Variation In Instar Number Research Articles

Variability in development of the striped rice borer, Chilo suppressalis (Lepidoptera: Pyralidae), due to instar number and last instar duration.

The striped stem borer, Chilo suppressalis (Walker), is an important insect pest of rice which shows substantial variation in developmental duration among individuals. This variation is currently poorly characterized but it is important from a control perspective because pesticides can only target early sensitive instars. It is unclear whether there are key stages that determine the length of developmental duration of individuals and/or whether variation in instar number contributes to this variation. In this study, a laboratory population and a population recently established from the field were used to test variation in development time across instar stages. The duration of developmental time of C. suppressalis started to diverge from the 5th instar onward. Individuals pupated at the 5th, 6th, 7th or even 8th instar stage. In both populations, both the instar at which the larva pupated and the duration of the last larval instar stage determined total developmental time of an individual. There was little impact of the developmental time of early instars on total developmental duration or on instar number prior to pupation. Sex influenced the number of instars but not development time within this number. The biological and applied significance of uneven development in C. suppressalis are discussed.

Developmental Patterns inStagmomantis limbata(Mantodea: Mantidae): Variation in Instar Number, Growth, and Body Size

Development affects many components of life history and fitness, including body size. The present study examined the influence of developmental pattern, specifically the number of nymphal instars, on body size (pronotum length) in the praying mantid Stagmomantis limbata Hahn. Mantids were reared in the laboratory from hatching, on standardized diet, to examine variation in instar number. These lab data were then used to assess developmental patterns forfield-collected female nymphs. Laboratoryreared males and females varied in number of instars. Most females required 6 nymphal instars to reach adulthood (64%), whereas 36% underwent 7 instars. Seven-instar females reached the 4th, 5 th, and 6th instars faster than six-instar females, but had shorter pronota than the six-instar females at each of these stages. Seven-instar females were longer than six-instar females at adulthood. Interestingly, the total developmental period from hatching to adulthood was similar for lab-reared seven-instar and six-instar females. In the lab, most males (91%) underwent 6 instars, with the remaining 9% following a five-instar pattern. By the 4th instar, differences between the sexes began to appear. From the 4th instar onwards, females typically took less time than males to reach each instar. From the 5th instar onwards, females were longer than the males, and were longer as adults. Variation in developmental pattern (number of instars) was evident among siblings from the same ootheca; such intra-clutch variability in number of instars may be a bet-hedging strategy by ovipositing females in a variable environment. The laboratory data allowed for the detection of six-instar and seven-instar patterns among the field-collected females. The field-collected data suggest that females undergoing 6 nymphal instars reach adulthood later in the season, and at smaller body size, than seven-instar females.

Developmental Plasticity in Tenebrio molitor (Coleoptera: Tenebrionidae): Analysis of Instar Variation in Number and Development Time under Different Diets

The variation in instar number and the pattern of sequential instar development time of Tenebrio molitor L. (Coleoptera: Tenebrionidae) was studied under 4 different diet regimes. Addition of dietary supplements consisting of dry potato or a mix of dry potato and dry egg whites significantly reduced the number of larval instars and total development time. The pattern of sequential stadia length showed regularity and low variability in instars 5 - 9. Stadia length continuously increased between instar 10 and the last instar before pupation (P−1). Instar number significantly impacted total development time and was also significantly correlated with stadia length. The length of the 1st, 2nd, 3rd, and 4th stadia was not significantly affected by diet indicating that they may represent a stable part of the life cycle of T. molitor. The length of the last stadium (P−1) was regular within food treatments regardless of sequential instar number, but P−1 was also significantly affected by nutrition in a similar way as all the sequential stadia. The pattern of instar variation is discussed in regard to the insertion of new instars based on variation of stadia length.

