Sunburst Diving Beetle (Thermonectus marmoratus)
Image credit: © San Diego Zoo Wildlife Alliance. All rights reserved.
Phylum: Arthropoda — arthropods
Class: Insecta — insects
Order: Coleoptera — beetles
Family: Dytiscidae — predaceous diving beetles
Species: Thermonectus marmoratus — sunburst diving beetle, yellow-spotted diving beetle
Sources: Evans and Hogue (2006), ITIS (2020), Nilsson and Hájek (2020)
Colymbetes marmoratus, Hydaticus flavomaculatus, Acilius maculatus (Nilsson and Hájek 2020)
10.0 to 15.0 mm (0.4 to 0.6 in) (Evans and Hogue 2006)
Adults: Body shape oval and flattened; streamlined for swimming. Powerful hind legs; paddle-like appendages for swimming. Upper body shiny black with 10 to 11 bright yellow spots on each wing cover (e.g., Morgan 1995; Evans and Hogue 2006). Underside bright orange to reddish orange. Yellow head with variable M-shaped marking. Individuals from southern California and Baja California generally larger, more elongate, and have darker coloration, with more smaller yellow spots, than other populations (Evans 2014).
Larvae: Soft, elongated body, with well-developed legs and antennae (campodeiform). Small flattened head with sickle-shaped pincers (Morgan 1995; Roughley and Larson 2000).
Similar coloration in Thermonectus zimmermani (Goodhue-McWilliams 1981; Larson 1996).
Adults beetles have typical compound eyes of insects; however, larval eyes are especially noteworthy. Larvae have among the most structurally complex and specialized eyes of any insect or animal (Mandapaka et al. 2006; Stowasser and Buschbeck 2014). They have twelve simple-lens eyes (stemmata) with more than 25 retinas (Mandapaka et al. 2006). Two pairs of tubular eyes with bifocal lens (Stowasser et al. 2010). “Camera-like vision,” with high-resolution imaging (Stowasser and Buschbeck 2012), and green wavelength, UV, and polarization sensitivity (e.g., Maksimovic et al. 2009; Stecher et al. 2010; Maksimovic et al. 2011; Stowasser and Buschbeck 2012). Able to visualize prey and judge distance to strike very accurately (Buschbeck et al. 2007; Stowasser et al. 2010; Bland et al. 2014; Stowasser and Buschbeck 2014; Werner and Buschbeck 2015; Stahl et al. 2017). Larval eyes reabsorbed and replaced with adult compound eyes during metamorphosis (see Life Cycle) (Salvatore and Morgan 1992, as cited by Morgan 1995; Sbita et al. 2007).
Antennae and jaws with chemoreceptors and mechanoreceptors (Morgan 1995).
Distribution & Status
Behavior & Ecology
Southwestern California and southern Utah to western Texas, Mexico, and northern Central America (Anderson 1962; Morgan 1992; Baron et al. 1998; Challet 2003; Shaverdo 2005; Evans 2014). In California, primarily found in canyon streams in Peninsular Ranges (Evans and Hogue 2006). Elevation range: 750 to 1,800 m (2,500 to 6,000 ft) (Morgan 1992; Evans 2014).
Macrohabitat: Montane pine or pine-oak forests (above 750 m, or 2,500 ft). Spring-fed pools within intermittent desert, chaparral, and upland streams (Evans 2014).
Microhabitat: Clear, shallow, slow-moving streams or stream pools at the base of mountains, with open sand or gravel bottoms (Evans 2014). Found near edges of ponds/lakes or in open water (Roughley and Larson 2000).
Not listed (IUCN 2020).
Not listed (UNEP 2020).
Populations in the Wild
No range-wide estimates. Thermonectus spp. may be common in pools, creeks, and streams within their range (Miller and Bergstem 2016).
Threats to Survival
Long-term threats: groundwater extraction, pollution, overuse of recreation areas, invasive species (Bogan et al. 2014; Yee 2014), climate change (Yee 2014; Boersma et al. 2016).
Active day and night but appear to be more nocturnal (Morgan 1995; Evans 2014).
Generally, solitary predators (Kelly Miller, personal communication, 2021). Sometimes found in loose groups or aggregations; several to “dozens” of individuals observed in stream pools (Morgan 1992, 1995).
Not well studied. Likely communicate with one another through combination of visual and chemical cues (Kelly Miller, personal communication, 2021) and possibly water-borne vibratory signals (Randy Morgan, personal communication, 2021).
Diet and Feeding
Dytiscids generally feed on aquatic invertebrates (especially mosquito larvae, small crustaceans, zooplankton), tadpoles, and fish (Culler et al. 2014; Yee and Kehl 2015).
T. marmoratus is a generalist predator–scavenger. Feeds on a variety of live or freshly dead invertebrates (Morgan 1995; Velasco and Millan 1998; Bogan 2012). Seems to prefer mosquito larvae (Morgan 1995). Larval cannibalism likely (Yee 2014).
T. marmoratus larvae have specialized distance vision for hunting. Stalk prey, then strike with great force (Morgan 1995; Buschbeck et al. 2007; Stowasser and Buschbeck 2014). Hollow mandibles for injecting digestive enzymes to liquefy prey (Yee and Kehl 2015).
Dytiscids preyed on by odonate nymphs, fish, amphibians, reptiles, birds, and mammals (Culler et al. 2014; Yee 2014)
Behaviors include evasive swimming/flight, biting, and using defensive chemicals (Whitman et al 1990, as cited by Morgan 1995; Morgan 1995).
