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Scientists are piecing together how and why live-bearing animals evolved from egg-laying ones — and why they might evolve in the other direction on rare occasions. Egg laying almost certainly came before live birth; the armored fish that inhabited the oceans half a billion years ago and were ancestral to all land vertebrates seem to have laid eggs. But the rest of the story is far from straightforward.


The ovipositor, or egg-laying organ in the female, is often very long and may be modified for piercing, sawing, or stinging. Metamorphosis is complete; i.

Sex is usually determined by whether or not an…. In the sawflies, the ovipositor is modified into a sawlike tool used for making slits in the leaves or stems of plants in which the eggs are deposited, but in all other Hymenoptera it is modified for stinging or piercing. Depending on the species, the eggs remain in the pouch between 10 days and 6 weeks. During this time the….

A strategic choice

The ovipositor, a pair of basal plates and three pairs of elongate bladelike structures, generally is used to pierce or drill slots in plant tissue for oviposition. The variable external genitalia of the male,…. The ovipositor of Scolops pungens is more primitive than that found in many of the Orthoptera.

Thus the Fulgoridae are…. Gon produce the gametes and hormones essential for reproduction. Associated ducts and glands store and transport the gametes and secrete necessary substances.

In addition to these structures, most male and female vertebrates have a cloaca, a cavity that serves as a common…. In butterflies and moths Lepidoptera a second copulatory canal independent of the vagina has been evolved, so that the sperm enter by one route, and the eggs are deposited by another.

Winged existence may last only a few hours, although Hexagenia males may live long enough to engage in mating flights on two successive days, and female imagos that retain their eggs may live long enough to mate on either of two successive…. Attached to the oviduct or vagina is a sac called the spermatheca for storage of male sperm; as eggs move down the oviduct, they are fertilized by the sperm.

The typical male contains paired testes that produce…. Ovipositor anatomy.

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