Page:Insects - Their Ways and Means of Living.djvu/192



Immediately after being laid in the fall, the germ nucleus of the aphis egg begins development, and soon forms a band of tissue lying lengthwise on the under sur- face of the yolk. Then this scarcely-formed embryo undergoes a curious process of revolution in the egg, turning on a crosswise axis head foremost into the yolk and finally stretching out within the latter with the back down and the head toward the original rear end of the egg. Thus it rernains through the winter. In March it again becomes active, reverses itself toits first posi- tion, and now compIetes its development. The date of hatching of the apple aphis eggs depends much upon the weather and will vary, therefore, ac- cording to the season, the elevation, and the latitude; but in latitudes from that of Washington north, it is some time in April, usually from the first to the third week of the month. The eggs of most insects resemble seeds in

Fz. 9-- Eggs of the green apple aphis with outer cover- ings split before hatching; below, an egg removed from its covering

their capacity for lying inert un- til proper conditions of warmth and moisture bring forth the creature biding its time within. The eggs of one of the apple aphids, however, are killed by premature warm weather, or if artificially warmed too long be- fore the normal time of hatching. In general, the final development of the aphis embryos keeps pace with the development of the apple buds, since both are con- trolled by the same weather con- ditions, and this coordination usually insures the young aphids against starvation; but the eggs commonly hatch a little in ad- rance of the opening of the buds, and a subsequent spell of cold

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