Tenebrio molitor is a major pest of stored grain, although its larvae have potential for use as food. However, little is known about the characteristics of individual larval instars of this species, and the number of instars remains disputed. Therefore, we assessed T. molitor for the average number of instars and its characteristics at different larval stages. The focus of this study was to establish a foundation for further studies on the characteristics of each larval instar. All of the T. molitor larvae showed incubation periods of 7 to 8 d and a period of 3 to 4 d for the 1st instar. Beyond the 1st instar, there were relatively large variations in the number of days in each instar period. Before emergence, most of the larvae had typically gone through 15 to 17 instars. The highest rate of pupae formation, 28.32%, was observed in the 17th instars. The body length gradually increased with each successive instar, reaching its maximum at the 17th instar. Beyond the 17th instar, however, the body length decreased. The larvae were white in the first instar, and gradually turned brown after the 2nd instar.
The physiological features of
At the time of mating, the pheromone 4-methyl-1-nonanol is emitted by
The optimal temperature for the reproduction of
In the process of larval ecdysis, molting hormones play an important function to enable larvae to proceed to the next instar (Delbecque
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Measurement of body length of each instar
After hatching, larval exuviae were counted to determine the number of instars and subsequently removed from the petri dishes. When the larva pupated, instar counting was stopped.
Another set of experiments was performed to measure the body length of each instar. The body lengths of 10 larvae were measured using Vernier Caliper each time an exuvium was observed. Generally, the width of the head capsule was measured because it exhibits distinct variation between larval stages (Hsia and Kao, 1987). In this experiment, however, the body lengths of larvae were measured owing to the small head capsule size during the early larval stages. Finally, photographs of each instar were taken using a DSLR camera.
As shown in Table 1, the incubation period was 7-8 d and the duration of the 1st instar was 3-4 d, on average. Interestingly, the larvae were not significantly different in terms of the incubation period or the duration of the 1st instar. Between the 2nd and 20th instars, the duration of each instar was not uniform among larvae. Very few larvae were detected in the 19 th -20th instar. The lack of uniformity among larvae between the 2nd and 20th instars may be attributed to malnutrition. An experiment on
Periods in each instar, number of pupae for each of the 14th to 21st instars, and body lengths of each larval stage in Tenebrio molitor.
Pupation occurred after the 14th instar. Approximately 69.69% of total pupation was observed between the 15th and 17th instars. The largest proportion of pupae (28.32%) was observed in the 17th instar. In other words, most of the larvae used in this experiment exhibited 15 to 17 instars in their life cycle. Before the 19th instar, most of larvae pupated. Based on a study by Ludwig (1956), the average number of instars 25°C ranged from 11 to 15 (Fiore, 1960), which is smaller than what we found. This discrepancy may be explained by many factors, such as the nutritional status of the larvae and parents (Ludwig and Fiore, 1960). Repeating this study over a wider range of temperature may explain how ambient temperature influences the number of larval stages.
The body length of
Although the 1st instar was white, the larvae gradually turned brown from and after the 2nd instar. Except for the change in color, no significant differences in the morphological characteristics of the larvae were detected (Fig. 1)
Through this experiment, we confirmed the incubation period, the duration of the 1st instar, and the average number of instars in