Why are there so many raptorial limbs?

Watch the video or jump to the end of this article for the complete list of insects with raptorial limbs.

Insects are the most diverse group of organisms with more species than any other. With that much variation it is not surprising that they have settled on similar traits repeatedly throughout evolution. One of those traits is raptorial limbs. They seem to crop up in several major groups of insects and some have become so similar to each other that they are often misidentified. What makes raptorial limbs great? Let’s answer that question and look at all the groups of insects that have them.

Convergence

Before getting into it, we need to clarify a few things. First convergent evolution. If organisms that are not closely related to one another are under similar evolutionary pressures, they might develop similar traits to one another and so give the appearance of being closely related, even when they are not. One of the most overused examples are the similarities sharks, dolphins, and ichthyosaurs share that help them swim more effectively in the water, despite not being very closely related to each other. Like how streamlined they are, and their fins and tails are similarly shaped.

Raptorial limbs

The other thing we should probably define is what a raptorial limb is! A raptorial limb is where opposing leg segments (usually the femur and tibia) have developed into a pinching structure and they are primarily used for grabbing and holding their prey. In fact, raptorial or raptor comes from the Latin root for seizing and is also where birds of prey get their name too. Raptor also means plunderer or thief which is what is meant when applied to the non-avian dinosaur raptors. Raptorial limbs are different from a crabs or scorpion's claws which are modified digits rather than leg segments, these claws are technically called a chela. The technical term for raptorial limbs is subchela but we will continue to refer to them as raptorial.

There are two orientations for raptorial legs, vertical or horizontal. As you’ll see today most insects have evolved the vertical legs as the limbs have the potential of being used for walking as well as seizing prey. 

Let’s go through all the groups of insects that have raptorial limbs, if you notice I’ve left a group out let me know in the comments and I’ll add a section!

Mantodea

First off the group everyone is probably most familiar with, Mantodea or the mantises. These are classic examples of multi purposing their raptorial limbs for walking and hunting. All members of this order have raptorial limbs, in fact while other orders have few species or families, mantids are the only ones where every species in the order has raptorial limbs. They demonstrate many of the adaptations that are needed to create these limbs. 

Obviously there are the adaptations for the raptorial limb itself. The fore tibia and femur have been beefed up and there are small spines running down both sides. The Coxa has elongated out, whereas in most insects its a relatively small segment. If you’re not familiar with insect morphology the Coxa usually acts similar to the way your shoulder or hip works. It attaches the rest of your limb to your body in a way that allows additional movement rather than the side to side movement your arms otherwise have. The first segment of the thorax, the prothorax is elongated as well. Which makes sense: you don’t want to get your raptorial legs all tangled up with your other legs as you walk and you need a place to house those large muscles used to power such large limbs. Notice that the mantids still retain many of their tarsal segments, we’ll see variation in other insects depending on if the raptorial limbs are used for walking.

Hemiptera (Heteroptera)

The next group is the Hemipterans, or the true bugs. Not every member of this order has raptorial limbs, however, several groups within this order have evolved them independently not only from each other but from mantids as well, I mean just look at how distantly related they are to the mantids!

Within the Reduviidae or assassin bug family we have a few that have raptorial limbs; notice that in both of these examples (this one is a thread legged assassin and the other is an ambush bug) that we have a reduction of the tarsal segments into a single segment. But many of the same adaptations are there. Enlarged femur and tibia. Elongated coxae and large prothoraxes. Looking at the family of damsel bugs or Nabidae, they look very similar but with the tarsal segments present.

The aquatic families within Hemiptera have really gone in with the raptorial limbs. The Nepidae or water scorpion has pretty much everything that was going on with our previous groups, however the raptorial limbs are super thin and the teeth are small and frequent. This reminds me a lot of animals that eat primarily fish like gharials and spinosaurus, that have thin jaws and frequent teeth that are good for catching slippery prey. This is also a good time to introduce horizontal orientation of the raptorial limbs. I don’t have a specimen of a water scorpion that has horizontal limbs but these species look similar to the Belostomatidae and Naucoridae, both of which have the horizontal orientation. 

