2011

Wasp Wednesday: Wasp/Not Wasp

Previously on “Wasp Wednesday” we featured keyhole wasps of the genus Trypoxylon. Like most stinging insects, these wasps are models for other harmless insects that escape predators by pretending to be something more dangerous than they are: a sheep in wolf’s clothing if you will. Meet Pseudodoros clavatus, a near perfect imposter of a keyhole wasp.

A biologist friend once cautioned me on jumping to conclusions about which insects serve as models for various mimics, but just look at this fly. The elongated abdomen even has pale markings to make it appear narrower than it actually is. At 7-12 millimeters long, it is also similar in size to many Trypoxylon wasps. Besides looking like the wasp, the fly even behaves like it. Male keyhole wasps often hover in front of vertical objects, and the fly hovers just as well, if not better than, the wasp.

Ok, so how do you tell them apart, anyway? Above is the wasp. Note that it has two pairs of wings (though connected to each other they are usually still discernible as separate). The antennae of the wasp are thick and relatively long. They eyes are large, but do not take up the entire head or face of the wasp. Now look at the fly below. It has only one pair of wings. The antennae are so short they are scarcely visible. The eyes cover most of the head of the fly, and are not notched on the inner margin like those of the wasp.

Pseudodoros clavatus is a member of the family Syrphidae, collectively known as “flower flies” (“hover flies” in Europe). They are frequent, abundant visitors to flowers of all kinds. While they may be insignificant pollinators, they play their part in perpetuating wildflowers. Their good deeds extend to the larval stage as well. The maggots of Pseudodoros are voracious predators of aphids.

Seeing one of these slug-like larvae on your rose bush might lead you to think that it is also eating the plant, but watch one closely and you will see it methodically slaying aphids, seizing the tiny sap-suckers and hoisting them off the stem. The fly larva then sucks the hapless pest dry and discards the empty husk of its exoskeleton.

Look for this species from coast to coast in the United States and southern Canada. It may be confused with the similar genera Baccha and Ocyptamus in some parts of its range, but Ocyptamus species have at least faint dark markings on the wings (if only a bold leading edge to the wing). Baccha is more difficult to distinguish, but at least one species has a distinctly banded abdomen. The image below is still a Pseudodoros.

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