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Scale Insects — Part 1

(Family Coccidae.)

The insects of this family, popularly known as scale insects or bark-lice, include forms of great morphological interest as well as very many of great economic importance. In the latter respect they form very injurious enemies to most fruit trees as well as to many other shade and ornamental trees. They occur upon bushes and vines as well and are even in some instances found upon grasses. The group as a whole is a most abnormal one in its life history. In general terms it may be stated that the eggs are laid by the adult female either immediately below her own body or at its posterior extremity. Certain species do not lay eggs but give birth to living young as do the plant-lice. This abnormal habit is not characteristic of any particular group of forms but is found with individual species in one or more genera. The young on hatching from the eggs are six-legged, mite-like creatures, which crawl rapidly away from the body of the mother, wander out upon the new and tender growth of the plant or tree, and there settle, pushing their beaks through the outer tissue of the leaf or twig and feeding upon the sap. Even at this early stage the male insect can be distinguished from the female by certain differences in structure. As a general rule the female casts its skin from three to five times before reaching the adult condition and beginning to lay eggs or to give birth to young. With each successive molt the insect increases in size and becomes more convex in form. Its legs and antennae become proportionally reduced, its eyes become smaller and are finally lost. As a general rule it is incapable of moving itself from the spot upon which it has once become fixed after the second molt, although certain species crawl throughout life. The adult female insect then is a wingless, motionless, degraded, and for all practical purposes legless and eyeless creature. She seems simply an animated drop of protoplasm enclosed in a skin. In the armored scales she is absolutely legless and eyeless. The mouth-parts through which she derives nourishment remain functional and become enlarged from molt to molt. Her body becomes swollen with eggs or young and as soon as these are laid or born she dies.

The life of the male differs radically from that of the female. Up to the second molt the development remains practically parallel in the two sexes, but after this molt the male larva transforms to a pupa in which the organs of the perfectly developed fledged insect become apparent. This change may be undergone in a cocoon or under a male scale. The adult male, which issues at about the time when the female becomes full grown, is an active and rather highly organized creature with two broad functional wings and long vibrating antennae clothed with hairs. The legs are also long and stout. The hind wings are absent but are replaced by rather long tubercles to the end of which is articulated a strong bristle hooked at the tip and fitted into a pocket on the hind margin of the wings. The eyes of the adult male are very large and strongly facetted. The mouth-parts are absent and curiously enough their place is taken by supplementary eye spots or simple eyes (ocelli). The function of the male seems simply to find the female, to fertilize her and then die.

The number of generations in scale insects varies greatly and no general statement can be made.

The scale insects found in the United States belong to three large groups, the most important and the most abundant in species of which is the group known as the armored scales (subfamily Diaspinae), so termed because the insect soon after settling begins the secretion of a scale by means of threads of wax exuding from pores in the body which eventually blend together and form an impervious covering separated from the insect's body. The shape and character of this scale is of importance in the classification of the group but the most important characters are found in the anal plate of the body of the female insect. A second large group well represented in the United States is the group of naked barklice, known as the subfamily Lecaniinae. In this group no true scale is formed and the body is usually well arched so as to become almost hemispherical. The third group (subfamily Coccinae) comprises those forms known as mealy-bugs and related forms. These insects have no differentiated scale but are all covered with a white waxy secretion, some of them having conspicuous waxy egg sacs at the end of the body of the female.