Explaining wing pattern polyphenism in male Processionary caterpillar

27 Jul 2012

Wing pattern polyphenism (the occurrence of different colour patterns in the same species) is a form of phenotypic plasticity that is widespread among Lepidoptera. However, the phenomenon is only well documented and understood among butterflies. In butterflies, wing pattern polyphenism often relates to the occurrence of different seasonal forms, e.g. wet- and dry-season forms, within different generations. Variations between these forms are the result of changes in the expression of elemental features of the wing “groundplan” which are manifested through the butterflies’ scales. The phenomenon in moths has largely been overlooked despite probable phylogenetic parallels between the taxa.

Dr Martin Steinbauer, Department of Zoology, La Trobe University, has been examining the possible causes of wing pattern polyphenism in the charismatic Processionary caterpillar, Ochrogaster lunifer Herrich-Schäffer (family Notodontidae). This species is well known because the caterpillars form long head-to-tail processions when dispersing between hosts (see Steinbauer 2009) and because some populations form large, highly apparent silken-bag nests in the canopies of Acacia and Senna species. The external surfaces of the forewings of male moths exhibit striking variation in colour and pattern. In more southern regions of their distribution (< 35°57’ S), all males have the same wing pattern as females, i.e. a single spot of white scales just below the areole (see two LHS images). This “spot” element of the groundplan occurs in all phenotypes and forms a focus for pattern elaboration (another is the region between the first and second anal veins). In more northerly regions, up to 67% of males have wings with continuous areas of intervenous silvery, white scales (see two RHS images). Between 35°30’ S and 34°18’ S, males with fewer “streaks” on their wings also occur (see middle two images); their occurrence varies between a few percentage up to 67% of individuals from a given location or group of caterpillars.

Since O. lunifer is a univoltine species, these wing pattern polyphenisms are unlikely to equate to the seasonal forms seen in butterflies. Hence, another explanation is required. Because variations in wing pattern in some butterflies represent physiologically induced phenotypes, the influence of caterpillar body temperature on wing pattern expression is being investigated. Measurements in autumn 2009 found that groups of caterpillars nesting beneath leaf litter could be between 1.3°C to 5.5°C higher than ambient. (Note, O. lunifer caterpillars live colonially either in canopy-nests or ground-nests with the former only found north of the Murray River or > 34°38’ S.) This summer and autumn, temperatures inside groups of canopy- and ground-nesting caterpillars were monitored using Thermochron iButtons to determine the influence of nest type on internal temperature and subsequently on wing pattern expression in male moths. The analysis of this data is ongoing while moths from nests returned to the laboratory have only just begun eclosing. The ecological studies have been supplemented with DNA barcoding of caterpillars to search for possible genetic explanations of wing pattern polyphenism. This work is being conducted in collaboration with Dr Andrew Mitchell, Australian Museum, Sydney, and is also ongoing.

Steinbauer MJ (2009) Thigmotaxis maintains processions of late instar caterpillars of Ochrogaster luniferPhysiological Entomology 34: 345-349.