Geographic variation in adult body size in an herbivorous insect: Testing the role of food processing and relative head size in morphological evolution

Background/Question/Methods

Body size may be a target of selection, but it may also be affected by selection on characters correlated with size that are functionally important.  For example, selection on herbivorous insects to maximize foraging opportunities or efficiency often favors larger individuals because of the positive correlation between feeding morphology (heads and mandibles) and body size.  In addition, the positive relationship between feeding morphology and body size might differ among populations because of differences in foraging strategies and nutritional requirements of different populations.  Head width is correlated with an ability to consume tougher vegetation in grasshoppers.  We test predictions of the general hypothesis that variation in feeding morphology is adaptive by testing for a relationship between feeding morphology and consumption of tougher vegetation.  We also tested for differences in the morphology-consumption relationship among populations that differ in average adult body size. Morphological measurements and timed feeding trials were performed on adult eastern lubber grasshoppers (Romalea microptera) from three populations, during which the mass consumed for two food plants differing in toughness was assessed in two different experiments: one with commercially available food (cabbage - tough; bok choy - soft) and one with native food plants (Crinum americanum– tough; Ludwigia peruviana- soft).

Results/Conclusions

            Sizes of both sexes from an eastern population were greater than for a central and a western population, but head linear dimensions change isometrically.  Populations differ in relative masses of head muscle and exoskeleton, with larger individuals from the east having relatively heavier head musculature than individuals from central and western populations.  Consumption rates for large individuals did not differ from small individuals when feeding trials were performed with two commercially available plants that differed in toughness (both sexes tested).   Feeding trials with two food plants from the field sites in south Florida yielded a significant difference in relative and absolute consumption among populations:  females from the eastern (larger) population consume more Crinum and less Ludwigia than do females from the central and western (smaller) populations (males not tested).  These results are consistent with the hypothesis that large size is advantageous for consuming tough vegetation; however, the advantage only appears when extremely tough plants are encountered and eaten (i.e., Crinum).