Evolutionary studies on animal body size have primarily focussed on selective pressures operating during the adult life. In contrast, ontogenetic pathways leading to differently sized adults have received less attention. In the present study, based on a common garden experiment, we report considerable genetic differences in body size among European populations of Ematurga atomaria (L.) (Lepidoptera: Geometridae). In terms of body mass, the moths from a southern (Georgian) population are twice as large as their northern (Estonian) conspecifics. Detailed monitoring of larval growth schedules revealed that the size difference arises through a longer development period of the Georgian larvae, with no difference in the number of instars. Differential (instantaneous) growth rates of the larvae do not differ between the populations. Eggs and newly hatched larvae are larger in the Georgian population but the difference vanishes in the second instar. The larger size of the Georgian moths is regained through higher relative mass increments during each of the three final instars. Such gradual 'accumulation of the difference' confirms the idea about constraints on substantial evolutionary changes in growth patterns within a single instar. The larger Georgian moths were found to be considerably more fecund which implies a strong selection for large female size. It remains unclear which counteracting selective pressures have favored the smaller size of Estonian conspecifics. As the associated difference in egg size appears not to be carried over to larger larval size, the adaptive value of larger eggs is not likely in contributing to the prospective large adult size. The larger eggs of the Georgian population should have an adaptive value per se, or represent a mechanistic consequence of larger maternal body size.

Latitudinal and elevational trends in body size are found in numerous animal taxa, with various adaptive explanations proposed. It is however debatable whether geographic trends in adult body size are accompanied by corresponding differences in juvenile growth rate (= mass gain per unit time). Respective studies have been complicated by conceptual and methodological problems related to defining and measuring this variable, particularly in organisms with discontinuous growth like arthropods. Using an original method for estimating differential (instantaneous) juvenile growth rates, we compared geographically distant European populations of six insect species in a common garden experiment. We found no among population differences in differential growth rate in any of the species. This result is in concert with concurrent increase in both adult size and developmental time towards the south. While opposite examples exist, we interpret our results as challenging the view that growth rate is a trait that readily responds to environmentally based selective pressures. Our results thus advocate the more classical view of growth rate maximisation within its physiological limits. We discuss the advantages of using differential (rather than integral) measures of growth rate in evolutionary ecological studies and evaluate the reasons for the detected latitudinal trends in life history traits.

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Seasonal polyphenism constitutes a specific type of phenotypic plasticity in which short‐lived organisms produce different phenotypes in different times of the year. Seasonal generations of such species frequently differ in their overall lifespan and in the values of traits closely related to fitness. Seasonal polyphenisms provide thus excellent, albeit underused model systems for studying trade‐offs between life‐history traits. Here, we compare immunological parameters between the two generations of the European map butterfly (Araschnia levana), a well‐known example of a seasonally polyphenic species. To reveal possible costs of immune defence, we also examine the concurrent differences in several life‐history traits. Both in laboratory experiments and in the field, last instar larvae heading towards the diapause (overwintering) had higher levels of both phenoloxidase (PO) activity and lytic activity than directly developing individuals. These results suggest that individuals from the diapausing generation with much longer juvenile (pupal) period invest more in their immune system than those from the short‐living directly developing generation. The revealed negative correlation between pupal mass and PO activity may be one of the reasons why, in this species, the diapausing generation has a smaller body size than the directly developing generation. Immunological parameters may thus well mediate trade‐offs between body size‐related traits.

The knowledge about the diversity and ecological role of entomopathogenic fungi is primarily based on agroecosystems whereas information derived from natural insect populations is much more limited. To contribute to filling this gap, we recorded the prevalence of fungal infections in laboratory rearing experiments with five species of Lepidoptera, and in a field rearing experiment including one of these moths. The diversity of detected fungi was found to be high; we isolated 25 species of fungi from insects that had died in the course of these experiments. Six species belonged to the family Cordycipitaceae known to include unambiguous insect pathogens. The trophic niche of the representatives of other taxa is less clear and requires further studies. Regarding the most abundant species, Cordyceps farinosa, in which this question could be addressed, there was no indication of specialization on particular insect hosts, whereas several of the less common species may have been recorded from lepidopteran hosts for the first time. Across the subsets of the data, the prevalence of fungal infections generally remained below 5%. Our results are thus consistent with the idea that entomopathogenic fungi are always present in insect populations but rarely reach epizootic levels. The detected species richness shows that much is to be gained from mapping the diversity of fungal species associated with folivorous insects in natural populations.