Two Gall Flies

     Most every niche on Earth is occupied by some living being, plant or animal.  And, I am sure, there are occupied niches we humans don't know about, yet.  And there will be niches in the future that don't exist yet, but will be by some plant or animal when those habitats are present.  The larvae of two kinds of flies in North America are examples of animals using niches that we can't imagine until we become aware of them.  One species of fly is the goldenrod gall fly and the other is the Canada thistle gall fly.  The larvae of both species of flies create galls, or swellings, in the tissues of their plant hosts that they are named after.  There they live, eating the tissues of those plants until they pupate and emerge as adult flies.  
     Goldenrod plants stand up to five feet high and have pretty clusters of tiny, golden flowers at the top of their stems by August and into autumn.  Some goldenrod stems have round or elliptical, green swellings called galls in summer.  Turning brown when their plant tissues die in fall, those galls are caused by the larvae of goldenrod gall flies.  Those larvae, which are protected in the galls, eat the tissues in them, hollowing them out.
     Goldenrod gall flies are native to most of the United States.  In spring, adult male goldenrod flies emerge from their galls, followed a bit later by female goldenrod flies.  Small, brown flies that blend into their surroundings, males and females pair off and mate.  Then females use their ovipositors, located on the ends of their abdomens, to insert fertilized eggs into goldenrod plant tissues near the developing buds of growing goldenrod plants.      
     In a few days the goldenrod larvae hatch and eat their way to the bases of goldenrod leaf buds, which induces the tissues to form into bulbous chambers, each about the size of a purple Concord grape- the gall, with a hardened covering.  One larva lives in each gall, which provides the small, white grub with shelter, food and liquid for the summer, into fall.
     Some of the larvae of goldenrod flies are eaten while they are at home in their galls.  Downy woodpeckers and at least two kinds of chickadees have learned to chip into galls to extract the grub inside and eat it.  The larvae of a couple of kinds of parasitic wasps, deposited as eggs in goldenrod galls by their mothers, also consume goldenrod gall fly larvae.  
     By mid-September, each goldenrod gall fly larva instinctively chews a tunnel through its gall almost to the outside.  They must do that as larvae when they have the bodily equipment and strength to make that burrow.  Upon finishing its tunnel, each larva settles down to being a brown pupa, the stage in which it overwinters. 
     During winter in its gall, each goldenrod fly larva accumulates sorbitol and glycerol in its tissues which protects it from freezing.  And freezing temperatures allow the cold-blooded larvae to conserve their energy.   
     In the warmth of the following spring, each pupa develops into an adult fly, crawls out the tunnel it created the autumn before, and pushes through the extremely thin wall of its gall to the outside world.  There it finds a mate and another generation of goldenrod gall flies is born.
     Canada thistle gall flies are originally from Europe where their larvae feed on the tissues in elliptical galls of their own making on Canada thistle stems.  This kind of fly was introduced to North America to control the growth and spread of its host, the invasive Canada thistles.  A kind of fruit fly, Canada thistle flies are pretty, little critters with black thoraxes and abdomens, and white wings, with black patterns on each one.   
      Female Canada thistle flies lay eggs on Canada thistle stems during warmer months.  The larvae hatch and burrow into the stems where they form the galls they live and dine in. 
     The larvae overwinter in those galls and are reddish-brown pupae early the next spring when still in their galls.  The adult flies emerge from their galls to the outside world, via each one's tunnel, from June to October.  The flies mate and the females place eggs into Canada thistle stems where they hatch into another generation of Canada thistle larvae.
     These are just two of the innumerable ways that life survives on Earth.  Each species adapts to specific conditions; its niche.