Bug o’the Week – Golden Dung Fly

Greetings, BugFans,

From the start, the main criterion for any BOTW episode has been a reasonably good picture of its protagonist (second is whether the BugLady can discover its story).  Alas – it’s January, and it’s 439 episodes later, and the bar is getting lower.  Meet the Golden/Yellow dung fly.

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Its geographic range includes cooler, temperate regions throughout much of the world, and it probably arrived in North America from Europe early-on, with shipments of cattle.  It is associated with agricultural areas but is found wherever there is dung from large-ish mammals for its eggs and larvae.  Although references list golden dung flies as fairly common (especially if you hang around farmyards), the BugLady has only photographed them a few times over the past eight years, and never well.

These are small, spiny flies, maybe three-eighths of an inch long (and here’s a better picture of one http://bugguide.net/node/view/626435/bgimage).  Males are a little larger than females; females are not as golden and their front legs aren’t as fuzzy as the males’ are.  Appearance can vary geographically, seasonally, and because of a variety of other factors including larval food availability (it’s called “phenotypic plasticity”).  Their internet presence consists of publications for horse-owners, because of the barnyard services they provide (decomposition and pest control); photo sites, because, as Eric Eaton (Field Guide to Insects of North America) says, “The insect positively glows when the sunlight strikes its body at just the right angle;” and scientific sites, because of their thoroughly-studied and slightly bizarre reproductive practices.

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The scientific name, Scathophaga stercoraria, says it all.  Scatho, from the Greek “skatos,” meaning “excrement,” and phaga, “to eat,” plus “stercoraria,” from the Latin “stercoris,” meaning “of dung, found in dung.”

The fly’s life is dung-centered.  Adults locate dung by “scenting” it with their antennae and flying upwind to find it.  Mom and Dad meet there, mate there, lay eggs there, and take their meals there.  They feed on other flies that are attracted to dung, and they also consume nectar (and, if times get tough, each other), but they leave man and beast alone.  Larvae hatch in the manure’s warm, moist depths and consume both it and some other small insects they find in there, like the larvae of house flies and stable flies.

Females are more likely to be found in vegetation, but males, which outnumber females, hang out on manure all the time, like a gamey Singles Bar.  The arrival of a female causes such an uproar that males will sometimes grab other males in the confusion.  When that happens, the aggrieved male will “go up on tiptoes” on his front and rear sets of legs and use his middle legs to pry his would-be paramour off (the same tactic is used when an interloper tries to disturb a mating pair).

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She sets her sights set on larger males (body size = “fitness” in her compound eyes), and she has a peculiar Superpower – she has three sperm-storage chambers (spermathecae) within her abdomen.  She mates with multiple partners (which results in some internal “sperm competition”), and she can delay fertilizing and laying her eggs.  Some authors say that sperm from an assortment of males is mixed in the spremathecae, and that producing genetically diverse eggs with variety of males is most advantageous to the species (the final male in the queue, though, will father 80% of her young).

Other authors speculate that she may be able to favor sperm from the males she finds most attractive, possibly having the ability to sequester “A-list” sperm in the two spermathecae on the right side of her abdomen and to empty those first when fertilizing her eggs.

Eggs are laid in manure, and a female golden dung fly is a manure connoisseur.  Too old/crusty and her ovipositor will not penetrate it.  “Topography” matters – manure “peaks” may dry out before her eggs hatch, and valleys may flood in the rain.

The pioneering work on golden dung flies was done by English ecologist Goeff Parker, who, according to author Menno Schilthuizen, “spent his summers propped on an elbow next to a cowpat quietly amassing a volume of work that has since become a classic of ecology.”  Schilthuizen continues, “Completing his doctoral studies at Bristol University in the late 1960’s, he realized that dung flies on cow pats were in many ways the ideal system to base ones behavioral ecology PhD on.  First of all, the work was inexpensive: he needed only ‘a stopwatch, ruler, thermometer, tape recorder, glass vials, entomological pins, a notebook, and a pencil.’  Second, the flies and their habitat were ubiquitous; doing his fieldwork involved little more than stepping into his local cattle field…”  (Grad students take note).

As part of her research, the BugLady likes to find out where her subject fits in the food web.  Golden dung fly predators include birds, bats, and a variety of insects including beetles that frequent the dung pats, and patrolling robber flies.  Most surprising is an insect-eating fungus called Entomopthora muscae, which is actually a fly STD.  Once on-board, the fungal strands (hyphae) grow within the fly’s body cavity, digesting the fly’s guts, bursting through the softer tissue between the segments, ultimately appropriating the fly’s nervous system and “ordering” it to crawl upwards (so the fungal spores can be broadcast most effectively).  Flies that come to investigate the corpse may get more than they bargained for.

And so, finally, in the “Take-Pictures-of-Everything-and-Throw-Away-Nothing” category – the BugLady photographed this house fly in distress a few years ago, and now she knows what she was looking at.

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Kate Redmond, The BugLady

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