Now here is some interesting variation with the horizontal orientation. Notice that you do not need as long of a prothorax or coxa. To me the raptorial limbs seem to be more beefy but I don’t know if that has anything to do with the positioning of the raptorial limbs or not. Of course in the water you don’t need to walk around so it’s not surprising that in all these aquatic groups there’s a reduction of the tarsal segments and is another reason they are held horizontally. 

Neuroptera

Ok let’s move on to the next group! This family is probably my favorite. They so clearly demonstrate the power of convergent evolution. You might be thinking that these are mantids. But they aren’t (though don’t worry a lot of people get that wrong!). These belong to the order Neuroptera and are much more closely related to lacewings or antlions then they are to mantids. They are the mantisflies or Mantispidae. These guys are more arial then mantids and I’ve seen them hunt on the wing which is pretty cool. What is absolutely cool about this group is they not only converged with mantids but many of the diurnal species are disguised as wasps to trick predators! How amazing is that! Obviously they have those same traits we’ve been talking about, I mean just look at them! But notice again, the reduction of the tarsal segments. This is because these guys are doing more flying than walking, they don’t need them.

Diptera

Moving to the end of the phylogeny there are also 2 genera of true flies (Diptera) that also have raptorial prolegs. The Ochthera and Chelipoda. Do you notice something that’s missing from the group of traits we’ve been talking about? Yeah, they don’t have elongated prothoraxes! Why is that? Well evolution works with what’s already there, meaning that it usually modifies traits that already exist. Dipterans, the flies, went all in on very controlled flight and reduced the thorax to house better more efficient flight muscles. This means that true flies have an extremely small prothorax and if you didn’t have much of a prothorax to begin with then you won’t magically get one if you evolve raptorial legs later. You might have also noticed the tarsal segments. Without a long prothorax to get those raptorial legs up off the ground you have to use them for walking!

Hymenoptera

I purposely skipped a group before because wouldn’t be surprised if most entomologists would say these aren’t true raptorial limbs because of how different they are, but I wanted to include them because they are probably the coolest use of raptorial limbs (if that’s what you’d call them) ever! They are the Chalcididae wasps within Hymenoptera. And hopefully you see right off the bat what’s different with these. Their raptorial limbs are backwards! It’s their hind limbs or metalegs not the prolegs that are raptorial. Now this family was recently found to be paraphyletic so it might not stay together for much longer BUT you have to admit they are some awesome looking insects! 

Now if the point of raptorial limbs is to catch and seize prey, why have the raptorial limbs as far as you can from your head?! Well if you know about wasps you might have already guessed that these wasps are parasitoids. Parasitoids are different from parasites in that the host will die after serving its purpose. Whereas with parasites the host doesn’t normally die. But like many parasites, parasitoids need to get inside their hosts and to do that a mother chalcidid wasp will seize and restrain the host while it deposits an egg inside the host! Not a good day for the host. There’s one species (maybe others) that specialize in antlion larva and will use their hind limbs to grapple the antlion jaws open so that the female can lay an egg inside the throat of the antlion! How amazing is that! And maybe just a tad bit disturbing!

Now they have many features that are shared with raptorial limbs, the same limb sections are what make up the raptorial limb and the coxa is elongated just like we’ve seen everywhere else. Of course the prothorax isn’t expanding because these limbs are on the metathorax and because they are the metalegs they need to be used for walking so we aren’t going to see the reduction of the tarsal segments.

So why are raptorial legs so great?

The benefits to having raptorial legs must outweigh the cost of growing more robust and thick limbs as many insects have developed them on separate occasions. Most raptorial limbs have serrated edges that immobilize prey, these teeth are pointed inward which further traps the prey. While this is advantageous, claws do the same thing. Claws can also be used for defense easier than raptorial limbs as claws can hold off attackers at a greater distance from the animal's body. And we do see claws popping up convergently multiple times within arthropods, and so are also a beneficial tool to evolve.

