Wasps in the genus Pepsis lay their eggs in a specific region on a species of tarantula and their larvae eat the tarantula organs in a specific sequence to keep it alive as long as possible.

How could this process have evolved, considering that a mistake at any stage results in death?

This is the description of the egg laying behaviour:

Upon finding a tarantula, the wasp locks its mandibles onto a foreleg and brings its stinger into position so as to paralyze it.

During this process, the tarantula tries unsuccessfully to dislodge the wasp, but the wasp ultimately locates the right part of the tarantula in which to insert its stinger.

Upon inserting it, the wasp injects a venom that paralyzes the tarantula. Note that the tarantula is ten times the weight of the wasp, so the wasp takes very big risk in trying to attack such a tarantula.

The wasp then drags the tarantula into a hole that it has already dug. It pulls the tarantula down into the hole, attaches its egg to a specific part of the tarantula's body and then covers up the hole so that predators won't find it.

When the egg hatches, the larva eats the organs of the tarantula in a specific sequence so as to keep the tarantula alive for several weeks. Eating in the wrong sequence results in the death of the tarantula, and its subsequent putrefaction, which means the death of the wasp larva.

When the larva successfully eats in the right sequence, it is able to obtain enough nourishment to prepare for metamorphosis, ultimately resulting in a full-grown wasp, which then exits the hole and becomes a predatory wasp.

These wasps are solitary, which means there are no conspecifics to teach the new wasp how to do all this. Thus, the entire behavioral program described above must be encoded in the wasp's genome.

The description above is based on:

Punzo, F. (2005). Experience affects hunting behavior of the wasp, Pepsis mildei Stål (Hymenoptera: Pompilidae). Entomologica Americana, 113(3), 222-229.

Petrunkevitch, A. (1952). The spider and the wasp. Scientific American, 187(2), 20-23.

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    $\begingroup$ Please use paragraphs to make text legible to people, especially with complicated phrases. The question could be only 2-3 lines. It's a bit like AI. Evolution is usually about gradual change, so the wasp has always followed a path which has given it good survival rates. The behaviors become complex over time, from simpler to more complex. $\endgroup$ Commented Dec 20, 2019 at 18:37
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    $\begingroup$ Welcome to Biology.SE! This is an interesting question, but I suspect very hard to answer in just a few paragraphs. We encourage you to do some research on your own and then, informed by what you have learned, ask any questions you still have (ideally with references to reliable sources) or even answer your own question. For example, I found several freely available review articles on this subject by searching using your title. ——— Please also take the tour and then go through the help pages starting with How to Ask questions effectively on this site. Thanks! 😊 $\endgroup$
    – tyersome
    Commented Dec 20, 2019 at 19:24
  • $\begingroup$ @com.prehensible and tyersome thank you for editing my question. $\endgroup$ Commented Dec 20, 2019 at 19:41
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    $\begingroup$ While it doesn't directly address this particular example, you should find this related question and answer to be of interest. In general, complex behaviors (and structures) evolve over very long periods of time as improvements on simpler behaviors. A look at related wasps with simpler "lifestyles" should help you understand how such an "optimized" (note it is probably sloppier than you think) behavioral program can evolve. $\endgroup$
    – tyersome
    Commented Dec 20, 2019 at 19:48
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    $\begingroup$ The common ancestor is thought to have laid eggs on or in wood grubs 247 million years ago. Since then they have evolved into as many as 500,000 species, mostly with a specifif host species. The wasps have had that many generations to make the jump from one type of insect to another, from larvae to crickets to spiders, and they can lay 100ds of eggs. The specifics are very complicated, i don't know, here are some docs: annualreviews.org/doi/10.1146/annurev.ento.51.110104.151029 and sciencedirect.com/science/article/abs/pii/… $\endgroup$ Commented Dec 20, 2019 at 20:34

2 Answers 2


Context is everything. Lets start by working back to front.

larvae eat the tarantula organs in a specific sequence to keep it alive as long as possible.

