The eggs of butterflies consist of a membranous shell containing a fluid mass composed of the germ of the future caterpillar and the liquid food which is necessary for its maintenance and development until it escapes from the shell. The forms of these eggs are various. Some are spherical, others hemispherical, conical, and cylindrical. Some are barrel-shaped; others have the shape of a cheese, and still others have the form of a turban. Many of them are angled, some depressed at the ends. Their surface is variously ornamented. Sometimes they are ribbed, the ribs running from the center outwardly and downwardly along the sides like the meridian lines upon a globe. Between these ribs there is frequently found a fine network of raised lines variously arranged. Sometimes the surface is covered with minute depressions, sometimes with a series of minute elevations variously disposed. As there is great variety in the form of the eggs, so also there is great variety in their color. Brown, blue, green, red, and yellow eggs occur. Greenish or greenish-white are common tints. The eggs are often ornamented with dots and lines of darker color. Species which are related to one another show their affinity even in the form of their eggs. At the upper end of the eggs of insects there are one or more curious structures, known as micropyles (little doors), through which the spermatozoa of the male find ingress and they are fertilized. These can only be seen under a good microscope. [Illustration FIG. 3.--Egg of _Papilio turnus_, greatly magnified.] [Illustration FIG. 4.--Egg of _Anosia plexippus_, magnified 30 diameters (Riley).] [Illustration FIG. 5.--Egg of _Anosia plexippus_, natural size, on under side of leaf (Riley).] [Illustration FIG. 6.--Egg of _Anthocharis genutia_, magnified 20 diameters.] [Illustration FIG. 7.--Turban-shaped egg of _Lycæna pseudargiolus_, greatly magnified.] [Illustration FIG. 8.--Egg of _Melitæa phaëton_, greatly magnified.] The eggs are laid upon the food-plant upon which the caterpillar, after it is hatched, is destined to live, and the female reveals wonderful instinct in selecting plants which are appropriate to the development of the larva. As a rule, the larvæ are restricted in the range of their food-plants to certain genera, or families of plants. [Illustration FIG. 9.--Upper end of egg of _Pieris oleracea_, greatly magnified, showing the micropyle.] [Illustration FIG. 10.--Egg of _Grapta comma_, laid in string-like clusters on the under side of leaf. (Magnified.)] The eggs are deposited sometimes singly, sometimes in small clusters, sometimes in a mass. Fertile eggs, a few days after they have been deposited, frequently undergo a change of color, and it is often possible with a magnifying-glass to see through the thin shell the form of the minute caterpillar which is being developed within the egg. Unfruitful eggs generally shrivel and dry up after the lapse of a short time.