Despite multiple lineages of arthropods convergently evolving claws, I am only aware of one group of insects, the Dryinidae in Diptera, that have evolved claws. I don’t believe there has been any studies on why insects don’t evolve claws so take what I'm about to say with a grain of salt. One advantage that raptorial limbs have over claws for insects is that the femur and tibia are already relatively large and strong compared to the tarsi, which for most insects are small. Therefore less modifications would be required to change the baseline leg into a raptorial one than for the tarsi to expand. This makes sense as evolution usually works with what it already has.

However, what we do know is that raptorial legs have leverage and an energy efficient way of creating a fast snapping motion. We can use a mantisfly’s raptorial leg as a model for how raptorial legs store and release energy. Of course there will be differences from order to order but this will get the point across. 

The inside workings

To load the spring a mantisfly opens its raptorial limb by contracting a muscle called the M25 (blue). Attached to the inside of the tibia are internal structures (outlined) that push down and up on a tendon (green) at the same time. Tendons act similarly to a rubber band elastic and are capable of storing lots of energy. One of the internal structures called the ventrotibial complex or VTC has a groove down the middle, to prevent the tendon from sliding into that groove two muscles, M24c (yellow), on the other end pull the tendon in opposite directions.

The tendon is already storing an incredible amount of energy but the final muscle complex, M24ab (red), pulls on the opposite end stretching it even further! When ready to release the two M24c’s relax, causing the tendon to slip through the groove into its relaxed state. The pulling of the M24ab as well as the stored energy in the tendon makes it snap closed faster and with less energy than it would if it were being powered by muscle alone.

Again there is going to be a lot of variation. Mantids, for example, load the energy with their raptorial limbs closed. But in all cases the raptorial limbs are storing and releasing energy in very effective ways.

So next time you are out and about and you see one of these amazing insects, give a little more appreciation to the amazing feats their feets are performing every day!

A list of raptorial limbs

Insects

• Mantodea (mantids)

• Hemiptera (true bugs)

  • Nabidae (damsel bugs)

  • Belostomatidae (water bugs)

  • Nepidae (water scorpions)

  • Reduviidae (assassin and ambush bugs)

  • Naucoridae (creeping water bugs)

• Neuroptera

  • Mantispidae (mantisflies)

• Coleoptera (beeltes)

  • Philonthus marginatus (type of rove beetle, Staphylinidae)

• Diptera (true flies)

  • Ochthera spp.

  • Chelipoda spp.

• Hymenoptera (ants, bees, wasps)

  • Chalcididae (chalcid wasps)

Non-Insects

• Stomatopoda (Mantis shrimp)

• Amblypygi (Tailless whip scorpions)

• Opiliones (Harvestme) genus Taracus

Honorable mentions

• Cicada nymphs (Cicadidae) - have fossorial for limbs used for digging but look very similar. Check cicada mania's page about this.

• Vinegaroons (Uropygi) - bring both limbs together to make the pinching. Imagine someone using a single chopstick in each hand.

  
  

References

• Weirauch, C., Forero, D., & Jacobs, D. H., 2011. On the evolution of raptorial legs – an insect example (Hemiptera: Reduviidae: Phymatinae). Cladistics, 27(2), 138-149

• Poivre, C. 1976. Observations sur la biologie, le comportement et le phénomène de convergence chez les Mantispidés [Planipennes]. Entomologiste, 32(1), 45341

• Piek, T. ed 2013. Venoms of the Hymenoptera: biochemical, pharmacological and behavioural aspects. Chapter 4 Stinging Behavhiour of Solitary Wasps (Stiener, A. L.). Elsevier

• Baumler, F., Gorb, S. N., & Busse, S. 2023. Extrinsic and intrinsic musculature of the raptorial forelegs in Mantodea (Insecta) in the light of functionality and sexual dimorphism. Journal of Morphology, 284(6), e21590