Most of the time if the larva eat the organs in the wrong order they don't die, they just end up with less food. It is pretty easy to see how this could evolve to be better, larva that eat in the correct sequence end up with more food thus have an advantage, larva that eat in hte absolute wrong sequence don't survive.

on a [specific] species of tarantula

Again easy many wasps are not as picky they will lay their eggs in/on a wide variety of hosts, but specialization is favored a lot in evolution as it helps your lineage exploit a resource better and better, you only have to deal with the defenses of a single species as opposed to many different ones which might have mutually exclusive solutions.

lay their eggs in a specific region.

again failing to do this does not necessarily mean their larva die, it just means they are worse off becasue they don't get to eat as much. Again easy to see how this evolves, a wasp that picks its target end up with more well fed offspring. Many don't even lay their eggs inside the host but outside and let them eat the host from the outside in, this is what this species does.

Upon finding a tarantula, the wasp locks its mandibles onto a foreleg and brings its stinger into position so as to paralyze it.

Again not every parasitic wasp does this many don't paralyse their host, but they jab them with the ovipositor relying in brute force to injure, paralyse, or kill the host. Others just lay their eggs on them and fly away. Others simply kill the host jam it in a hole and lay their eggs on it. But wasps that paralyse the host are less likely to get attacked. but even if they kill the host instead the larva still get a meal. hunting always tends to be risky, but as long as the reward outweighs the risk it can evolve. Additionally many wasps do not target adults but larva and eggs that can't fight back. venomous Stinging is actually fairly rare in wasps occuring only in the ones that attack the most dangerous hosts. Of course once a wasp lineage gets good ant not killing its host then there is little reason for the larva to be able to eat more putrid remains of the host, as such adaptations are costly so you can see how after they stop killing the prey the larva might lose the ability to keep eating dead hosts.

The wasp then drags the tarantula into a hole that it has already dug.

Again not every species does this, some just lay their eggs on a host, without doing anything else, but this carries more risk of your larva getting dislodged or injured, but insects are not people if there is a parasite on their back there is not a lot they can do about it.

These wasps are solitary, which means there are no conspecifics to teach the new wasp how to do all this. Thus, the entire behavioral program described above must be encoded in the wasp's genome.

Even your own source disagrees with this, it shows wasps get better at subduing the host the more often they do it, laying eggs in NOT a once in lifetime thing they may do it many many times, so not being perfect can still work.

Once you see the huge variety in parasitic wasp behavior it is much easier to the step by step sequence that can happen to get to each one. You are not even at the most extreme, there are wasps that go even further, like having viral mutualists, larva that can't poop so as not to contaminate a host they live inside, and even leaving chemical markers to keep other wasps from laying eggs in/on the same host. Parasitic wasps are rather well studied I invite you to look more into them, they are really good models of evolution.


parasitoid wasp evolution:

Started radiating about 127 million years ago, at the same time as Gymnosperms (flowering plants). Today they represent 10-20% of all insect species.

They have accompanied the other insect species in their diversification since that time, and often undergo co-speciation... When a host species divides into different types, the wasp species divides too.

Species that lay inside living insects are thought to be derived from wasps that lay eggs on or in other insect's eggs or pupae. Endoparasitoids have evolved from ectoparasitoids independently in different hymenopteran lineages.

Some species also lay microeggs on leaves near a larvae, and when the host eats the leaf and the egg, it develops inside them.

They have both a sting and an ovipositor, which permits different species a variety of specialized and opportunistic behaviours to lay on or inside eggs, and to inject genes and venoms which inject into the host to modulate host immunity, metabolism and development. Up to 70 different venom proteins have been found in a single wasp species.

Parasitoids can also inject substances like polydnaviruses (PDVs), virus-like particles (VLPs), ovarian fluids and teratocytes.

This kindof reflects the complex evolution of the Pepsis species described... The different components of venoms and behaviours came as little changes over 100ds of millions of years, and the specialization on a single species of tarantula kindof gives the wasp exclusivity from intra clade competition.

Digging a hole to put the spider in does seem overly complex. apparently, it must have first learnt to pull the animal into a hidden recess... Afterwards it started to scratch and clear the recess, and that foraging movement eventually turned into excavation of a pit prior to dragging the prey.

here's one of the most readable references: https://www.nature.com/articles/srep19604


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