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65 PART IV Truly Flies About True Flies (Diptera) True flies are a group with which everyone is familiar, in one way or another. Mosquitoes, black flies, crane flies, and midges (sometimes called gnats) are among the best known aquatic Diptera because they are sometimes irritating to humans, often very abundant, and occasionally economically important. All true flies require some sort of moisture, which can come from quite a variety of sources like the inside of a plant or animal; for the purposes of this book, we are interested in those that live in bodies of water, primarily flowing water. Diptera have adapted to just about every habitat imaginable, except for open ocean. To reach their extraordinary levels of success, flies have developed a great range of adaptations for eating, breathing, and moving in extreme conditions. Morphology True fly larvae are highly specialized. They do not have segmented legs, but some have one or more pairs of short prolegs; others have abdominal creeping warts or turbercles that help them get around. Dipteran larval heads are very small at best. There are two major divisions of aquatic flies, distinguished by those with fully exposed heads in contrast to those with highly reduced heads. Larvae with well-developed heads have stout, toothed mandibles that bite or chew their food, as opposed to the clawor hook-like mandibles larvae with reduced heads use to pierce or slash their prey. The highly modified mouthparts of black fly larvae are fans that they extend into the water for filtering food particles. The thorax usually has three segments that sometimes have respiratory openings called spiracles. Along the abdomen there may be a wild array of nubby tubercles, creeping welts, setae, or pubescent hairs. At the hind (or posterior) end of many larvae are various numbers of spiracles that are sometimes 66 Truly Flies adorned with spines, making the posterior end of the animal more elaborate than the opposite, anterior end, which might have a barely visible, reduced head. One feature that distinguishes the adults of true flies is a pair of knob-like halteres behind the larger, single pair of wings. The halteres are remnants of the second pair of wings and are used for balancing the insect in flight. Though some families of true flies spend adulthood skimming the water surface, most do not remain in direct contact with water. However, most aquatic adult flies stay close to water before laying their eggs in an aquatic habitat, with the exception of those that require a blood meal (such as mosquitoes, horse flies, or black flies). Life History Generation time for true flies vary greatly. Some species have one generation (univoltine) or two (bivoltine) per year; in these groups, adults are usually present in the spring and summer. Others have many generations per year (multivoltine). True flies undergo complete metamorphosis, having four distinct life stages. Usually there is a brief egg stage (a few days to weeks), followed by three to four molts as larvae when most are freeliving . Pupae may be free-swimming, attached to the bottom, burrowed in the substrate, or simply pupated within the last instar skin. They emerge as adults either at or below the surface, and live anywhere from a few days to several weeks or months. Bioindicators As diverse as true flies are in their adaptations to the aquatic environment, so are their levels of sensitivity or tolerance to pollution and habitat alteration. Most Diptera are considered to be relatively tolerant of polluted conditions. However, the blepharicerids and Deuterophlebia described in our book are among the exceptions to the notion of dipteran tolerances. As a general rule, the longer-lived (e.g., univoltine) species comprise those that are sensitive, whereas the shorter-lived (e.g., multivoltine) species, such as mosquitoes, are tolerant of a variety of water quality conditions. ...

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