1. The bacteria within the ‘bacterial cells’ of Calandra are in the form of bacilli. 2. The bacilli do not pass into the alimentary tract of the larva. 3. In the adult the bacilli pass from their host-cells into the lumen of the gut, mix with the food there, and pass out with the faeces mostly in the form of cocci. 4. The relation between Calandra and the intracellular bacteria is obscure and so far cannot be described as symbiotic. 5. ‘Bacterial cells’ have been found at the anterior tips of the ovarioles. 6. The ovarian eggs are invaded at a very early stage during their growth by bacteria coming from the ‘ovarian bacterial cells’. 7. The bacteria remain in the cytoplasm of the egg scattered in between the yolk globules throughout the early embryonic life. 8. In all developing eggs the ‘alimentary bacterial cellmass’ appears during the latter part of embryonic life. 9. In the eggs destined to give rise to females the genital rudiments are associated with ‘bacterial cells’. 10. The developmental history of the ‘ovarian bacterial cells’ has been followed out.

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Summary The relation between certain insects and the intracellular micro‐organisms they harbour is obscure and cannot be described as symbiotic for the following reasons: Sterile mycetomes are known to occur in some species closely related to others with heavily infected mycetomes. Defloration experiments have been carried out successfully without any harmful effect to the host insect. Intracellular micro‐organisms in xylophagous insects cannot be considered as playing an important role in the digestion of wood for the following reasons: In weevils with wood‐eating habits which have intracellular micro‐organisms, the latter only pass into the lumen of the gut of their host during the adult stage when the insect is not feeding on wood. Some wood‐eating species of insects harbour intracellular micro‐organisms, while closely related species with similar feeding habits are free from these. The intracellular micro‐organisms of some wood‐eating forms have been cultivated in vitro and found to be unable to break down cellulose. The relatively high content in nitrogenous substances of different kinds of wood, and the general occurrence of proteolytic enzymes in insects, make the assumption that intracellular micro‐organisms fix atmospheric nitrogen for the use of their host superfluous. The extracellular intestinal micro‐organisms of certain lamellicorn larvae and of some termites play no role in breaking down cellulose for the use of their host. They are utilised as a direct food source. Such insects are therefore better referred to as micro‐organism‐feeders. True wood‐eating insects derive the necessary carbohydrates from the wood they live on through the activity of their own enzymes. The enzyme complex in such insects has been found to vary from species to species. The carbohydrate components of wood vary in quantity in different kinds of wood. Some true wood‐eating forms depend upon starch and soluble sugars for their source of carbohydrates. Such insects have no cellulase and can consequently only live in kinds of wood comparatively rich in starch and sugars. Other true wood‐eating forms secrete cellulase and are therefore able to live in kinds of wood comparatively poor in starch and soluble sugars. The occurrence of hemicellulase has been demonstrated only in a very few cases and the value of the hemicelluloses as food for wood‐eating insects has not been thoroughly investigated.

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