Esperk et al. (2007) reviewed the literature showing that a variable number of instars is taxonomically widespread among insects. They cite 173 studies in which extra instars have been reported for 145 insect species in 12 orders and 49 families. They cited five studies for stored-product insects in which the average number of instars was significantly higher as a result of adverse conditions (poor diet or extreme humidities or temperatures). For Tenebrio molitor, extra instars can be a result of drinking water or high temperature. In the Pittendridge et al. (1997) study, all of the larvae with an extra instars were in the low humidity treatment. In additional studies for 13 species of stored-product insects cited in Esperk et al. (2007) larvae had a variable number of instars. For nine species of dermestids, sex or exposure to extreme relative humidities seemed to result in extra instars. An additional 8 stored-product insect species can be added to their list for which extra instars were a result of adverse conditions. Knowing that a species may have extra instars is important because individuals with extra instars tended to have longer developmental times.
Esperk, T., T. Tammaru and S. Nylin. 2007. Intraspecific variability in number of larval instars in insects. J. Econ. Entomol. 100: 627-645.
Extra Instars found to be Results of Adverse Conditions
Azab, A. K. 1953a. The effect of various types of food upon the rate of development of the larvae of Stegobium paniceum L. Bull. Soc. Fouad 1er Entomol. 37: 127-147. (Stegobium paniceum had extra instars as a result of poor diet, range 3-5 instars)
Beckett, S. J., and D. E. Evans. 1994. The demography of Oryzaephilus surinamensis (L.) (Coleoptera: Silvanidae) on kibbled wheat. J. Stored Prod. Res. 30: 121-137. (Oryzaephilus surinamensis had extra instars as a result of suboptimal high (35oC) or low (20oC) temperature, range 3-5 instars)
Fleming, D. A., and T. A. Jacob. 1986. The influence of temperature and relative humidity upon the number and duration of larval instars in Dermestes lardarius L. (Col., Dermestidae). Entomol. Mag. 122: 43-50. (Dermestes lardarius had extra instars as a result of suboptimal high (28oC) or low (17.5oC) temperature, range 5-10 instars)
Gautam, S. G.; Opit, G. P.; Shakya, K. 2016. Population Growth and Development of the Psocid Liposcelis fusciceps (Psocoptera: Liposcelididae) at Constant Temperatures and Relative Humidities. Environ. Entomol. 45(1): 237-244. (males have two to four nymphal instars (28% two, 70% three, and 2% four instars, whereas females have two to five instars (2% two, 33% three, 63% four, and 2% five)
Hafez, M., and A. F. M. Wakid 1967. Biological studies on Oryzaephilus surinamensis L., in Egypt (Coleoptera: Cucujidae). Soc. Entomol. Egypte Bull. 51: 273-288. (95% had 3 or 4 instars and 5% had 5 or 6 instars. Poor diet (sieved polished rice, dried fruit or >70% yeast), high temperatures, low relative humidity at high temperatures and high humidity at low temperatures) increased the number of instars.
Ludwig, D. 1956. Effects of temperature and parental age on the life cycle of the mealworm, Tenebrio molitor Linnaeus (Coleoptera: Tenebrionidae). Ann. Entomol. Soc. Am. 49: 12-15. (Tenebrio molitor had extra instars as a result of higher temperature, range 12-16 instars at 25oC and 16-24 instars at 30oC)
Miles, M. 1933. Observations on growth in larvae of Plodia interpunctella Hubn. Ann. Appl. Biol. 20: 297-207. (Plodia interpunctella had extra instars as a result of poor diet, range 3-8 instars)
Mills, Robert B. 1965. Early Germ Feeding and Larval Development of the Angoumois Grain Moth. Journal of Economic Entomology 58(2): 220-223. (extra instars (4-7) as results of poor diet)
Mukerji, Durgadas and Sinha, Ranendra Nath. 1953. Effect of Food on the Life History of the Flour Beetle, Tribolium castaneum Herbst. Journal of the Kansas Entomological Society 26(3): 118-124. (Tribolium castaneum had extra instars as a result of poor arrow root diet, range 6-9 instars)
Niiho, C. 1984. Ecological study of the tobacco beetle, Lasioderma serricorne (F.) 2. Growth of tobacco beetles fed on bread crumbs. Jpn. J. Appl. Entomol. Zool. 28: 209-216. (Lasioderma serricorne had extra instars as a result of lower temperature, range 5-6 instars)
Pittendrigh, B. R., J. E. Huesing, R. E. Shade, and L. L. Murdock. 1997. Monitoring of rice weevil, Sitophilus oryzae, feeding behavior in maize seeds and the occurrence of supernumerary moults in low humidity conditions. Entomol. Exp. Appl. 83: 225-231. (Sitophilus oryzae had extra instars as a result of low relative humidity, range 4-5 instar, all 15 larva with 5 instars were in the low relative humidity treatment)
Russell, V. M., G.G.M. Schulten, and F. A. Roorda. 1980. Laboratory observations on the development of the rice moth Corcyra cephalonica (Stainton) (Lepidoptera: Galleriinae) on millet sorghum at 28_Cand different relative humidities. Z. Ang. Entomol. 89: 488-498. (Corcyra cephalonica had extra instars as a result of low relative humidity, females had one more instar than males, range 6-18 instars)
Schwardt, H. H. 1927. The effect of different foods on the larval periods of three grain insects. MS Thesis, Kansas State University Department of Entomology (Tribolium confusum had extra instars as a result of poor diet, range 5-14 instars)
Takahashi, F. 1961. On the effect of population density on the power of the reproduction of the almond moth, Ephestia cautella, VII. The effect of larval density on the number of larval molts and the duration of each larval instar. Applied Entomology and Zoology Nihon Oyo Dobutsu Konchu Gakkai 5: 185-190 (in Japanese with English summary). (Cadra cautella had extra instars as a result of poor diet in crowded cultures poor diet, range 4-7 instars)
Titschack, E. 1926. Untersuchungen über das Wachstum, den Nahrungsverbrauch und die Eierzeugung. II. Tineola biselliella Hum. Gleichzeitig ein Beitrag zur Klärung der Insektenha¨utung. Z. Wiss. Zool. 128: 509-569. (Tineola biselliella had extra instars as a result of poor diet, range 5-16 instars)
Urs, K.C.D., and T. L. Hopkins. 1973. Effect of moisture on growth rate and development of two strains of Tenebrio molitor L. (Coleoptera, Tenebrionidae). J. Stored Prod. Res. 8: 291-297. (Tenebrio molitor had extra instars as a result of drinking water, range 13-18 instars one strain and 11-16 instars another strain)
Variation in Number of Instars
Allegret, P. 1964. Interrelationship of larval development, metamorphosis and age in a pyralid lepidopteran, Galleria mellonella (L.), under the influence of dietetic factors. Exp. Gerontol. 1: 49-66.
