The _Diptera_, or two-winged insects, form an extensive order, containing many species, and these are composed of very numerous individuals. Flies and mosquitos are exceedingly annoying to man and animals, and many species, in the larval state, consume vegetable matters; but even here they are often of use in consuming decayed vegetation, and like many others of the order may be considered scavengers, consuming, as they do, immense quantities of filth and carrion, that would otherwise continue to taint the air and produce disease. Some of the most destructive insects of this order are the _Gall-gnats_, among which are the Wheat-fly and the Hessian-fly, which often sadly interfere with the farmer's prospects. A few insects will now be noticed more in detail. =Selandria cerasi=, or _Blennocampa cerasi_, is the common Slug of the cherry and pear trees, and quite a troublesome hymenopterous insect. In some parts of the United States these little creatures are so numerous as to strip the substance from the foliage of pears and cherries. Our Slug resembles the _Selandria oethiops_ of Europe, but is declared to be different. The larvæ are at first white, but the slimy substance that oozes from their bodies covers them with an olive coating. They have twenty very short legs; when fully grown, the largest are about nineteen-twentieths of an inch long. The head is concealed under the fore part of the body, which is largest before, and tapers behind. They attain their growth in twenty days, casting their skins five times, eating them until the last time, after which they remain free from viscidity, and are of a clear yellow color. They leave the tree and enter the ground to the depth of one or three inches, to form their chrysalids. In three days they come up as flies, in July and August, to lay eggs for a second brood, the pupæ of which remain in the ground during the winter. Another insect of this genus is very destructive to our rose bushes; it is called _Selandria rosæ_. =Selandria vitis=, is a species that appears upon our grape vines, and is quite troublesome in some vineyards in July. They feed in companies of a dozen or more. REMEDIES.--Shaking them off the leaves has been recommended, but does not promise to be effectual. When few, they should be sought for and crushed, to prevent their increase. Though troublesome, this may be effectually done, and their ravages leave traces that will direct us to the leaves which contain them. When more numerous, the foliage may be syringed with common soapsuds, or with the whale oil soap, two pounds to fifteen gallons of water. Air-slaked lime has been dusted upon them with good effect; ashes, and even dry dust from the road, will destroy them, by adhering to the slimy surface. These applications are best made when the foliage is wet after a shower, or with the dew. The great difficulty consists in their habit of going under the leaves, and thus being protected.