How the Insect Body Evolved

Check out the YouTube Video herehttps://www.youtube.com/watch?v=ow6PpJ_Br0o

Insects are the most successful group of animals, having more species than all others put together. Part of what makes them so successful is how evolution was able to play with their worm-like ancestors to shape them into what they are today. But how exactly did a worm become an insect?

One of the amazing things about the proposed evolutionary path that insects took, is that each of the intermediate stages have modern animals that are probably similar in appearance to those ancient insects. This gives science a lot of unique glimpses into the behavior and morphology of these early ancestors. The earliest insects might have looked something like segmented worms, much like the annelids that still are around today. Because of the similarity of these worms to annelids we can take the basic body plan of annelids and apply this to the ancestors of insects. For annelid worms each segment is separated from the others by thin membranes called septum. A pair of nerve cords, a large blood vessel and the gut are the only things that run through all the segments of the worm. Essentially each segment is self contained, and will even have their own organs, such as excretory organs, that are not connected to the next segment’s. The nerve cords that penetrate each segment connect to a ganglia in each segment. Ganglia are a mass of nerves that react to stimuli without communicating with the brain. It might seem weird to have a “mini brain” in various parts of your body making decisions without your brain, but you too, have ganglia throughout your body and this is part of the reason your reactions are so quick, for example when you jerk back after touching something hot.

Some of these segmented worms started to develop two pairs of appendages for each segment. These appendaged would have been simple but helped the animals move through the water as they swam, similar to polychaete worms. Eventually some of these polychaete-like ancestors began to move around on the ocean floor, using their appendages more like legs than paddles. While the legs grew more musculature they were still basically just fleshy sacks looking very similar to today's Onychophorans or velvet worms. To avoid predation some of these ancestors started to develop a hardened cuticle made from chiton. The process of an animal’s tissues becoming hard is known as sclerotization. As they developed this hardened cuticle, their sack-like legs would no longer have their full range of movement, being restricted to a pegleg-like movement. Instead of going full pirate, early insects developed joints of fleshy material between the sclerotized sections.

This allowed insects to keep a lot of mobility while also having amazing protection. This is usually a trade-off in evolution, the more protection you have the more mobility you lose. Think of a tortoise or snail, amazing protection but low mobility. Arthropods found a way around this problem, and this solution is a major reason they are so successful today. In fact, jointed legs are a defining character of all arthropods, and arthropod literally means joint-foot.

The ancestor of insects during this stage would be a creature that looked similar to that of a millipede (Myriapoda), a long, segmented, worm-like body with a hardened exoskeleton and two pairs of jointed legs per segment. This is where evolution really took off, modifying this basic plan into the 1 million described body plans we see in insects today!

New arthropod groups came into being as segments started to lose their individuality, fusing into what is known as tagmata. Tagmosis is the fusion of multiple segments to specialize in a given process. For insects, the segments fused into three tagmata (sometimes mistakenly called segments), head, thorax, and abdomen. However, the ancient segments can still be seen as the segments within the tagmata. For example, the thorax region of an insect is a group of 3 segments: the pro-, meso-, and meta- thoracic segments. These segments have specialized in locomotion, containing mostly muscles. In many cases, the original segments still contain their own ganglia, although they lose many of the other individual organs and structures.

As segments became tagmata, legs could be dropped or developed to serve other functions. Insects are highly modified crustaceans and so we can use a crustacean leg, like this Anaspididae, to see what the ancestral state may have looked like on ancient insects.

Let’s first look at the modifications that took place to make a typical insect leg. Each part is referred to with the entomological name of the ancestral state, but I included the names that carcinologists, those who study crustaceans, have given them in parentheses. For the insect leg to evolve, the endite lobes shrunk along with one of the trochanters. The femur, tibia, and tarsus lengthened out, making it easier for them to walk on land. It is important to note that both the tarsus in the ancestral state and the current insect leg have many segments that are not shown here. Now onto the evolution of the mouthparts, which is even more complicated and fun!