• Busse, S., Baumler, F., & Gorb S. N. 2021. Functional morphology of the raptorial forelegs in Mantispa styriaca (Insecta: Neuroptera). Zoomorphology, 140(2021), 231-241

Pictures used for the video (in order of appearance)

Dolphin By steve b - https://www.inaturalist.org/photos/268667502, CC0, https://commons.wikimedia.org/w/index.php?curid=134060077

Shark By Stormy Dog - https://www.flickr.com/photos/the-lees/134610871/, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=4549629

Pics by Becca = By Weevil Games - www.weevilgames.com

Flower mantis By zleng - https://www.flickr.com/photos/60134003@N04/21682936523/, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=136745007

Belostomatidae By Frank Vassen from Brussels, Belgium - Giant water bug (Belostomatidae), Vohimana reserve, Madagascar, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=43770104

Defensive Mantid By Tibor Duliskovich at English Wikipedia, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=15530893

Mantis video by The Nature Box 

Gharial By Mike Prince from Bangalore, India - Gharial, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=112389706

Spinosaurus By I, Steveoc 86, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=2082681

Water scorpions By Traian Brad, Sanda Iepure and Serban M. Sarbu - https://www.mdpi.com/1424-2818/13/3/128#:~:text=Movile%20Cave%20hosts%20one%20of,primary%20productivity%20by%20chemoautotrophic%20microorganisms., CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=149065096

Horizontal water scorpion By 奶油蛋糕 at Chinese Wikipedia - Own work, GFDL, https://commons.wikimedia.org/w/index.php?curid=32871299

Green Lacewing By Alvesgaspar - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=40939372

Antlion By gailhampshire from Cradley, Malvern, U.K - Antlion. Distoleon annulatus. (Neuroptera. Myrmeleontidae).., CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=50566267

Owlfly By © 2010 Jee & Rani Nature Photography (License: CC BY-SA 4.0), CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=19141824

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Mantisfly eating video Granier, C. [yackphoto], (11. 08. 2019) Mantispa styriaca 1920x1080 Qf. https://www.youtube.com/watch?v=hQTSrLUSqGo.

Mantisfly on twig By © 2016 Jee & Rani Nature Photography (License: CC BY-SA 4.0), CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=54005114

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Wasp mantisfly 2 By Stygioberotha - Own work, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=149664104

Ochthera By Frank Vassen from Brussels, Belgium - Ochthera manicata (ID to be confirmed), Parc de Woluwé, Bruxelles, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=116492479

Chelipoda By xpda - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=67749750

Dead Chelipoda By Ivković M, Perović M, Grootaert P, Pollet M (2021) - Ivković M, Perović M, Grootaert P, Pollet M (2021) High endemicity in aquatic dance flies of Corsica, France (Diptera, Empididae, Clinocerinae and Hemerodromiinae), with the description of a new species of Chelipoda. ZooKeys 1039: 177-197. https://doi.org/10.3897/zookeys.1039.66493, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=130708240

Chalcididae wasp By AfroBrazilian - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=70700684

Pinned chalcid wasp By K. McCormack - https://bdj.pensoft.net/article/8013/element/2/2872826//, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=118301274

Ichneumonidae By Charles J. Sharp - Own work, from Sharp Photography, sharpphotography, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=49601799

Caterpillar parasite By Nikhil More - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=116617506

Ovipositor By Charles J. Sharp - Own work, from Sharp Photography, sharpphotography.co.uk, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=146540445

Lobster By Filippo antinori1223 - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=142991316

Crab By © Hans Hillewaert, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=518367

Shrimp By Clinton & Charles Robertson from RAF Lakenheath, UK & San Marcos, TX, USA & UK - Big-claw river shrimp (Macrobrachium carcinus)Uploaded by Jacopo Werther, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=25200598

Drawing Dryinidae By Elke Freese - selbst gezeichnet (Elke Freese) nach Abbildung von M. Olmi, 1999, Public Domain, https://commons.wikimedia.org/w/index.php?curid=660764

Dryinidae By maxson.erin - cf Anteoninae F, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=50989903