Thermonectus marmoratus

Thermonectus marmoratus is a relatively colorful North American species of diving beetle known by the common names sunburst diving beetle and spotted diving beetle.[1] The behavior of this diving beetle has been compared to a scuba diver, since it carries with it a bubble of air as it dives down into the water.[2] The beetle has recently become notable when it was discovered that its aquatic larval stage is the first ever recorded use of bifocal technology in the animal world.[3] The beetle uses in its principal eyes two retinas and two distinct focal planes that are substantially separated, in the manner of bifocals to switch their vision from up-close to distance, for easy and efficient capture of their prey.

Thermonectus marmoratus
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Coleoptera
Family: Dytiscidae
Genus: Thermonectus
Species:
T. marmoratus
Binomial name
Thermonectus marmoratus
Gray, 1832

Because of their bright colors, they are often displayed in zoos, sometimes together with Abedus herberti (also found together in the wild[4]) and other aquatic beetles.[5]

Physical description
An enclosure for sunburst diving beetles

The adult beetle has a length of 0.8–1.5 cm (0.31–0.59 in),[6][7] with females slightly larger than males. The sunburst diving beetle has a black and streamlined carapace covered with bright yellow or golden spots. The male has a suction disk on each foreleg.[8][9]

Diet

Sunburst diving beetles are predators and scavengers of small animals, especially invertebrates such as other aquatic insects and snails, but also small vertebrates such as young fish and tadpoles. If available, they prefer to feed on small, already dead soft-bodied animals.[4] These beetles are useful because they eat mosquito larvae and pupae. They have also been observed swarming a prey item and feeding en masse. In captivity, these beetles will feed on flake fish food and live crickets.[10][11]

Behavior

Swimming

Sunburst diving beetles live in water and swim well.[1][8] The diving beetle swims by thrusting its hind legs simultaneously, and can remain underwater for extended periods of time by storing an air bubble beneath its wings.[12] They inhabit various slow-moving freshwater habitats, especially shallow, temporary or intermittent pools and creeks (arroyos) with little or no aquatic vegetation.[7][13] When their water source dries up they will fly to a new one.

Protection

The sunburst diving beetle's distinctive yellow spots also serve as a warning sign to predators that the insect can release a foul tasting chemical.[14] The milky appearing chemical is ejected from specialized glands found behind the insect's head, and has been found to be made primarily out of the steroid hormones cybisterone and mirasorvone. It has been further hypothesized that mirasorvone functions as a fish deterrent.[15]

Distribution

Sunburst diving beetles are found in extreme Southern California (mainly Peninsular Ranges),[7] southern Utah,[16] Arizona, New Mexico, Texas (where first recorded in 1996), and Mexico.[17]

Development

Early development

Eggs that are laid by the mother are initially 3.5 to 4 mm in length and lack pigmentation or any special features – causing them to be white and opaque. As the embryo develops, it begins to appear more translucent. Around a third of the way into development, the eyes become externally visible, however, the head region will only become discernible nearly halfway into development.[18]  The larvae of the organism have legs, a soft body, antennae, and are aquatic - preying primarily on mosquito larvae and other aquatic animals. The beetles pupate on land, and can develop from an egg to an adult in 28 days.[19]

Visual system

Multiple retinas

Scientists have discovered the larval visual system of Thermonectus marmoratus have incredibly complex eyes that resemble a camera's bifocal capabilities. The beetle has a total of 12 eyes (six eyes on either side of its head), but a total of twenty eight retinas.[20] Early in the organism's development, there are many conserved functions and cell lineages between the diving beetle and other insect species such as the fly (Drosophila). However, there are also some stark differences, including larger numbers of cells which will form the two retinas per eye, as well as a large scale reorganization of the eye tissues. This is especially observed with the migration of photoreceptors in later stages of development.[21] The eyes are also not attached to any muscles, which restricts them from moving. Instead, the beetle scans its visual field by moving its head from side to side.[22] The larvae eyes are mapped to six distinct neuropils (clusters of interwoven nerves) that function as the optic lobe. As the beetle reaches adulthood – these neurophils degenerate and are replaced with a mature optic lobe. Adult beetles develop compound eyes, similar to many other insects.[23]

