The Two-Winged Flies
There are a number of small flying insects which belong to the same family as the Bees and Wasps, such as the Saw-flies, which destroy our vegetables, and the Gall-flies which make those curious galls we found on the oak-tree and other trees. But though we call these "flies" you may always know them from true flies because they have four wings, while all real flies have only two.
Try to collect as many two-winged flies as you can. There will be the common House-fly, the Blue-bottle or Blow-fly, Gnats, Midges, Daddy-long-legs, Horse-flies, and many others.
The House-fly and the Blue-bottle are both very useful in their right place, for they eat decaying matter and dead animals. But they do great harm if we allow them to multiply in the wrong place.
If you have a great number of flies in your house you may be sure that there is dirt somewhere, for the House-fly lays her eggs in dung heaps, dust-heaps, or on any dirt she can find behind a shutter or door, or in an unswept corner.
She lays about 150 at a time, and in a day or two the little legless grubs are hatched, and feed on the dirt. In four or five days they leave off eating and rest in their grub-skin, which grows hard and brown. Then in summer they come out as full-grown flies in about a week. But in winter the hard pupa often lies for months, and people who do not clean their house thoroughly in autumn are likely to have a plague of flies next year.
The Blue-bottle or Blow-fly (1, p. 61) lays her eggs (a) on meat of any kind, or on the bodies of decaying animals. When her grubs are hatched they are very useful in ridding us of bad-smelling creatures, for they give out a kind of liquid which makes the flesh decay more quickly so that they may eat it.
All boys know "gentils" (b) used for fishing. These are the maggots of the Blue-bottle, and when they have done feeding they grow soft inside and draw themselves up into an egg-shape. Then they give out a liquid which hardens their skin into a shiny reddish brown case (c). Inside this the Blue-bottle forms, and then pushes its head out between two little lids at the top of the cocoon.
If you catch a Blue-bottle and put it under a bell-glass with a few grains of sugar you may watch it put out its trunk and feed. You will see that it turns and twists the sugar as if it were playing with it. But all the time it is wetting it with some liquid which it sends down its trunk so as to work the hard lump into syrup which it can suck up. If you press the thorax of a Blue-bottle very gently with your finger and thumb it will put out its trunk and you can see the thick lips at the end with the sucker (A) between them. But you will want a magnifying glass or a microscope to see a little lancet (l) which it has inside its trunk, and which it uses to pierce the skins of fruits, when it wants to suck their juice.
There are two kinds of flies which are much more hurtful than the common fly or the blue-bottle. These are the Gad-flies and the Bot-flies. You know one of the small Gad-flies quite well, for it drops on our hands, or our neck, when we are sitting out of doors, and lets us know that it is there, by giving a sharp bite to suck our blood. We call it the Horse-fly because it teases the horses so much in summer; but there are many others we do not know so well. The largest English Gad-fly (4, p. 61) is about an inch long.
The Bot-flies are more dangerous than the Gad-flies, for instead of biting with their mouths they prick with a sharp tube at the end of their abdomen, so as to lay their eggs under the skin of an animal. The Bot-fly or Warble-fly of the ox (2, p. 61) looks very like a humble bee, only she has two wings instead of four. She has a pointed tube at the end of her body, with which she pricks the skin of the ox, and lays her eggs underneath it. In a short time the eggs hatch, and the maggot irritates the flesh so much that large lumps are seen on the side or back of the animal. If the farmer does not press out the maggots from these lumps, and put a proper dressing on them, the beef of the ox will be poor and bad, and no feeding will make it any better. When the maggot is full-grown it drops to the ground to make its change.
The horse Bot-fly (3, p. 61) does not put her eggs under the skin, but sticks them, with a little slime from her mouth, to the hairs of the horse on his shoulder or under the knee.
When the egg is ready to break, the warmth of the horse's tongue, as he licks himself, makes it crack and the grub slips down the horse's throat to his stomach. There it feeds, and when it is full-grown passes out with the dung.
The way to check this grub is to keep the skin of the horse clean and the hair short. This Bot-fly is rather larger than the House-fly, with bright yellow markings and a very hairy body.
I wonder how many grubs you know of those gnat-like flies, with thin feelers and legs, which fly in the fields and over the rivers. We read about the gnat in Book II., but you should know the midges, which attack wheat and other grain.
The Wheat-midge is a little orange yellow fly, about the size of a very small gnat. Early on a June morning, when the wheat is in flower, you may shake these midges off the stalks and see them flying near the ground. The mothers have a sharp tube as thin as a hair, with which they lay their eggs in the wheat blossom. There they hatch into little red maggots, which feed on the grain and often destroy half a crop.
You ought to know, too, the grubs of the Daddy-long-legs, which do so much harm to our crops. If you see a Daddy-long-legs clinging to a blade of grass she is most likely thrusting her egg-tube into the ground to lay her eggs.
These hatch into legless brown grubs with strong jaws and a pair of short horns. Farmers call them "Leather Jackets," and you may find them when you are ploughing damp fields. Or you may find the hard pupa, which is shaped like a Daddy-long-legs with its wings folded, its legs drawn up, and two horns on its head. It has spines on its abdomen, with which it will drag itself up when the fly wants to come out.
The best way to get rid of these hurtful grubs is to plough the ground deeply and bury the eggs or maggots, so that they die, or cannot get to the surface, or to put a dressing of gas-lime or other insect-killer on the land. Starlings are very useful in pulling them out of the ground and eating them.
Find grub and pupa of Blue-bottle. Examine a Blue-bottle—legs, body, and proboscis. Try to find House-fly eggs. Bring in a Horse-fly. Try to find the Bot-flies of the ox and the horse. Find a Wheat-midge and its grub; also the grub and pupa of the Daddy-long-legs.