The insect mouth has many movable parts and each one probably came from legs, we will focus on two of these mouthparts, where it is easy to see where each part comes from, the labium and maxillae. The maxillae are what insects use to chew their food. It also has palps to help with food manipulation, similar to the way we use our hands, but these palps also have sensory organs within that do something like smelling and tasting. The maxillae evolved from trochanter 2, stretching and smushing the endite lobes together. The endite lobes became harder and sharper as they became the means of chewing food. The femur and tibia became shorter as the tarsal segments became longer, so that the insects could use these to manipulate their food. Being on the end of the legs the tarsal segments already were equipped with sensory structures and so longer tarsi were selected for the palps over the femur and tibia. The second mouthpart, the labium, is easily the most interesting. The labium is the backmost part of an insect mouth and helps hold the food inside (somewhat like our lips), it too has palps but these are mainly used for manipulation of the food. Interestingly, the coxipodites of two different legs came together and fused forming one large plate. Both trochanters also fused together and in many chewing insects you can still see the gap where the trochanters have not entirely fused. The endites became sensory structures similar to our tongue. Because sensors were not as important the tarsal segments could be reduced to one and are of similar size to the femur and tibia.

This is just a few examples of what insects have used the extra body segments and legs for in their evolution. Other structures that come from legs could include the other mouth parts we did not cover, the reproductive systems, gill structures on some larval forms, and, though it is heavily debated, their wings. Of course evolution didn’t stop there and continued to play with these now basic insect parts. Certain orders, families, and even species lost or changed all of these parts over and over again! And I promise we will talk about all these crazy modifications in later videos. It is very important to know that, what became what is hotly debated, I went with the most widely accepted model, and what makes the most sense to me, but by the time this video is actually out there, there could be more information that changes the way we currently see insect evolution. Despite this all entomologists agree that this evolution, from annelid worm to the legged soft bodied velvet worms to jointed hardened legs of crustaceans, is what has made insects the most successful animals this planet has ever seen!

References

• Physiopedia Ganglia https://www.physio-pedia.com/Ganglion#:~:text=A%20ganglion%20is%20a%20collection,the%20ganglia%20and%20then%20exits. Accessed June 4, 2021

• Sarnat, H. B., & Netsky, M. G. (2002, December). When does a ganglion become a brain? Evolutionary origin of the central nervous system. In Seminars in Pediatric Neurology (Vol. 9, No. 4, pp. 240-253). WB Saunders.

Pictures from the Video

1. Grasshopper and water CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=59068817

2. Insect diversity ESA (2017) Protecting the Web of Life ESA Position Statement on Insects and Biodiversity https://www.entsoc.org/sites/default/files/files/Science-Policy/EntSocAmerica_PolicyStatement_ArthropodBiodiversity.pdf

3. Scorpionfly By Mathias Krumbholz - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=10364828

4. Beetles By Insects Unlocked, University of Texas at Austin - Crop of File:Assorted Coleoptera in the University of Texas Insect Collection.jpg, CC0, https://commons.wikimedia.org/w/index.php?curid=98004783

5. Water striders Phonon.b, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

6. EarthWorm By Donald Hobern from Copenhagen, Denmark - Lumbricus terrestris, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=50128937

7. Tube worms By NOAA Okeanos Explorer Program, Galapagos Rift Expedition 2011 - Flickr NOAA Photo Library, Public Domain, https://commons.wikimedia.org/w/index.php?curid=35246911

8. Earthworm sketches By Anonymous - Encyclopædia Britannica 1911 - Chaetopoda article, Public Domain, https://commons.wikimedia.org/w/index.php?curid=48748532

9. Earthworm cross section By Zi X. Ou - Gall L (2019). Invertebrate Zoology Division, Yale Peabody Museum. Yale University Peabody Museum. Occurrence dataset https://doi.org/10.15468/0lkr3w accessed via GBIF.org on 2019-10-05. https://www.gbif.org/occurrence/1570711136, CC0, https://commons.wikimedia.org/w/index.php?curid=82857721

