This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
These two-winged insects of the genus Scaeva are very numerous from July to September, and have received their name from the fact that they hover over flowers like a hawk; but they vary the hovering by suddenly darting about. They are deadly enemies to aphides, including American Blight. The eggs of the Hawkflies are laid amongst the aphides, upon the bodies of which the young Hawkfly maggots feed voraciously, each one being capable of destroying one hundred aphides in an hour. The Hawkfly maggots are to be recognized by their relatively large, fleshy, and thin-skinned bodies resting among the aphides or slowly crawling about. They are whitish, pale green, or yellow, and in some cases lined or streaked with orange. When fully developed the maggots assume a pear-like shape, and attach themselves by the tail to some part of a plant and then pupate. In a few days the perfect insect comes forth again to carry on the war amongst the aphides. The perfect insects and the larvae, therefore, as shown in fig. 158, should never be destroyed, if possible, as they perform such beneficial work.