THE GROWTH AND ACTIVITY OF THE CORPORA ALLATA IN THE LARVAL FIREBRAT, THERMOBIA DOMESTICA (PACKARD) (THYSANURA, LEPISMATIDAE)

¹ CSIRO, Division of Entomology, Canberra, Australia

1. The integument of the firebrat, Thermobia domestica, lacks scales until the molt from the third to the fourth larval stage, but retains them in all subsequent instars.

2. Implantation experiments confirm earlier findings that the scaleless integument of first stage larvae will develop scales prematurely when implanted into a molting adult, implying that the formation of scales is humorally determined.

3. The number of cells in the corpora allata increases progressively throughout the life of the firebrat, and the relationship between the number of allatal cells and the weight of the firebrat is expressed by the regression:

y = 10.61 + 1.94x, where y = the number of cells in each corpus allatum and x = the weight of the insect in mg. The regression does not intersect the origin.

4. The size of nuclei in the corpus allatum remains constant throughout life, but the volume of cytoplasm can alter. The minimal cytoplasmic volume coincides with the deposition of the first scale-bearing cuticle. The amount of cytoplasm then increases abruptly, the level continuing to rise slowly throughout the rest of larval life. A further increase may occur in mated females.

5. The juvenile hormone activity of the corpora allata, when assayed on pupae of the silkmoth Antheraea polyphemus, correlates with the volume of cytoplasm in the gland, and is minimal in the third stage.

6. The activity of the corpora allata is influenced by the intake of food, specifically protein.

7. Dedifferentiation of the epidermis is prevented by the pattern of growth in the corpus allatum, which indicates that the secretory potential of the gland, expressed as the volume of allatal cytoplasm per milligram body weight, is greater during the first through third instars than at any later stage, and by the inactivation of juvenile hormone during the molting cycle.