Adult coloration probably multimodal (Randy Morgan, personal communication, 2021).
Strong swimmers (all age classes). Adults use paddle-like hind legs in unison to swim and dive. Front and middle legs retracted into underside groove while swimming. Hairs on swimming legs fold flat to improve hydrodynamic ability. Adults fly using hind wings. Specialized swimming legs impedes movement on land, especially smooth surfaces (Morgan 1995; Roughley and Larson 2000; Evans and Hogue 2006). Breathe through spiracles, like land insects. Adults breathe at water’s surface or from air bubble reservoir held under wing covers while underwater; also use physical gill.
Agile arrow-shaped larvae able to dart forward short distances during prey capture. Breathe at the surface through specialized spiracles at tip of abdomen rather than carrying air reservoir, as in adults (Morgan 1995).
Only leave water to lay eggs or fly (Morgan 1995). Adults disperse to locate new bodies of water, leave ponds that have dried up, or locate overwintering sites (Evans and Hogue 2006). Long-range flight seems infrequent (Morgan 1995). Ability to find suitable water sources not yet studied; polarized light reflecting off water surfaces may play a role (Yee 2014).
Reproduction & Development
Breeding and Courtship
Breed year-round, with a peak during summer (Morgan 1995). See Kelly (2001) for detailed description of female reproductive system. Courtship not well described. Mates may be located through searching, sounds produced by male to attract females, or use of pheromones (Yee 2014).
In Thermonectus spp., male grasps female while female swims erratically to escape (Miller 2003; Yee 2014). Male T. marmoratus mounts female by grasping her wing covers with specialized adhesive disks on lower front legs (Morgan 1995; Bergsten and Miller 2007; Evans 2014). Male may perform additional behaviors, such as rocking or rubbing, so female will permit intromission (Yee 2014). Copulation may last a few second up to a minute (Morgan 1995).
Egg Laying and Hatching
Female leaves water at night to lay eggs in moist rocky crevices, under bark or in shore debris. Eggs pearly white and oblong in shape (about 1 x 3mm, about 0.04 x 0.1 in). Deposited in clutches of 20 eggs on average (range: 5 to 30). Hatch after about 6 days, on average (range: 4 to 8 days) (Morgan 1995). Influenced by water temperature (Morgan 1992, 1995; Beutel and Leschen 2016).
4 stages: egg, larva, pupa, adult (Roughley and Larson 2000). In warm conditions with ample food, development from egg to adult can occur in as little as 28 days. Longer under less optimal conditions (Morgan 1995).
Peak larval period in spring (Gray 1981). Three larval instars before pupation. Pupate just above water line in a cell made of soil and plant material (Roughley and Larson 2000; Yee and Kehl 2015). Pupae plump and cream colored, with dark eyespots, and embryonic wings, legs, and other adult features. Immobile; drown if pupal cell floods. Adults wait to leave pupal cells and enter water until exoskeleton fully hardened. Feed almost immediately and begin mating after a few weeks (Morgan 1992, 1995).
Typical Life Expectancy
Wild populations: Not reported. “Long-lived” (Evans and Hogue 2006).
Managed care: No published reports. Possibly 1 to 2 years or longer (Kelly Miller, personal communication, 2021).
For bibliography, click the tab at the top of this page.
© 2021 San Diego Zoo Wildlife Alliance
How to cite: Sunburst Diving Beetle (Thermonectus marmoratus) Fact Sheet. c2021. San Diego (CA): San Diego Zoo Wildlife Alliance; [accessed YYYY Mmm dd]. http://ielc.libguides.com/sdzg/factsheets/sunburst-diving-beetle.
(note: replace YYYY Mmm dd with date accessed, e.g., 2019 Dec 31)
Disclaimer: Although San Diego Zoo Wildlife Alliance makes every attempt to provide accurate information, some of the facts provided may become outdated or replaced by new research findings. Questions and comments may be addressed to firstname.lastname@example.org.
Many thanks to Randy C. Morgan for providing expert content review of this fact sheet.
Randy Morgan is a leading expert on the sunburst diving beetle, being one of the few people to have studied this species in the wild and in managed care. He has authored/co-authored several seminal papers on sunburst diving beetle biology and husbandry, including studies of T. marmoratus eye structures, larval feeding behavior, and defense physiology.
Randy is an internationally recognized insect husbandry expert, having pioneered new keeping practices for many challenging species, such as the giant tropical bullet ant (Paraponera clavata). He serves as Emeritus Curator of the Insectarium at the Cincinnati Zoo & Botanical Garden, where he worked for 32 years as an entomologist, and Curator of the Insectarium and herpetology departments.
Randy is also deeply involved in public science education and sustainable agriculture initiatives in the Peruvian rain forest. He holds a Master of Science degree in Entomology from the University of Wisconsin, as well as being a master beekeeper — 40 years and counting.
Thank you to Prof. Kelly B. Miller for contributing useful comments. Prof. Miller is Professor of Biology at the University of New Mexico and Curator of Arthropods and Assistant Director of the Museum of Southwestern Biology.
A sunburst diving beetle's eyes are well adapted for life above and below water.
This species' larvae have some of the most complex and specialized eye structures of any animal, making them capable predators (for their small size). See Senses to learn more.
Image credit: © San Diego Zoo Wildlife Alliance. All rights reserved.