Insect identification > Homoptera > Scale insects

Scale insects

Family Coccidae (scale insects). - These are remarkable insects, having been much modified and changed in appearance from the more ordinary forms.
Without attempting an accurate classification, they may be grouped under three heads: the armored scales, the soft scales and the mealybugs.

The mealybugs are the least degenerate of the three groups. In them the females preserve their body segments, eyes, antennae and legs and can move about. They secrete a waxy .material, usually as long cottony threads or plates, more or less covering their bodies and sometimes forming a large egg sac at the hinder end. In the female soft scales the antennae and legs are not lost but they become reduced to such an extent that, though the adult can move about somewhat, it seldom does so. Wax, when secreted, is usually to form a sac at the hinder end of the body enclosing the eggs, and the skeleton on the back of the insect becomes very much thickened, forming a scale, often very convex, strong and protective, though seemingly softer than in the armored scales. In the first-named group the female loses antennae, eyes and legs and secretes a waxy scale, with which the molted skins from the body are felted together, forming generally a rather flat and very tough scale. The metamorphosis in the females of all three groups is incomplete.

In some cases the females are fertilized before they have attained full size and grow considerably afterwards. The males develop much as do the females, at first, though not losing any of their parts by degeneration. After reaching full size. however, they pupate and emerge from the pupa as very tiny insects with only one pair of wings and no mouth-parts. Thus in the scale insects we have the remarkable fact that, while in the males there is a complete metamorphosis, in the females it is incomplete.

Whether the former was the original condition in the group, and the females through the degeneration connected with their mode of life have changed to an incomplete meta­morphosis, or this was the primitive condition and complete metamor­phosis has been developed in the males, is unknown, though the other Homoptera all have an incomplete metamorphosis.

About two thousand species of scale insects are known, attacking nearly all kinds of trees and shrubs, and sometimes other plants as well. Many have an almost incredible rapidity of increase, and when under favorable conditions this results in the death of the plant they are on. A few are beneficial to man. Thus the bodies of a scale feeding upon cactus, when dried and prepared, furnish the dye known as cochineal. Shellac is obtained from the excretions produced by another scale, and China wax, used as furniture polish, comes from a third species. Most scale insects, however, are injurious and fail to compensate for the injury they cause by producing anything of value.

Among so many serious pests, only a few can be considered in detail here. Taking the armored scales first, these are the oyster-shell, the scurfy and the San Jose scales, with brief reference to a few others.