Developmental variation in the forest tent caterpillar: life history consequences of a threshold size for pupation

In insects, size and age at adult emergence depend on larval growth that occurs in discrete steps or instars. Understanding the mechanisms controlling stepwise larval growth and the onset of metamorphosis is essential to the study of insect life history. We examined the patterns of growth of forest tent caterpillars Malacosoma disstria to quantify variation in the number of instars that larvae undergo before pupation, to identify the mechanisms underlying variation in larval development, and to evaluate the life history consequences of this variation. All caterpillars were reared under the same conditions; at each molt, the date, the head capsule width and the mass of the freshly molted insect were recorded. Logistic regression analysis showed that a threshold size (measured either as mass or head capsule width) must be reached at the beginning of a stadium for pupation to occur at the next molt. This threshold size was higher for females than for males, and as a result, females attained a higher pupal mass than males. To achieve this larger size, females often required more instars than males, despite a higher growth ratio (size increase within an instar). Within each sex, slow growing individuals exhibited more larval instars and longer larval development time, but attained the same pupal mass as faster growing individuals. The combination of a threshold size for pupation, discrete growth steps and variation in the number of these steps can thus complicate relationships between growth rate, pupal mass and larval development time. In our study, growth ratio and number of instars were correlated with development time but not with pupal mass, and no relationship was observed between development time and pupal mass. These findings imply that, in species with variable instar number, one cannot extrapolate overall larval growth from growth during a single instar. Given the constraints of discrete larval growth, variation in instar number provides greater flexibility for insects to compensate for poor growing conditions. In this case, inferior larval growth conditions don't necessarily lead to smaller adult size.

Life Cycle and Growth of Senita Moths (Lepidoptera: Pyralidae): A Lepidopteran with Less Than Four Instars?

Abstract Despite great variation in instar number among Insecta, no Lepidopteran has been observed to have less than four larval instars. I report in this work on the life cycle and growth of the senita moth, Upiga virescens Hulst, which forms an obligate pollinating predispersal seed-eating mutualism with senita cacti (Lophocereus schottii Engelmann) in the Sonoran Desert of North America. From 1996 to 1999, I studied larval growth and life cycle associations of U. virescens with L. schottii in the field by labeling cohorts of eggs laid in flowers and following them through pupation. All life stages of U. virescens were associated with flowers, fruit, or cactus stems of L. schottii. Among the five cohorts studied, larval growth consistently conformed to Dyar’s rule. Only three larval instars were identified among the >500 larvae for which head capsule widths were measured. I discuss and dismiss the feasibility of a fourth undetected instar. I then discuss selection pressures that may have contributed to ...

The effect of temperature on maturation threshold body-length in Daphnia magna

Aquatic invertebrates are usually larger at maturity when water temperatures are lower. For the freshwater cladoceran Daphnia, it has been suggested that a threshold size must be attained to initiate maturation, which results two instars later in the deposition of eggs into the brood chamber. This threshold size is believed to temperature on maturation threshold body-length in Daphnia magna. Daphnids were raised from birth to maturity under three constant-temperature regimes (12°C, 16°C, 22°C), and two food-level conditions. Animals were measured daily, and a body-length based maturation threshold determined for each individual. We demonstrate that mean maturation threshold length is negatively correlated with ambient water temperature. Further, daphnids with a larger threshold length tended to be larger at maturity. A maturation threshold linked to body length suggests that reduced variation in size at maturity is adaptive, even at the cost of additional variation in instar number or age at maturity.

Developmental Polymorphism in the Red-legged Grasshopper, Melanoplus femurrubrum (DeGeer) (Orthoptera: Acrididae)

Instar number in Melanoplus femurrubrum (DeGeer) is variable. Virginia populations were found to be developmentally polymorphic, having from five to eight nymphal instars. Grasshoppers with five instars were rare. In the field, fewer instars were associated with years with low May–June temperatures. Variation in instar number among populations, between years, and within seasons is due to variation in early season temperatures. A laboratory population had six to eight instars, with fewer instars occurring at lower rearing temperatures. Females went through more instars than males. Antennal segment number was correlated with instar number. Virginia populations of M. femurrubrum are developmentally adapted to a relatively long growing season.

Variation in Number of Instars, Longevity, and Fecundity of Cyrtorhinus lividipennis Reuter (Hemiptera: Miridae)

The biology of the predaceous mirid, Cyrtorhinus lividipennis Reuter, was studied with emphasis on the number of instars and the relationship of the observed variation in number of instars to adult longevity and fecundity. Nymphs underwent three, four, or five molts before becoming adults. Thus, C. lividipennis may be classified into three-instar, four-instar, and five-instar groups. Females of the four-instar group lived longer and were more fecund than those of the five-instar group. There was no statistical difference in the longevity of the males. The number of instars, longevity, and fecundity of the different geographical populations of C. lividipennis were compared.