Impurest's Guide to Animals #133 - Efferia aestuans

I couldn’t decide if it was vanity or curiosity that made me google ‘Impurest Cheese’s Guide to Animals’ but the results were surprising indeed. It appears that the IGTA Octopus has spread to pintrest and facebook without me realising, and speaking of octopuses, last week we met a strange one known as the Greater Argonaut. This week we have a rapacious request from @cbishop, hope you guys enjoy.


Issue #133Efferia aestuans


Kingdom – Animalia

Phylum – Arthropoda

Class – Insecta

Order – Diptera

Family – Asilidae

Genus – Efferia

Species – aestuans

Related Species –Efferia aestuans is one of the 7000 species that make up the Robber Fly family(1)

Range – Efferia aestuans can be found in meadows and rough grassland ranging from Ontario in the north down to Florida and New Mexico in the South

Highway Robbery

Efferia aestuans is a medium sized slender bodied grey fly which reaches a maximum length of just over a 2cm. The species, like all robber flies, has a body covered in fine hairs and the antennae are bristle like and positioned between the flies massive eyes (2). In addition to two large compound eyes, E. aestuans also has a secondary set of eyes recessed between the main eyes which aid the fly in detecting the differences caused in light and darkness when a silhouette of a flying insect moves over it.

Like all robber flies Efferia aestuans is an obligate predator that feeds on other flying insects, usually intercepting them mid-air from its hunting perch. The range of prey taken by E. aestuans is fairly diverse and includes butterflies, beetles, dragonflies and grasshoppers but the main part of its diet is made up of bees and wasps. When seized, the prey is cut open with the dagger like proboscis and injected with a neurotoxic saliva which quickly paralyzes the caught insects so it can be dragged back to the hunting perch and consumed. To defend itself from any retaliatory attacks from the prey before it’s paralysed, the robber fly has bristle like hairs, known as the ‘mystax’, that act as a defensive screen preventing stinging prey from striking the predators face and eyes.


Male Efferia aestuans attract female attention by vibrating back and forth from the tops of their hunting perches. If the female approaches the male flies off his perch and seizes her mid-air before locking his genitals into hers, with the pair mating on the move and on the wing (3). Once completed, the female will find a hole in the ground and lay her eggs inside, with the grubs tunnelling down upon hatching until they find a beetle tunnel. From here the predatory maggots spend up to two years eating other insect larvae before returning to the surface and pupating, ready to emerge as an adult up to a year later.

Five Foul Flies

While the Mydas Fly (Gauromydas heros) and Timber Fly (Pantophthalmus bellardi) may be competing for the largest fly, the species with the longest leg span is the giant crane fly Holorusia brobdignagius which with legs spread out, reportedly reaches a length of 23cm

While we all know flies can be found everywhere, one group the wingless snow flies (Chione sp) can only be found on the tops of mountains walking across snow fields even in the depths of winter


The Petroleum Fly (Helaeomyia petrolei) takes it a step further, with the larvae of this species living in pool of naturally occurring crude oil at a single site in California. In addition to feeding on insects that fall into the oil pools, the maggots appear to ingest a large amount of oil as well, although the species doesn’t appear to gain any nutritional value from these chemicals (4).

Male Stalk Eyed Flies (Teleopsis sp) use their long eye stalks in ritualised displays to attract females as well as for jousting (in much the same way male deer do) with other males for mating access to groups of females.


The Ant Decapitating Fly (Pseudacteon sp) is a parasite on the Red Imported Fire Ant (Solenopsis invicta) who lays its eggs on the thorax of an ant. On hatching the maggot buries into the ant’s head and feeds on its flood whilst producing an enzyme that weakens the ant’s body. After about two to four weeks (depending on species), the ant’s head falls off and the maggot pupates in the severed head of its host before eventually emerging as an adult.



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4 - Thorpe, William Homan (1930). "The biology of the petroleum fly (Psilopa petrolei)". Transactions of the Entomological Society of London 78: 331–344

Picture References

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Talk about foul flies indeed! Next week we gingerly head out onto the water once again, but until then make sure to critic, comment and suggest future issues as well as making sure you check past issues in Impurest’s Bestiary.

Many Thanks

Impurest Cheese

Want more IGTA? Uhm well there was that issue from two weeks ago the Mydas Fly, click here to see that. And for more ultra-predatory insect shenanigans click here to check out the Wasp Mantidfly