10. By Oligochaeta_anatomy.svg: Reytan / *derivative work: B kimmel (talk)Reytan - Own workThis file was derived from: Oligochaeta anatomy.svg:This image is a derivative work of the following images:File:Oligochaeta_anatomy.svg licensed with PD-user2006-12-16T12:38:49Z Reytan 620x461 (50288 Bytes)2006-12-12T01:20:15Z Nux 620x461 (24811 Bytes) +xlink namespace declaration (used in radialGradient)2006-12-04T13:24:02Z Reytan 0x0 (24768 Bytes)2006-12-02T21:42:48Z Reytan 620x461 (29383 Bytes)2006-12-02T20:56:22Z Reytan 0x0 (24644 Bytes)2006-12-02T20:16:41Z Reytan 0x0 (24588 Bytes)2006-12-02T12:43:34Z Reytan 0x0 (24585 Bytes)2006-12-02T12:19:46Z Reytan 0x0 (24598 Bytes)2006-12-02T00:24:34Z Reytan 0x0 (24583 Bytes)2006-12-02T00:20:09Z Reytan 0x0 (25271 Bytes)2006-12-02T00:17:09Z Reytan 0x0 (25287 Bytes) {{FReytan}}, Public Domain, https://commons.wikimedia.org/w/index.php?curid=18177012

11. Worm side section By Alexander Klepnev - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=79632885

12. Ganglia picture By Z-P. Feng, Z. Zhang, R. E. van Kesteren, V. A. Straub, P. van Nierop, K. Jin, N. Nejatbakhsh, J. I. Goldberg, G. E. Spencer, M. S. Yeoman, W. Wildering, J. R. Coorssen, R. P. Croll, L. T. Buck, N. I. Syed &amp; A. B. Smit - Feng Z-P., Zhang Z., Kesteren R. E. van, Straub V. A., Nierop P. van, Jin K., Nejatbakhsh N., Goldberg J. I., Spencer G. E., Yeoman M. S., Wildering W., Coorssen J. R., Croll R. P., Buck L. T., Syed N. I. &amp; Smit A. B. (23 September 2009) &quot;Transcriptome analysis of the central nervous system of the mollusc Lymnaea stagnalis&quot;. BMC Genomics 10: 451. doi:10.1186/1471-2164-10-451 Figure 1, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=8658487

13. Earthworm video Nesnad, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

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15. Vertebra By Laboratoires Servier - Smart Servier website: Images related to Vertebra, Skeleton and bones and Bones -- Download in Powerpoint format.Flickr: Images related to Vertebra, Skeleton and bones and Bones (in French)., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=82641369

16. Ganglia By BruceBlaus - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=46621399

17. Polycheate worm (red) By Auckland Museum Collections from Auckland, Aotearoa New Zealand - Annelida polychaeta, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=109342340

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19. Red long polychaete By Arnstein Rønning - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=41021961

20. Crawling polychaete worm By YVC Biology Department - bonaire085, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=97572613

21. Water bear Andrei Savitsky, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons https://commons.wikimedia.org/wiki/File:%D0%A2%D0%B8%D1%85%D0%BE%D1%85%D0%BE%D0%B4%D0%BA%D0%B8.webm

22. Velvet worm on leaf By Geoff Gallice - Flickr: Velvet worm, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=14589894

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24. Crayfish By Anna N Chapman - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=112855488

25. Blue velvet worm By Ypna - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=61739516

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28. Snakefly By Richard Bartz, Munich aka Makro Freak - Own work, CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=4112122

