Tenebrio molitor is a species of darkling beetle (Phylum Arthropoda, Class Insecta) of the Family Tenebrionidae (Myers et al. 2017). The genus name Tenebrio (latin for "one who shuns light") is likely derived from the beetle's nocturnal behaviour (Jacobs & Calvin 1988).
Description:
Like all insects, adult T. molitor have a three-part body (head, thorax and abdomen) that is covered by a dark chitinuous exoskeleton that provides support and protection. Adult beetles have a body length of approximately 12-16 mm (Hill 2003). They have three pairs of jointed legs for locomotion. Adults have a pair of compound eyes that can detect light (Yinon 1970), and likely distinguish differences in colour (Brisco & Chittka 2001). The beetles have a pair of antennae, which act as tactile organs and organs of smell (Triplehorn & Johnson 2005). T. molitor also have sense organs located in their body wall that can detect chemical, mechanical and potentially auditory stimuli (Triplehorn & Johnson 2005).
Ecology:
T. molitor is native to the Mediterranean region, but has been introduced to most temperate regions (Bosquet 1990), including North America. In the natural environment, they may occur under stones and logs, beneath loose bark, and within rotten wood and fungi (Bosquet 1990; Triplehorn & Johnson 2005) or other decaying vegetation, but they are primarily found in granaries, grain elevators, mills, bakeries and food stores (Bosquet 1990). Adult beetles are scavengers that feed on decaying vegetation, fungi and animal material, but they prefer moist and decaying grain products (Bosquet 1990). Predators of T. molitor include vertebrates (e.g. birds, rodents, lizards) (Skelhorn & Rowe 2006) and invertebrates (e.g. spiders) (Strohmeyer et al. 1998).
Life History:
The life cycle of T. molitor has four stages: egg, larva, pupa, and adult beetle. Adult beetles emerge in late spring or summer. Females lay eggs in the soil or other substrata (Jacobs & Calvin 1988) and eggs hatch into larvae within 1-4 weeks (Jacobs & Calvin 1988).
The larvae, also called mealworms, are pale white or cream coloured and are worm-like. When fully grown, larvae can reach body lengths of ~ 28 mm (Hill 2003). They lack compound eyes but instead have a pair of simple eyes, called ocelli, which can detect changes in light intensity (Triplehorn & Johnson 2005). Larval T. molitor feed throughout the summer and fall but will enter hibernation in early winter (Jacobs & Calvin 1988). In late spring, larvae transform into pupae (Jacobs & Calvin 1988) and within 1-3 weeks pupae will develop into adults (Jacobs & Calvin 1988). The adults live for approximately 2-3 months (Hill 2003).
References Cited:
Bousquet Y. 1990. Beetles associated with stored products in Canada: an identification guide.
Agriculture and Agri-Food Canada. Publication 1837. [accessed 2017 Jan 3]. http://esc-sec.ca/wp/wp-content/uploads/2017/03/AAFC_bousquet1990.pdf.
Brisco AD, Chittka L. 2001. The evolution of colour vision in insects. Annual Review of Entomology. 46: 471-510.
Hill DS. 2003. Pests of stored foodstuffs and their control. Dordrecht: Kluwer Academic Publishers.
Jacobs S, Calvin D. 1988. Dark and yellow mealworms. Entomological Notes. PennState College of Agricultural Sciences. Department of Entomology (online). [accessed 2017 Jan 3]. http://ento.psu.edu/extension/factsheets/pdf/dkyellowmealworm.pdf/view.
Myers PR, Espinosa CS, Parr TJ, Hammond GS, Dewey TA. 2017. Tenebrio molitor. Animal Diversity Web (online). [accessed 2017 Jan 3]. http://animaldiversity.org/accounts/Tenebrio_molitor/classification/.
Skelhorn J, Rowe C. 2006. Predator avoidance learning of prey with secreted or stored defences and the evolution of insect defences. Animal Behaviour. 72: 827-834.
