[Illustration: After Howard, _Year Book U.S. Dept. Agr._, 1894.
FIG. 13.--Apple Scale Insect (_Mytilaspis pomorum_). a, Scale from beneath showing female and eggs; b, from above, magnified--; c and e, female and male scales on twigs, natural size; d, male scale magnified.]
[Illustration: From Osborn (after Denny), _Bull._ 5 (N.S.), _Div. Ent. U.S. Dept. Agr._
FIG. 14.--Louse (_Pediculus vestimenti_). Magnified.]
[Illustration: From Osborn (after Schiödte), _Bull._ 5; (N.S.), _Div. Ent. U.S. Dept. Agr._
FIG. 15.--Proboscis of Pediculus. Highly magnified.]
In all the above-mentioned families of Homoptera there are three segments in each foot. The remaining four families have feet with only two segments. They are of very great zoological interest on account of the peculiarities of their life-history--parthenogenesis being of normal occurrence among most of them. The families _Psyllidae_ (or "jumpers") with eight or ten segments in the feeler and the _Aleyrodidae_ (or "snowy-flies") distinguished by their white mealy wings, are of comparatively slight importance. The two families to which special attention has been paid are the _Aphidae_ or plant-lice ("green fly") and the _Coccidae_ or scale-insects. The aphids (fig. 11) have feelers with seven or fewer distinct segments, and the fifth abdominal segment usually carries a pair of tubular processes through which a waxy secretion is discharged. The sweet "honey-dew," often sought as a food by ants, is secreted from the intestines of aphids. The peculiar life-cycle in which successive generations are produced through the summer months by virgin females--the egg developing within the body of the mother--is described at length in the articles APHIDES and PHYLLOXERA. The _Coccidae_ have only a single claw to the foot; the males (fig. 12 _a_) have the fore-wings developed and the hind-wings greatly reduced, while in the female wings are totally absent and the body undergoes marked degradation (figs. 12, _e_, 13, _a_, _b_). In the Coccids the formation of a protective waxy secretion--present in many genera of Homoptera--reaches its most extreme development. In some coccids--the "mealy-bugs" (_Dactylopius_, &c.) for example--the secretion forms a white thread-like or plate-like covering which the insect carries about. But in most members of the family, the secretion, united with cast cuticles and excrement, forms a firm "scale," closely attached by its edges to the surface of the plant on which the insect lives, and serving as a shield beneath which the female coccid, with her eggs (fig. 13 _a_) and brood, finds shelter. The male coccid passes through a passive stage (fig. 4) before attaining the perfect condition. Many scale-insects are among the most serious of pests, but various species have been utilized by man for the production of wax (lac) and red dye (cochineal). See ECONOMIC ENTOMOLOGY, SCALE-INSECT.