Gall-inducing Aphididae may feed directly on phloem, whereas Eriophyidae and Nematoda feed on cells lining the gall chambers. We assume that a variation in structural complexity will occur within galls induced by each taxon, and that the complexity of the galls could be related to the types of storage tissue they have. Histological, histometric, and histochemical analyses were used to compare six gall systems with different levels of complexity. Such levels are not taxon-related, even though eriophyid galls are usually simpler than nematode and aphid galls. The histological features of galls allowed the classification of storage tissues into three types: typical nutritive tissues (TNT), common storage tissues (CST), and nutritive-like tissues (NLT). The TNT and NLT have cells with dense cytoplasm and a prominent nucleus. The CST cells are vacuolated, and may store starch and other energy-rich molecules, as do the NLT cells. In contrast to NLT or CST, the TNT serves as a direct food source for gall inducers, and it is present in nematode and some eriophyid galls. NLTs may be present in some aphid galls, but are not the direct feeding site. The CST occurs on galls of all three inducing taxa.

The developmental anatomy of the stems of Marcetia taxifolia (A. St.-Hil.) DC. fits the patterns described for other Melastomataceae. The galling effect of the Lepidoptera caused discrete structural alterations and conspicuous histochemical profiles. Epidermis and cortical parenchyma were hyperplasic with hypertrophied cells. The vascular system showed smaller changes. Tracheal elements did not change in width, refuting the constriction hypothesis, i.e., no improvement in water supply occurs in this gall system. Fiber lignification increased, providing additional support for the gall structure. A true nutritive tissue was redifferentiated from pith cells and accumulated two groups of metabolites. The first, consisting of starch, reducing sugars, and polyphenols, was detected in the outer cell layers, and the second, consisting of lipids and terpenoids, was detected in the inner ones. Histochemical analysis revealed that the distribution of these compounds formed gradients, with the first group being more concentrated outwards, and the second being more concentrated inwards. These gradients differ from those described for other insect galls and seem to be specific for lepidopteran galls. This is the first description of such a gradient in Lepidoptera-induced galls and shows that the current view that these galls are simple and nonnutritive should be changed.