Do Insects Enjoy Sex?

By SophieWeiner on at

As species, we have little in common with bees, fruit flies and beetles. Bugs are so alien to us that it’s hard to know how exactly they experience the world. Do they feel pain? Do they experience pleasure? What is sex like for them? Do they enjoy it in any way—physical or otherwise?

This week on Giz Asks, we asked neuroscientists, biologists, and etymologists to help us understand what it’s like to be a horny insect. We found out that the world of insect sex is mysterious, fascinating, and sometimes disturbing as hell.

David J. Anderson

Professor of Biology, Director of the Tianqiao and Chrissy Chen Institute for Neuroscience, Caltech

It’s not possible to know whether insects “feel” pain, fear, etc. because feeling requires conscious experience, and we don’t know if insects are conscious.

There is some evidence that mating can serve as a reinforcer for fruit flies. It’s been shown that male flies prefer an odour that they smelled while mating with a female, over an unfamiliar odour. Whether that really means that flies “like” sex, (i.e. that sex provides a hedonic reward), or just that they formed a Pavlovian learned association between the odour and female pheromones, is something we’re currently trying to investigate.

Monica Dus

Assistant Professor, Department of Molecular, Cellular, and Developmental Biology, University of Michigan

It is hard to answer because we do not speak fly or spider or even mouse or monkey. We do not know how they are feeling; we can measure their behaviour in quite objective and quantitative ways, but that is their behaviour, not their feelings.

Even if we base it on facial expressions (hard to do in insects!), all we can say is that in humans that expression correlates with a good feeling, and that if another animal has the same, it is probable that they are also liking/loving/hating/blank that way, but, even with the animal we are the most “linked,” our dogs (mine are named Cupcake, Sprinkles and Brioche), we often guess their feelings wrong, as Alexandra Horowitz’s studies on guilt show.

What I can say for sure, is that fruit flies, the insect our lab uses to understand the genetics of obesity, do seem to love sugar, especially males—most times, the sweeter, the better. And, although this is also a wild speculation, our experiments make us think that if they had hands and cookies, they wouldn’t be able to stop reaching for the second one.

W. Daniel Tracey PhD

Linda and Jack Gill Chair, Department of Biology, Indiana University Bloomington

It is safe to assume that insects don’t have feelings or emotions in the same way that humans have feelings. The insect brain is vastly simpler than a human brain and the evolutionary history of insects that are living today is quite distinct from that of humans. Nevertheless, the brain of insects operates using the same general principles as our own. In an insect brain neurons are “wired” together to form circuits much like the motherboard of a computer. Insects possess almost all of the same neurotransmitter molecules such as dopamine, serotonin, acetylcholine, glutamate and GABA. Many of the molecules used by insect neurons for controlling their activity are the same as those that are used by our neurons. David Anderson at Caltech is a proponent of the argument that insects have emotion like states or what he called “emotion primitives.” I agree with this idea.

When it comes to pain, insects have sensory neurons that detect potentially damaging heat and mechanical stimuli. These neurons are called nociceptors. These pain sensing neurons trigger defensive behavioural responses in the insect. One well known example occurs in the larvae of fruit flies. Activation of these neurons produces a rolling escape response. These pain sensors protect the larvae from encounters with predators such as female wasps that pierce the larvae with a sharp egg laying organ. If the wasp succeeds in laying its egg inside the larvae then the wasp larva will devour the fly larva from the inside out and a wasp will be produced rather than a fly. So the fly larval pain sensing neurons may have evolved to protect them from the attacks of these specific wasps.

Insects have well known abilities to form positive memories. Honeybees that are foraging at flowers receive a positive rewarding experience from the sweet taste of the sugars in the flowers that they visit. The honeybees can learn to associate the odour, and the colour of the flower with the sugar reward. They can even remember the time of day that a particular flower produces its nectar. They can also communicate this information to other members of the hive.

To my knowledge, it is not known whether or not sexual experience is rewarding to insects. There are certainly many dramatic examples of the importance of sex to insects. The mayfly lives most of its life in its larval stage in rivers and streams and when it emerges as an adult fly the adult male fly lives for just a couple of days and never feeds. His sole purpose is to find a female to mate with before he days.

While we don’t know as much about the pleasures of sex for insects, we know something about the negative aspects of rejection. Male fruit flies that are rejected by potential mates seem to form a “negative” memory of that rejection. These flies perform a complex courtship ritual to attract female mates. The males chase the female, lick the female’s genitalia (probably to make sure she is of the right species and to test whether she is a virgin), and they play a courtship song by vibrating their wings. If the female is not receptive to the advances of the male he will remember this rejection and he will then be inhibited from additional courtship attempts for several hours. This is probably an evolutionarily advantageous strategy as it prevents the male from wasting his time courting the same unreceptive female and to move on to find another. There is even one study that discovered that male fruit flies have a tendency to drink more alcohol when they are repeatedly rejected by females.

Derek A. Woller

Entymologist, Song Laboratory of Insect Systematics and Evolution at Texas A&M University

Short answer? Doubtful. It all depends on how you define the sensation of pleasure and its connection to emotions, which are typically relegated to vertebrates, particularly us, of course.

This question ties well into a more common question that entomologists are frequently asked about whether or not insects feel pain. I’m on the side of “more observations needed” because, simply put, insects are extraordinarily alien in myriad ways compared to the vertebrate experience.

I have had the opportunity (pleasure is probably far too strong a word) to observe a plethora of sexy grasshopper interactions (aka copulations), and I have yet to see any reactions from either party involved that I would definitively classify as “enjoyment,” at least as far as the way we humans experience the same wonderful couplings.

