July is hoverfly season.
David Beeson, 11th July.
Flies have a single pair of wings and, before them, a pair of halteres (shown). Halteres are a pair of small club-shaped organs on the body of two orders of flying insects that provide information about body rotations during flight. They have compound eyes and a breathing (gas exchange) system of tubes (trachea) leading from external holes (spiracles) in their semi-rigid chitin exoskeleton.
Hoverflies, hover and look similar to bee drones, but the latter have two pairs of wings. As a group, they have striking colouration.
The diagram below is NOT a fly, but it shows the gas exchange system clearly. With a captive, large grasshopper the movement of the abdomen to drive oxygen-rich air through the breathing tubes can be easily watched. A dead animal, opened under water will show the silver looking (because they are air filled) trachea clearly. And, by sealing an animal with light cotton wool (or similar) in a tube you can expire air into the tube and see the effect of a slightly enhanced carbon dioxide level on the breathing / ventilation rate.
Adult hover flies live for about a month and their life cycle takes place in four stages (complete metamorphosis). The stages include egg, larva, pupa and imago (last stage is the adult).
Adults feed mainly on nectar and pollen. Beside nectar, hoverflies feed on honeydew produced by aphids. Hoverflies are one of the few kinds of insects that can digest pollen, which is a protein rich source for the eggs. The surface coating of pollen is resistant to most insect digestive juices. The yellow patterning can reflect the amount and type of pollen which the insects have eaten, they are often seen hovering or nectaring at flowers.
Larvae may feed externally on plants or they may be internal feeders, attacking the bulbs; for example the narcissus fly (Merodon equestris). In other species, the larvae are insectivores and prey on aphids, thrips, and other plant-sucking insects. In the latter case they are the gardener’s friend. In the former, there is little to be done about it! So, relax and enjoy the animal.
In some species, the larvae are saprotrophs, eating decaying plant and animal matter in the soil or in ponds and streams. For example the rat-tailed maggot, larva of the drone fly (Eristalis tenax) is found in polluted pools. They obtain air by extending their snorkel like tail breathing tubes to reach the water surface, breaking it with feathery hairs which emerge from the tube.
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