Eye patch

Along with the six eyes on either side of the head, larvae also have an eye patch. This eye patch does not have a lens, but is composed of retinal tissue. Scientists theorize that this functions in filling the gap in vision at the top of the head, and can alert the beetle larvae of a change in light – which may be indicative of an overhead predator.[24]

Bifocal lens

The diving beetle has a unique bifocal lens which is asymmetric. The protein makeup of the middle of the lens is different from that along the edges.[25] Scientists have found that this disparity in the lens goes hand-in-hand with the multiple retinas in each eye. The bifocal asymmetric lens allows for two images to be formed within the eye – each focusing on one of the retinas. The unfocused image is also shifted due to the asymmetry – causing only the focused image to be displayed on one retina – further enhancing resolution.[26]

References
  1. MORGAN, R.C. (1998). "5". "Windows on the Water World," Backyard BUGwatching. pp. 4–6.
  2. "Los Angeles Zoo and Botanical Gardens | Beetle, Sunburst Diving". Los Angeles Zoo and Botanical Gardens. Retrieved 24 April 2019.
  3. Dawn Fuller (duly edited) (24 August 2010). "Bug With Bifocals Baffles Biologists". ScienceDaily. ScienceDaily LLC. Retrieved 25 August 2010.
  4. J. Velasco; V.H. Millan (1998). "Feeding Habits of Two Large Insects from a Desert Stream: Abedus herberti (Hemiptera: Belostomatidae) and Thermonectus marmoratus (Coleoptera: Dytiscidae)". Aquatic Insects. 20 (2): 85–96. doi:10.1076/aqin.20.2.85.4500.
  5. O’Sullivan, T. "Captive rearing study of the Thermonectus marmoratus" (PDF). St. Louis Zoo. Retrieved 9 April 2018.
  6. "Sunburst Diving Beetle – Thermonectus marmoratus". arizonensis.org. Retrieved 9 April 2018.
  7. A.V. Evans; J.N. Hogue (2006). Field Guide to Beetles of California. University of California Press. p. 73. ISBN 978-0-520-24655-3.
  8. EVANS, D.L.; SCHMIDT, J.O.; EDs. (1990). Insect Defenses, Adaptive Mechanisms and Strategies of Prey and Predators. State Univ. of New York Press, Albany. pp. 482 pp.
  9. MILNE, L; MILNE, M. (1980). "Marbled Diving Beetle (Thermonectus marmoratus)," The Dobbin Society Field Guide to North American Insects and Spiders. Alfred A. Knopf, NY. pp. 542+ plate 96.
  10. Arnett Jr, R.H.; Jacques Jr, R.L. (1981). "Thermonectus marmoratus," Simon and Schuster's Guide to Insects. New York, Toronto and London: Simon and Schuster Inc. pp. 86–87.
  11. MORGAN, R.C. (1992). "Natural History, Captive Management and Display of the Sunburst Diving Beetle Thermonectus marmoratus," AAZPA/CAZPA Annual Conference Proceedings. pp. 457–464.
  12. "Sunburst Diving Beetle | Insectarium". audubonnatureinstitute.org. Retrieved 24 April 2019.
  13. "Species Thermonectus marmoratus – Sunburst Diving Beetle". BugGuide. Retrieved 9 April 2018.
  14. "Sunburst Diving Beetle | Saint Louis Zoo". www.stlzoo.org. Retrieved 24 April 2019.
  15. Meinwald, Jerrold; Huang, Qing; Vrkoč, Jan; Herath, Kithsiri B.; Yang, Zhi-Cai; Schröder, Frank; Attygalle, Athula B.; Iyengar, Vikram K.; Morgan, Randy C. (17 March 1998). "Mirasorvone: A masked 20-ketopregnane from the defensive secretion of a diving beetle (Thermonectus marmoratus)". Proceedings of the National Academy of Sciences of the United States of America. 95 (6): 2733–2737. Bibcode:1998PNAS...95.2733M. doi:10.1073/pnas.95.6.2733. ISSN 0027-8424. PMC 19637. PMID 9501158.