Strohmeyer HH, Stamp NE, Jarzomski CM, Bowers MD. 1998. Prey species and prey diet affect growth of invertebrate predators. Ecological Entomology. 23: 68-79.
Triplehorn CA, Johnson NF. 2005. Borror and DeLong’s introduction to the study of insects. 7th ed. Toronto: Thomson Brooks/Cole.
Yinon U. 1970. The visual mechanism of Tenebrio molitor: some aspects of the spectral response. Journal of Experimental Biology. 53:221-229.
To Learn More:
Allen JL, Clusella-Trullas S, Chown SL. 2012. The effects of acclimation and rates of temperature change on critical thermal limits in Tenebrio molitor (Tenebrionidae) and Cyrtobagous salviniae (Curculionidae). Journal of Insect Physiology. 58:669-678.
Anderson RL, Mutchmor JA. 1971. Temperature acclimation in Tribolium and Musea at locomotory, metabolic, and enzyme levels. Journal of Insect Physiology. 17(11):2205-2219.
Balfour CE, Carmichael L. 1928. The light reactions of the meal worm {Tenebrio molitor Linn.). The American Journal of Psychology. 40:576-584.
Bartholomew A, El Moghrabi J. 2018. Seasonal preference of darkling beetles (Tenebrionidae) for shrub vegetation due to high temperatures, not predation or food availability. Journal of Arid Environments. 156:34-40.
Bjørge JD, Overgaard J, Malte H, Gianotten N, Heckmann LH. 2018. Role of temperature on growth and metabolic rate in the Tenebrionid beetles Alphitobius diaperinus and Tenebrio molitor. Journal of Insect Physiology. 107:89-96.
Cloudsley-Thompson JL. 1953. Studies in diurnal rhythms. IV. Photoperiodism and geotaxis in Tenebrio molitor L. (Coleoptera: Tenebrionidae). Physiological Entomology. 28:117–132.
Drickamer LC. 1971. The humidity responses of Tribolium confusum Jacquelin Duval in wheat flour, sand and air. The Ohio Journal of Science. 71(3):149-158.
Howard RS. 1955. The biology of the grain beetle Tenebrio molitor with particular reference to its behavior. Ecology. 36:262-269.
Kenagy GJ, Stevenson RD. 1982. Role of body temperature in the seasonality of daily activity in Tenebrionid beetles of eastern Washington. Ecology. 63(5):1491-1503.
McIntyre NE. 1997. Scale-dependent habitat selection by the darkling beetle Eleodes hispilabris (Coleoptera: Tenebrionidae). The American Midland Naturalist. 138:230-235.
Pielou DP, Gunn DL. 1940. The humidity behavior of the mealworm beetle, Tenebrio molitor. Journal of Experimental Biology. 17: 286-294.
Punzo F, Huff G. 1989. Comparative temperature and water relations and the effects of thermal acclimation on Tenebrio molitor and Tenebrio obscurus (Coleoptera: Tenebrionidae). Comparative Biochemistry and Physiology Part A: Physiology. 93:527- 533.
Punzo F, Mutchmore J. 1980. Effects of temperature, relative humidity and period of exposure on the survival capacity of Tenebrio molitor (Coleoptera: Tenebrionidae). Journal of the Kansas Entomological Society. 53: 260-270.
Agriculture and Agri-Food Canada. Publication 1837. [accessed 2017 Jan 3]. http://esc-sec.ca/wp/wp-content/uploads/2017/03/AAFC_bousquet1990.pdf.
Brisco AD, Chittka L. 2001. The evolution of colour vision in insects. Annual Review of Entomology. 46: 471-510.
Hill DS. 2003. Pests of stored foodstuffs and their control. Dordrecht: Kluwer Academic Publishers.