To put it another way, here is what the male-female interaction sure seems like to me after watching plenty o’ mating in the many species I study that reside in the southeastern U.S.: Male: “Ah-ha! Found you! Now, I’m gonna ride you like a bucking bronco all over the place until you calm down and let me stick this in you. Hey, don’t kick me, don’t kick me—you want this, I swear!” Female: “Whoa, where’d you come from?! You’re not the one I wanted, at least not unless you can survive all of....THIS [proceeds to thrash, jump all over the place, and kick wildly at her assailant]! Okay, okay, just go ahead and do it already. I’ve got plants to eat and other guys to wait for.” So, you see, the interaction can be quite antagonistic, which, of course, doesn’t mean it’s not pleasurable, but it sure doesn’t seem that way from, again, a common human perspective.

I have found micro structures on the aedeagus (fancy term for the complex penis analogue) in some of the grasshopper species I study. One hypothesis is that these are used to stimulate a female to be more receptive to fully mating once the male forcefully inserts himself into her vaginal opening. Naturally, plenty of further evidence is needed to determine the actual function of these structures, but, if vaginal stimulation is their function it could be argued that the female’s reaction to this stimulus is akin to our concept of enjoyment since she decides to allow a male to mate with her. Following this acceptance, she often calms considerably, allowing for a fairly lengthy process in which the male transfers a long packet of sperm (aka a spermatophore) to the female.

Bottom line answer: a boring “maybe”, but it highlights the need for more nuanced and creative studies to gather further evidence on how insects (and invertebrates, in general) perceive our collective world. They’re definitely strange, which makes them all the more fascinating and enjoyable to work with, in my admittedly-outlying opinion. Be an entomologist: see the world and study crazy and bizarre things! Plenty of room, apply today!

Edgar T. Walters, Ph.D.

Professor of Integrative Biology and Pharmacology, McGovern Medical School at UTHealth

Do insects feel? A few quick answers:

1) Maybe, but philosophers have argued that conclusive answers are impossible if one includes consciousness in the definition of “feel.”

2) Not like humans (their brains and behavior could not be more different), although they certainly show a few behavioural traits that people often take as signs of feeling (e.g., arousal by sexual stimuli – usually pheromones, or relatively long-lasting defensive arousal in some insects after injury).

3) Perhaps not, because strong selection pressures for the evolution of conscious feeling in insects are not apparent, at least not to this scientist. Would insects that had mutations promoting conscious feeling be likely to have more reproductive success than other insects? If not, any conscious feeling in today’s insects would be a biological accident rather than a product of natural selection. I doubt that it was selected for during insect evolution.

Nathaniel J. Himmel

Grad Student Researcher, Neuroscience Institute, Georgia State University

At the very least, we can compare what insects do to what vertebrates, mammals, or maybe humans, do when they—or we—are supposed to be experiencing pain.

Insects—in fact most, if not all, animals (and maybe some other forms of life!)–sense and respond to noxious, potentially harmful stimuli. We call this ability nociception.

For a variety of reasons, Drosophila melanogaster (the fruit fly) is perhaps the most extensively studied species of insect. We know that fly larvae contract—scrunch their body up—in response to noxious cold. They also roll in response to noxious high temperatures and noxious mechanical stimulation (stabbing, most importantly). Adults likewise make moves to avoid noxious temperatures.

These behaviours almost certainly serve some purpose—in the case of rolling, we believe it to be a behavior that evolved as a protective response to parasitoid wasps. These wasps lay their eggs in fly larvae; as the wasp larvae grow, they eat the fly from the inside out. By rolling toward the source of a noxious mechanical stimulus, fly larvae have a chance of tangling up, or perhaps breaking off, the sharp, needle-like ovipositor of aggressive wasps, thereby avoiding being eaten!

These sort of protective responses aren’t just fruit fly-specific, either. For example, we know that electric shocks, noxious high temperatures, and noxious mechanical stimuli elicit an aggressive “sting extension response” in honey bees, and that noxious mechanical stimuli induce a so-called “cocking” and striking behavior in the larval moths of the species Manduca sexta. What I find most interesting are not these behaviours, which may themselves be species unique. Rather, it’s the neural circuitry and the molecular mechanisms that underlie these nociceptive systems, and their similarity to ours.

The sensory neurons that mediate fly larval nociception have some similarity to ours (most specifically, vertebrate C-fiber nociceptors)—they play similar sensory roles, they look the same (naked dendritic projections to the epidermis), and they are unmyelinated. More interestingly, some of the molecular machinery at work in these neurons appears to be evolutionarily conserved between fruit flies and vertebrates (including humans). Transient receptor potential (TRP) channels are important players in vertebrate nociception, and we know of several which are conserved, and perform similar roles, in flies. There is a growing body of evidence that makes this likewise true for for anoctamin family channels. As before, this doesn’t appear to be totally restricted to fruit flies; at least in the case of honey bees, we know they express an order specific TRP channel which has been implicated in nociception.

That’s not to say we’re all the same, though. At first glance, it doesn’t seem like injury stops many insects from continuing to move, feed, or mate normally. At the very least, I’m not certain there’s much evidence of that. For example, I don’t think it’s clear if they might favor an injured leg in order to alleviate any sort of pain they might feel (although, if we want to make it more complicated, we can consider that some crustaceans, notably caridean shrimp, display increased grooming following injury).

But, let us just consider that the above is true–let’s assume that injury, in insects, doesn’t inform or cause the same sort of long-term cognitive and behavioural responses that we might see in humans. This might be some sort of evidence that they don’t feel or respond to pain in the same way that we do. But... so what? I like what Daniel Dennett has to say about animal consciousness: “Consciousness is not a black-or-white, all-or-nothing type of phenomenon, as is often assumed.”

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