The bizarre world of insect sex

Insects: tiny, alien. We tend to treat them like little machines, little nuisances, little scurrying things we call “It.” Swat It! Catch It! Don’t let the dog eat It! But we really should cry, Swat Him, Swat Her! – because insects, just like other animals, are biologically and genetically gendered as males or females. Insect sex determination is chromosomal; some insects have X and Y chromosomes just like humans. And insect gender is highly relevant to human lives. Only female mosquitoes and horseflies bite and drink blood, for instance. Only female honeybees sting, and their hives are dependent on the health and happiness of their queens. Only male cicadas produce the eardrum-perforating roar that typically echoes through July and August.

Certain insects have bizarre methods of reproducing, whether with regards to mating, to the genetics behind production of offspring, or to the care of young. The fascinating life histories of these tiny animals are weird, wonderful, and sometimes gross.

Weird mating rituals

You’ve probably seen dragonflies flying in tandem during the mating process. The male has claspers at the end of his abdomen, which he firmly attaches around the neck of the female. The female then curves the tip of her abdomen up to touch the underside of the male’s thorax, where he has stored his sperm in a neat little package. Something you may not have known is that dragonflies are a group of insects in which homosexuality is frequently observed. Males clasp their mates so tightly that they make little dents in the armature around the neck. Puzzled taxonomists found male dragonflies bearing the same characteristic dents and realized that male-male coupling in these insects is surprisingly common, and most likely the result of mistaken identity.

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Male (top) and female (bottom) dragonflies. Image: Tero Laakso. From Flickr Creative Commons.

A far more gruesome mating ritual takes place in praying mantises. Egg production requires protein and fat as building blocks for yolk, and is a highly energy-intensive process for a female insect. Female mantises turn to cannibalism to obtain that needed fuel – by chewing off the male’s head during copulation! Mating is a lengthy process for mantises, and incredibly, the headless male is still able to complete the process. How is this possible? The insect brain exists as a pair of nerve cords that run the entire length of the body, so decapitation is really more like lobotomy in these animals. The parts of a mantis’ brain needed for mating remain intact even after the head has been removed.

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Female (large) and male (small) mantises. Image: Shaun Lee. From Flickr Creative Commons.

Crazy cloning

Ordinarily, it’s advantageous for a species population to have some genetic diversity. This increases the population’s chances for survival in the event of a new disease or predator. In sessile animals such as aphids, there are advantages to reproducing clonally. Aphid populations remain on the same plant for several successive generations, where there is little environmental (i.e. habitat) change. Cloning is an easy way to produce large numbers of offspring without expending energy to find a mate, and genetic diversity isn’t necessarily advantageous if the habitat is homogenous and static.

In humans, gender is determined by sex chromosomes: fertilized eggs bearing XX become female, and fertilized eggs bearing XY are destined to be male. In the insect world, things can get a little more complicated. Certain bees, ants, and wasps undergo what is termed arrhenotoky, in which unfertilized haploid eggs become males and fertilized diploid eggs become females. These social insects are able to carefully balance gender ratios to suit the needs of their hive community, in which males and females play different roles. For instance, the role of male honeybees (drones) is to mate with the queen, female honeybees become pollen-gathering workers, and female larvae fed large quantities of royal jelly during infancy develop into sexually mature queens.

The flipside of arrhenotoky is thelytoky, in which males are rarely or never produced, and females arise from unfertilized diploid eggs that have never undergone meiosis. In more simple terms, females generate offspring that are genetically identical clones to themselves. Aphids and gall wasps are examples of insects that undergo thelytokous reproduction. In the case of aphids, stresses on the mother can also influence the phenotype of her offspring.  When the host plant becomes overcrowded and resources are scarce, winged sons and daughters are produced, which can fly away to colonize a new plant. Another strange feature of aphids is their ability to produce offspring while still juveniles, in what is termed pedogenesis. This means an aphid is already pregnant before she has even matured external genitalia. In fact, aphids that undergo this process are actually born in a pregnant state, meaning that their mothers were pregnant with children and grandchildren at the same time! Aphids that undergo paedogenesis are producing clones through thelytoky and do not need to mate, which makes this process possible.

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Mother aphid and offspring. Image: Steve Begin. From Flickr Creative Commons.

Care of young

Most insects deposit eggs and leave, abandoning their offspring to the forces of nature. There are a few interesting exceptions in which one parent sticks around to protect the eggs or to feed the larvae. In the case of the aquatic giant toe-biter, the male allows the female to stick her eggs on his back and he ferries them around carefully, occasionally brushing water over them with his hind legs to keep them oxygenated.

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Giant toe-biter male with eggs (family Belostomatidae). Image: kqedquest. From Flickr Creative Commons.

Tiny parasitoid wasps lay their eggs inside larger insect hosts such as caterpillars, which are eaten alive by the wasp offspring. The parasitoid wasp eggs have especially thin shells so that the developing wasp embryo can readily absorb nutrients from the host blood. Lacewings don’t stick around to protect their eggs, but they take care to protect them from predators such as ants by suspending them on long, elegant stalks.

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Lacewing eggs on stalks. Image: Samuel. From Flickr Creative Commons.

But in one of the most fascinating and disgusting processes, the tsetse fly, a blood feeder which transmits sleeping sickness to humans, gives live birth rather than laying eggs. Moreover, she feeds her larva a protein-rich, nutritious liquid from what is referred to as “milk glands” located in the uterus. Interestingly, tsetse flies have a symbiotic relationship with a particular bacterium Wigglesworthia glossinidia. This bacterium resides in the digestive tract, where it aids digestion, synthesizes essential B vitamins to supplement the tsetse fly’s nutrient-poor diet of vertebrate blood, and boosts the tsetse fly immune system. W. glossinidia also infects the female tsetse fly’s milk glands, ensuring the suckling tsetse fly larva becomes inoculated with the life-saving bacterium.

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A bloodfed, soon-to-be pregnant tsetse fly. Image: Oregon State University. From Flickr Creative Commons.

The incredible diversity of insects has led to these bizarre reproductive traits, and many more. The biology of these incredible animals are sometimes overlooked simply due to their diminutive stature, but on closer examination, their bizarre life histories are stranger than fiction. If this piece has piqued your interest, you might look up the strange fig wasp life cycle, check out the fruit fly with the longest sperm in the world, or watch Isabella Rossellini’s artistic and macabre interpretation of traumatic insemination in bedbugs!

Ruby HarrisonRuby Harrison is a Ph.D. graduate student in the Department of Entomology at the University of Georgia. Ruby studies endocrinology and reproduction of the yellow fever mosquito, Aedes aegypti – the same species that spreads Zika virus. In her free time she enjoys backpacking, camping, swing dancing, and cooking dumplings. Contact her at reh21923@uga.edu.. More from Ruby Harrison.