Most insects use fertilized, nutrient-rich eggs to reproduce. In some insects, however, eggs can develop into embryos without fertilization or egg nutrients. And in few insects, embryos develop directly inside the female insect’s body. Nutrient-rich eggs are a resource to parasitoids and predators, as well as to embryos, and so insects use a variety of methods to protect their eggs.
TYPICAL INSECT EGGS
Typical insect eggs contain nutrients to support embryogenesis and produce newly emerged first instars. Most eggs contain large amounts of lipid, for use as building material and energy, and yolk proteins, for the amino acids needed to build a larval insect body. Eggs also contain a cytoplasmic “starter kit” for development that includes cellular machinery such as ribosomes. In species with symbiotic bacteria or protists, eggs are inoculated with a small population of the mutualistic microbes.
Insects typically have internal fertilization, and fertilized eggs contain one set of chromosomes from each parent. Eggs are laid in protected places in environments where young are likely to find food. For example, many butterflies lay their eggs on larval food plants, mosquitoes lay their eggs in water in which larval food grows, and parasitoids lay eggs in, on, or near a host insect. Because mature eggs are usually covered by a thin shell, there must be a way for sperm to penetrate the shell before it is laid, and a way to accommodate water balance and respiratory needs afterward. Sperm enter through an opening called the micropyle. Water and air can pass through specialized regions of the eggshell and embryonic membranes. Finally, some insect groups, remarkably, produce offspring without sperm, egg nutrients, or both.
INSECTS FROM EGGS WITHOUT FERTILIZATION
The sex of hymenopteran insects normally is determined by the number of sets of chromosomes. Unfertilized, haploid eggs have only their mother’s set and develop as males. Fertilized, diploid eggs have chromosome sets from both their parents and develop as females. Mated females have control over when sperm is released from the spermatheca to fertilize eggs. Therefore, they can adjust their offsprings’ sex ratio in response to a variety of cues. Hymenoptera are particularly susceptible to manipulation of sex determination by parasitic microbes. Wolbachia, for example, can alter sex determination so that haploid, and therefore unfertilized, eggs develop as females.
Aphids have complex life cycles that often include female forms that reproduce parthenogenetically, producing female clones of themselves. In such aphids, diploid oocytes form in the germarium and begin development without fertilization.
INSECTS FROM EGGS LACKING NUTRIENTS
Stored nutrients are a major feature in typical insect eggs, but diverse insects have reduced amounts of yolk or lack it entirely. Obviously, if nutrients for embryonic development are not provided in eggs, they must come from another source. Two major alternate sources are the mother and other insects, which can serve as hosts for both embryonic and larval development.
Females that provide nutrients to their embryos, in addition to or instead of egg materials, are termed viviparous. The parthenogenetic aphids described earlier are viviparous: they produce first instars rather than eggs. Young aphid embryos shed their covering of follicle cells and break the strands of tissue that connect them to the germarium. Then, they absorb the necessary nutrients directly from the mother’s body.
The viviparous Pacific beetle cockroach, Diploptera punctata, produces eggs with insufficient yolk to support complete embryonic development. During pregnancy, females make a supplementary protein, termed roach milk, that is taken up by embryos. Females in a few groups, such as the order Strepsiptera, are neotenous. Neotenic insects do not go through a full metamorphosis, and they reproduce during the larval stage. Strepsipteran females lack oviducts, so that eggs produced in the ovaries are released into the blood. The eggs can contain some, but insufficient amounts of yolk. To fertilize these eggs, sperm must move from the genital canal into the blood. When embryogenesis is complete, larvae use the genital canals to leave the mother’s body.
Some parasitoids also develop inside other insects, but here the insect is the parasitized host. Female parasitoids that oviposit directly inside other insect eggs, larvae, or adults are likely to produce small eggs with little or no nutrients. The parasitoid embryos, lacking their own supply of nutrients, then use the host’s body to supply materials for their own development.
ECOLOGY OF EGGS
Eggs, the first life stage of insects, can be important ecologically. For example, eggs are the diapausing stage in many insects, with embryogenesis stopping at a species-specific point. Eggs of silkworms (Bombbyx mori) have an obligatory diapause that coincides with winter under natural conditions. Embryonic diapause has been studied extensively in silkworms because delayed development can be a nuisance from an industrial perspective. Gypsy moth eggs also diapause during winter, but they arrest development at a later stage. Embryos complete embryogenesis and overwinter as unhatched larvae.
Daylength is the most common cue for inducing diapause, but moisture, temperature, and food quality can also be important. The environmental cues that cause eggs to stop developing can be detected by females and then passed on to signal the eggs. Alternatively, the cues can be detected directly by the eggs and embryos. Later, eggs must break diapause in response to another environmental combination of daylength, temperature, and moisture.
Eggs are rich sources of nutrients and therefore pose a great “temptation” to parasitoids, parasites, and predators. Insects protect their eggs in a variety of ways. For example, eggshells can be thick and protective, or cryptic (difficult to detect). Eggs can be laid in protected places. Primarily females, but sometimes males, can contribute chemical repellents or toxins to eggs to deter attacks. A variety of insects stay with their egg masses and actively protect them.