Fig. 491.—Egg of Drosophila.—After Comstock.
Mode of deposition.—The exact process of oviposition has been rarely observed, or at least not observed in detail, and further observations are much needed. In the cockroach (Phyllodromia), Wheeler has seen the eggs pass out of the oviduct and become arranged in the oötheca, in a way similar to that in the account published by Kadyi on Periplaneta.
Fig. 492.—Rocky Mountain locust (aa) depositing its eggs (c); d, the earth partially removed, showing (e) an egg-mass already in place, and (d) one being placed; f shows where such a mass has been covered over. A, oviposition; j, position of oviduct; g, egg-guide; e, egg. B, egg-mass of the same; a, from side, b, from beneath, c, from above.—After Riley.
“When about to form the capsule, the female Blatta closes the genital armature, and the two folds of the white membrane which lines the oöthecal cavity close vertically in the middle line. Then some of the contents of the colleterial glands are poured into the chamber, and bathe the inner surface of the posterior wall. The first egg glides down the vagina from the left ovary, describes an arc, still keeping its germarium-pole uppermost, after having pressed the micropylar area against the mouth of the spermatheca, passes to the right side of the back of the chamber, and is placed perpendicularly two-thirds to the right of the longitudinal axis of the insect’s body. The next egg comes from the right ovary, describes an arc to the opposite side of the body, decussating with the path of the first egg, and is placed completely on the left side of the median line. The third egg comes from the left ovary, and is made to lie completely on the right side of the median line; and so the process continues, the ovaries discharging the eggs alternately, and each egg describing an arc to the opposite side of the capsule. The oöthecal chamber soon becomes too small to contain all the constantly accumulating eggs, so the anal armature opens and allows the end of the capsule to project. A raised line, the impression of the edges of the white membrane, runs down the end of the capsule. The last egg deposited comes from the right ovary, and lies two-thirds on the left, and one-third to the right, of the median line. As soon as the egg is laid, a further discharge from the colleterial glands spreads over the vaginal or anterior wall of the cavity, and becomes evenly continuous with the secretion which has before been spread over the back and the sides of the capsule by the white membrane.
“The crista, a cord-like ridge running the full length of the dorsal surface of the capsule, is a thick-walled tube, either half of which is formed by the edge of the side walls of the capsule split into two laminæ. The rhythmical clasping of the three pairs of palpi which guard the vaginal opening is registered in an exquisite pattern on the inner face of either half of the crista.”[[78]]
The mode of oviposition in the locust has been fully described by Riley, who states that the eggs pass down and out of the oviduct, and “guided by a little finger-like style” (Fig. 298), they pass in between the horny valves of the ovipositor, and issue at their tips amid the mucous fluid which forms the egg-capsule (Fig. 492).
Vitality of eggs.—It is well known that the eggs of phyllopod and other fresh-water Crustacea have wonderful vitality, withstanding extreme dryness for several years, at least from two to ten. Such cases are unknown among insects. It has been observed, however, by T. W. Brigham, and also by L. Trouvelot, that the eggs of the walking-stick (Diapheromera femorata) for the most part hatch only after the interval of two years.[[79]]
The eggs of Bittacus are said by Brauer to lie over unhatched for two years; indeed, the first condition of their hatching is a complete drying of the earth in which the eggs lie, the second is a succeeding thorough wetting of the ground in spring.
Appearance and structure of the ripe egg.—The eggs of insects are on the whole rather large in proportion to the size of the parent, especially so in many minute forms, as the fleas, lice, etc.