  16. Vinson, M. (15 May 2002). "Aquatic Ecosystems and Invertebrates of the Grand Staircase-Escalante National Monument" (PDF). Utah State University. Retrieved 9 April 2018.
  17. Shaverdo, H.V. (2005). Konstantinov; Tishechkin; Penev (eds.). "New Records of Some Species of Diving Water Beetles (Coleoptera: Dytiscidae) from the Palearctic and Nearctic Regions". Contributions to Systematics and Biology of Beetles Papers Celebrating the 80th Birthday of Igor Konstantinovich Lopatin: 327–331.
  18. Stecher, Nadine; Stowasser, Annette; Stahl, Aaron; Buschbeck, Elke K. (July 2016). "Embryonic development of the larval eyes of the Sunburst Diving Beetle, Thermonectus marmoratus (Insecta: Dytiscidae): a morphological study". Evolution & Development. 18 (4): 216–228. doi:10.1111/ede.12192. ISSN 1525-142X. PMID 27402568.
  19. Sbita, Sarah J.; Morgan, Randy C.; Buschbeck, Elke K. (December 2007). "Eye and optic lobe metamorphosis in the sunburst diving beetle, Thermonectus marmoratus (Coleoptera: Dytiscidae)". Arthropod Structure & Development. 36 (4): 449–462. doi:10.1016/j.asd.2007.08.003. ISSN 1467-8039. PMID 18089121.
  20. Mandapaka, Karunyakanth; Morgan, Randy C.; Buschbeck, Elke K. (10 July 2006). "Twenty-eight retinas but only twelve eyes: an anatomical analysis of the larval visual system of the diving beetle Thermonectus marmoratus (Coleoptera: Dytiscidae)". The Journal of Comparative Neurology. 497 (2): 166–181. doi:10.1002/cne.20974. ISSN 0021-9967. PMID 16705677.
  21. Stecher, Nadine; Stowasser, Annette; Stahl, Aaron; Buschbeck, Elke K. (2016). "Embryonic development of the larval eyes of the Sunburst Diving Beetle, Thermonectus marmoratus (Insecta: Dytiscidae): a morphological study". Evolution & Development. 18 (4): 216–228. doi:10.1111/ede.12192. ISSN 1525-142X. PMID 27402568.
  22. Buschbeck, Elke K.; Sbita, Sarah J.; Morgan, Randy C. (September 2007). "Scanning behavior by larvae of the predacious diving beetle, Thermonectus marmoratus (Coleoptera: Dytiscidae) enlarges visual field prior to prey capture". Journal of Comparative Physiology A. 193 (9): 973–982. doi:10.1007/s00359-007-0250-x. ISSN 0340-7594. PMID 17639412.
  23. Sbita, S. J.; Morgan, R. C.; Buschbeck, E. K. (December 2007). "ScienceDirect". Arthropod Structure & Development. 36 (4): 449–462. doi:10.1016/j.asd.2007.08.003. PMID 18089121.
  24. Mandapaka, Karunyakanth; Morgan, Randy C.; Buschbeck, Elke K. (2006). "Twenty-eight retinas but only twelve eyes: An anatomical analysis of the larval visual system of the diving beetle Thermonectus marmoratus (Coleoptera: Dytiscidae)". Journal of Comparative Neurology. 497 (2): 166–181. doi:10.1002/cne.20974. ISSN 1096-9861. PMID 16705677.
  25. Stahl, Aaron L; Baucom, Regina S; Cook, Tiffany A; Buschbeck, Elke K (November 2017). "A Complex Lens for a Complex Eye". Integrative and Comparative Biology. 57 (5): 1071–1081. doi:10.1093/icb/icx116. ISSN 1540-7063. PMC 5886344. PMID 28992245.
  26. Stowasser, Annette; Rapaport, Alexandra; Layne, John E.; Morgan, Randy C.; Buschbeck, Elke K. (24 August 2010). "Biological bifocal lenses with image separation". Current Biology. 20 (16): 1482–1486. doi:10.1016/j.cub.2010.07.012. ISSN 1879-0445. PMID 20691594.
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