Ali, M. F. 1993. Biological studies on immature and adult stages of Attagenus fasciatus (Thunberg) (Coleoptera: Dermestidae). J. Stored Prod. Res. 29: 237-242. (range 8-14 instar, one individual 15 instar)
Ali, M. F. 1997. Development and heat tolerance of Anthrenus coloratus (Reitter) (Coleoptera: Dermestidae) reared on a laboratory diet. J. Stored Prod. Res. 33: 251-255. (range 7-9 instar)
Archer, T. L., and R. G. Strong. 1975. Comparative studies on the biologies of Trogoderma: T. glabrum. Ann. Entomol. Soc. Am. 68: 105-114. (range 5-9 instar)
Armes, N. J. 1990. The biology of Anthrenus sarnicus Mroczkowski (Coleoptera: Dermestidae): I. Egg and larval development. J. Stored Prod. Res. 26: 11-22. (range 5-17 instar)
Baker, J. E. 1977. Growth and development of the black carpet beetle on the laboratory diet. Ann. Entomol. Soc.Am. 70: 296-298. (range 5-12 instar)
Beck, S. D. 1971. Growth and retrogression in larvae of Trogoderma glabrum (Coleoptera: Dermestidae). 1. Characteristics under feeding and starvation conditions. Ann. Entomol. Soc. Am. 64: 149-155. (Trogoderma glabrum, without food, after the 6th instar had 5-8 retrogressive molts over a year; Attagenus unicolor, Trogoderma inclusum and Trogoderma variable also can have retrogressive molts, but Dermestes maculatus does not)
Collins, P. J., J. C. Mulder, and D. Wilson. 1989. Variation in life history parameters of Oryzaephilus surinamensis (L.) (Coleoptera: Silvanidae). J. Stored Prod. Res. 25: 193-199. (field strain had 4 instars and laboratory strain had 5 instars)
Cotton, R. T. 1927. Notes on the biology of the meal worms, Tenebrio molitor Linne and T. obscurus Fab. Ann. Entomol. Soc. Am. 20: 81-86. (Tenebrio molitor had 9-20 instars, half of larvae observed had 17-19 instars; Tenebrio obscurus had 12-22 instars, half of larvae had 14-15)
Cymborowski, B., and M. I. Bogus. 1976. Juvenilizing effect of cooling on Galleria mellonella. J. Insect Physiol. 22: 669-672.
Iwata, R., and K. Nishimoto. 1985. Studies on the autecology of Lyctus brunneus (Stephens) (Coleoptera, Lyctidae). VI. Larval development and instars with special reference to an individual rearing method. Wood Res. 71: 32-45. (range 4-6 instar, one individual 7 instar)
Jones, C. R. 1913. The cigarette beetle in the Philippine Islands. Philippine J. Science Section D 8: 1-61. (range 5-6 instar and one larva had 7 instar)
Ludwig, D., and C. Fiore. 1960. Further studies on the relationship between parental age and the life cycle of the mealworm Tenebrio molitor. Ann. Entomol. Soc. Am. 53: 595-600. (parental age little or no influence on number of instars)
Partida, G. J., and R. G. Strong. 1975. Comparative studies on the biologies of six species of Trogoderma: T. variabile. Ann. Entomol. Soc. Am. 68: 115-125. (range 5-9 instar, one individual 10 instar)
Pipa, R. L. 1976. Supernumerary instars produced by chilled wax moth larvae: endocrine mechanisms. J. Insect Physiol. 22: 1641-1647.
Strong, R. G. 1975. Comparative studies on the biologies of six species of Trogoderma: T. inclusum. Ann. Entomol. Soc. Am. 68: 91-104. (range 5-10 instar)
Strong, R. G., and D. W. Mead. 1975. Comparative studies on the biologies of six species of Trogoderma: T. simplex. Ann. Entomol. Soc. Am. 68: 565-573. (range 6-10 instar)
Watson, T. F., and P. H. Johnson. 1974. Larval stages of the pink bollworm, Pectinophora gossypiella. Ann. Entomol. Soc. Am. 67: 812-814. (range 4-5 instar)
Xiong, X.-Z. 1992. Laboratory rearing method and life history of Falsogastrallus sauteri Pic (Coleoptera: Anobiidae). Coleopt. Bull. 46: 321-324. (range 4-12 instar)