Jacobs S, Calvin D. 1988. Dark and yellow mealworms. Entomological Notes. PennState College of Agricultural Sciences. Department of Entomology (online). [accessed 2017 Jan 3]. http://ento.psu.edu/extension/factsheets/pdf/dkyellowmealworm.pdf/view.
Myers PR, Espinosa CS, Parr TJ, Hammond GS, Dewey TA. 2017. Tenebrio molitor. Animal Diversity Web (online). [accessed 2017 Jan 3]. http://animaldiversity.org/accounts/Tenebrio_molitor/classification/.
Skelhorn J, Rowe C. 2006. Predator avoidance learning of prey with secreted or stored defences and the evolution of insect defences. Animal Behaviour. 72: 827-834.
Strohmeyer HH, Stamp NE, Jarzomski CM, Bowers MD. 1998. Prey species and prey diet affect growth of invertebrate predators. Ecological Entomology. 23: 68-79.
Triplehorn CA, Johnson NF. 2005. Borror and DeLong’s introduction to the study of insects. 7th ed. Toronto: Thomson Brooks/Cole.
Yinon U. 1970. The visual mechanism of Tenebrio molitor: some aspects of the spectral response. Journal of Experimental Biology. 53:221-229.
To Learn More:
Allen JL, Clusella-Trullas S, Chown SL. 2012. The effects of acclimation and rates of temperature change on critical thermal limits in Tenebrio molitor (Tenebrionidae) and Cyrtobagous salviniae (Curculionidae). Journal of Insect Physiology. 58:669-678.
Anderson RL, Mutchmor JA. 1971. Temperature acclimation in Tribolium and Musea at locomotory, metabolic, and enzyme levels. Journal of Insect Physiology. 17(11):2205-2219.
Balfour CE, Carmichael L. 1928. The light reactions of the meal worm {Tenebrio molitor Linn.). The American Journal of Psychology. 40:576-584.
Bartholomew A, El Moghrabi J. 2018. Seasonal preference of darkling beetles (Tenebrionidae) for shrub vegetation due to high temperatures, not predation or food availability. Journal of Arid Environments. 156:34-40.
Bjørge JD, Overgaard J, Malte H, Gianotten N, Heckmann LH. 2018. Role of temperature on growth and metabolic rate in the Tenebrionid beetles Alphitobius diaperinus and Tenebrio molitor. Journal of Insect Physiology. 107:89-96.
Cloudsley-Thompson JL. 1953. Studies in diurnal rhythms. IV. Photoperiodism and geotaxis in Tenebrio molitor L. (Coleoptera: Tenebrionidae). Physiological Entomology. 28:117–132.
Drickamer LC. 1971. The humidity responses of Tribolium confusum Jacquelin Duval in wheat flour, sand and air. The Ohio Journal of Science. 71(3):149-158.
Howard RS. 1955. The biology of the grain beetle Tenebrio molitor with particular reference to its behavior. Ecology. 36:262-269.
Kenagy GJ, Stevenson RD. 1982. Role of body temperature in the seasonality of daily activity in Tenebrionid beetles of eastern Washington. Ecology. 63(5):1491-1503.
McIntyre NE. 1997. Scale-dependent habitat selection by the darkling beetle Eleodes hispilabris (Coleoptera: Tenebrionidae). The American Midland Naturalist. 138:230-235.
Pielou DP, Gunn DL. 1940. The humidity behavior of the mealworm beetle, Tenebrio molitor. Journal of Experimental Biology. 17: 286-294.
Punzo F, Huff G. 1989. Comparative temperature and water relations and the effects of thermal acclimation on Tenebrio molitor and Tenebrio obscurus (Coleoptera: Tenebrionidae). Comparative Biochemistry and Physiology Part A: Physiology. 93:527- 533.
Punzo F, Mutchmore J. 1980. Effects of temperature, relative humidity and period of exposure on the survival capacity of Tenebrio molitor (Coleoptera: Tenebrionidae). Journal of the Kansas Entomological Society. 53: 260-270.