Sunburst Diving Beetle (Thermonectus marmoratus) Fact Sheet: Summary

Kingdom: Animalia

Phylum: Arthropoda — arthropods

Class: Insecta — insects

Order: Coleoptera — beetles

Family: Dytiscidae — predaceous diving beetles

Genus: Thermonectus

Species: Thermonectus marmoratus — sunburst diving beetle, yellow-spotted diving beetle

Sources: Evans and Hogue (2006), ITIS (2020), Nilsson and Hájek (2020)

Synonyms

Colymbetes marmoratus, Hydaticus flavomaculatus, Acilius maculatus (Nilsson and Hájek 2020)

Length

10.0 to 15.0 mm (0.4 to 0.6 in) (Evans and Hogue 2006)

General Appearance

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 Species

Similar coloration in Thermonectus zimmermani (Goodhue-McWilliams 1981; Larson 1996).

Senses

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).

Range

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).

Habitat

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).

IUCN Status

Not listed (IUCN 2020).

CITES Appendix

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).

Activity Cycle

Active day and night but appear to be more nocturnal (Morgan 1995; Evans 2014).

Social Behavior

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).

Communication

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).

Predators

Dytiscids preyed on by odonate nymphs, fish, amphibians, reptiles, birds, and mammals (Culler et al. 2014; Yee 2014)

Defense

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).

• Predators without color vision: color pattern of adult marmoratus likely serves as camouflage (to blend in with gravel and rocky stream beds)
• Predators with color vision: conspicuous bright yellow-and-black pattern almost certainly acts as warning coloration (advertising bad taste)

Locomotion

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).

Movements

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).

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).

Mating

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).

Life Cycle

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).

Feature Facts

• Adults breathe underwater by carrying air bubble beneath wing covers; periodically return to surface to renew air supply (Morgan 1995; Roughley and Larson 2000)
• Contrasting coloration provides camouflage with stream bottom (Larson 1996; Evans and Hogue 2006)
• Discharge an unpleasant milky fluid (from glands behind head) as defense (Whitman et al. 1990; Meinwald et al. 1998; Yang and Meinwald 1998)
• Considerable scientific study of larval visual system (see Senses) (Miller and Bergstem 2016)
• Popular species in insect zoos (Evans and Hogue 2006; Miller and Bergstem 2016)