29. Millipede By Vengolis - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=51368692

30. Pillbug millipede By Vengolis - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=60594516

31. Insect collage By Anax_imperator_qtl2.jpg: QuartlCetonia-aurata.jpg: CrumpsUnidentified_Mantid_Species_(6105939456).jpg: Thomas BrownPyrrhocoris_apterus_(aka).jpg: André Karwath aka AkaEctophasia_spp.jpg: BetacommandBotEastern Tiger Swallowtail Papilio glaucus on Milkweed 2800px.jpg: Ram-ManMantispidae_fg1.jpg: Fritz Geller-GrimmBee-collecting-pollen2.jpeg: P.manchevGrasshopper_2.JPG: Ryan WoodPhyllium_giganteum,_adult_femal_from_dorsal.JPG: DrägüsScorpion_Fly._Panorpa_communis._Mecoptera_(7837166610).jpg: Sebastian WallrothBlaberus_giganteus_MHNT_dos.jpg: Didier Descouensderivative work: Anaxibia - This file was derived from:Anax imperator qtl2.jpg:Cetonia-aurata.jpg:Unidentified Mantid Species (6105939456).jpg:Pyrrhocoris apterus (aka).jpg:Ectophasia spp.jpg:Eastern Tiger Swallowtail Papilio glaucus on Milkweed 2800px.jpg:Mantispidae fg1.jpg:Bee-collecting-pollen2.jpeg:Grasshopper 2.JPG:Phyllium giganteum, adult femal from dorsal.JPG:Scorpion Fly. Panorpa communis. Mecoptera (7837166610).jpg:Blaberus giganteus MHNT dos.jpg:, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=28239435

32. By Pearson Scott Foresman - This file has been extracted from another file, Public Domain, https://commons.wikimedia.org/w/index.php?curid=94759650

33. Madagascar hissing cockroach By Almabes at English Wikipedia - Transferred from en.wikipedia to Commons by Sreejithk2000 using CommonsHelper., Public Domain, https://commons.wikimedia.org/w/index.php?curid=10542089

34. By Unknown author - Popular Science Monthly Volume 39, Public Domain, https://commons.wikimedia.org/w/index.php?curid=12134260

36. Anasipidae By William Thomas Calman - Calman, W. T. (1911) Life of Crustacea, Category:New York: the MacMillan Company, Public Domain, https://commons.wikimedia.org/w/index.php?curid=53129414

37. Stonefly By Dave Huth from Allegany County, NY, USA - Stonefly nymph, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=64866261

38. Muscles By Anders Hedenström - http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001822, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=45572384

39. Nerves of bee By Unknown author - Popular Science Monthly Volume 39, Public Domain, https://commons.wikimedia.org/w/index.php?curid=12134260

40. Mantis video Pristurus, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

41. Grasshopper mouth parts By Westeros91 - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=19776596

42. Maxillae By Sikander Kiani - Gall L (2019). Invertebrate Zoology Division, Yale Peabody Museum. Yale University Peabody Museum. Occurrence dataset https://doi.org/10.15468/0lkr3w accessed via GBIF.org on 2019-07-07. https://www.gbif.org/occurrence/1571709592, CC0, https://commons.wikimedia.org/w/index.php?curid=80234694

43. Head_of_Chrysomelidae_SEM.jpg: By Louisa Howard (uploaded by gian_d)derivative work: Siga (talk) - Head_of_Chrysomelidae_SEM.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=14408069

44. Beetle eating Pristurus, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

45. Labium By Sikander Kiani - Gall L (2019). Invertebrate Zoology Division, Yale Peabody Museum. Yale University Peabody Museum. Occurrence dataset https://doi.org/10.15468/0lkr3w accessed via GBIF.org on 2019-07-07. https://www.gbif.org/occurrence/1571709588, CC0, https://commons.wikimedia.org/w/index.php?curid=80234692

46. Dragonfly nymph By Fredlyfish4 - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=31115872

47. Earwig on red leaves By hedera.baltica from Wrocław, Poland - European earwig, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=107944834

48. Insect moutparts By Xavier Vázquez - commons, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2924364

49. Camel Cricket By Pavel Kirillov from St.Petersburg, Russia - DSC_1303, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=64705391

50. Hover fly By Chec20 - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=91555884

51. Stag beetle By Pablo Baeyens - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=106801221