Microcosmos 2 - Diversity in the Quarry Corner

During my September séances in the Quarry Corner a surprising variety of insects appeared along the shoreline. The presence of water is not only essential to life but multiplies the habitat elements that make for a more diverse ecological community.

Green Bottle Fly, Lucilia sericata, family Calliphoridae

There were of course commonplace insects like this Green Bottle Fly. The opportunity to see it close up (macro-photography) presented a rather elegant portrait of a creature most of us have a dim view of around the house, with its uncanny knack for dodging screen doors and buzzing around the kitchen. I rinsed out some prejudices and read a bit about this fly's split-second flight reflexes and ability to come to light upside down on a ceiling.

Grass Fly, Thaumatomyia glabra, family Chloropidae

An eighth-inch speck on the tip of a leaf introduced me to this member of the Chloropidae family, a handsomely decorated type of Grass Fly. Before the month was out I had encountered representatives of nearly three dozen fly families at Halibut Point, each with potentially multiple genera and species. Clearly flies were a very diverse group. Among all the winged six-legged creatures, I wondered, what distinguishes a true fly?

A crane fly of the family Limoniidae,
nectaring on Lycopus virginicum, Virginia water-horehound

As I crouched in the Quarry Corner a remarkably large mosquito-looking creature drew nectar from the flowers of a plant growing on the shoreline. It was a crane fly, in no way equipped to siphon the blood of an animal (me). Both it and mosquitoes are members of the insect order Diptera (from the Greek meaning "two wings"), the True Flies. Diptera are distinguished by using only a single pair of wings to fly. Their wing arrangement gives them great maneuverability in flight. This order contains an estimated 1,000,000 different species of insects.

A crane fly showing the hind wings reduced to drumstick-shaped halteres *

The hind wings of True Flies have evolved into advanced mechanosensory organs known as halteres, which act as high-speed sensors of rotational movement and allow dipterans to perform advanced aerobatics. 

Flies have evolved remarkable features in their small bodies to enable their prolific abilities. A few of these are described in the footnotes below.

Female Tiger Crane Fly, Nephrotoma ferruginea, family Tipulidae

Many crane flies live in moist habitats. The one pictured above is hovering along the edge of the quarry repeatedly bouncing up and down on the waterline to deposit eggs beneath the surface. In both form and behavior the insect world has stretched inventiveness in ways that prompt human imagination to ever greater wonder. 

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* Photo and description from the Wikipedia article "Fly". 

Maneuverability

The halteres act as gyroscopic and are rapidly oscillated in time with the wings; they act as a balance and guidance system by providing rapid feedback to the wing-steering muscles, and flies deprived of their halteres are unable to fly. The wings and halteres move in synchrony but the amplitude of each wing beat is independent, allowing the fly to turn sideways. The wings of the fly are attached to two kinds of muscles, those used to power it and another set used for fine control.

Flies tend to fly in a straight line then make a rapid change in direction before continuing on a different straight path. The directional changes are called saccades and typically involve an angle of 90°, being achieved in 50 milliseconds. They are initiated by visual stimuli as the fly observes an object, nerves then activate steering muscles in the thorax that cause a small change in wing stroke which generate sufficient torque to turn. Detecting this within four or five wingbeats, the halteres trigger a counter-turn and the fly heads off in a new direction.

 

Vision

Flies have a mobile head with a pair of large compound eyes on the sides of the head, and in most species, three small ocelli on the top....For visual course control, flies' optic flow field is analyzed by a set of motion-sensitive neurons. A subset of these neurons is thought to be involved in using the optic flow to estimate the parameters of self-motion, such as yaw, roll, and sideward translation. Other neurons are thought to be involved in analyzing the content of the visual scene itself, such as separating figures from the ground using motion parallax. The H1 neuron is responsible for detecting horizontal motion across the entire visual field of the fly, allowing the fly to generate and guide stabilizing motor corrections midflight with respect to yaw. The ocelli are concerned in the detection of changes in light intensity, enabling the fly to react swiftly to the approach of an object.

 

Taste and touch

Like other insects, flies have chemoreceptors that detect smell and taste, and mechanoreceptors that respond to touch. The third segments of the antennae and the maxillary palps bear the main olfactory receptors, while the gustatory receptors are in the labium, pharynx, feet, wing margins and female genitalia, enabling flies to taste their food by walking on it....Flies that feed on blood have special sensory structures that can detect infrared emissions, and use them to home in on their hosts, and many blood-sucking flies can detect the raised concentration of carbon dioxide that occurs near large animals.