The formation of a hatching line in the serosal cuticle confers multifaceted adaptive functions on the eggshell of a cicada

Insect eggshells must meet various demands of developing embryos. These demands sometimes conflict with each other; therefore, there are tradeoffs between eggshell properties, such as robustness and permeability. To meet these conflicting demands, particular eggshell structures have evolved in diverse insect species. Here, we report a rare eggshell structure found in the eggshell of a cicada, Cryptotympana facialis. This species has a prolonged egg period with embryonic diapause and a trait of humidity-inducible hatching, which would impose severe demands on the eggshell. We found that in eggs of this species, unlike many other insect eggs, a dedicated cleavage site, known as a hatching line, was formed not in the chorion but in the serosal cuticle. The hatching line was composed of a fine furrow accompanied by ridges on both sides. This furrow-ridge structure formed in the terminal phase of embryogenesis through the partial degradation of an initially thick and nearly flat cuticle layer. We showed that the permeability of the eggshell was low in the diapause stage, when the cuticle was thick, and increased with degradation of the serosal cuticle. We also demonstrated that the force required to cleave the eggshell was reduced after the formation of the hatching line. These results suggest that the establishment of the hatching line on the serosal cuticle enables flexible modification of eggshell properties during embryogenesis, and we predict that it is an adaptation to maximize the protective role of the shell during the long egg period while reducing the barrier to emerging nymphs at the time of hatching.

“Intracascaral space” an eggshell structure of Caiman latirostris eggs

In recent decades, eggshells of eggs from large-bodied reptiles have been studied by many researchers, to describe the eggshell, to compare them to extinct lineages that once inhabited our planet and also to understand how the egg provides the embryo specific conditions during incubation. In previous studies we described and characterized normal and pathologic Caiman latirostris eggshells; we also evaluated how the eggshell changes during incubation. In a study relating temperature variation and eggshell structures of successful eggs, we observed empty structures not previously described that we termed “intracascaral space”. The aim of this study is to describe this structure of C. latirostris eggshells. We hypothesize about the possible functions which it would perform during incubation and for development of the embryos.

Convergent evolution of reduced eggshell conductance in avian brood parasites

Brood parasitism has evolved independently in several bird lineages, giving rise to strikingly similar behavioural adaptations that suggest convergent evolution. By comparison, convergence of physiological traits that optimize this breeding strategy has received much less attention, yet these species share many similar physiological traits that optimize this breeding strategy. Eggshell structure is important for embryonic development as it controls the flux of metabolic gases, such as O 2 , CO 2 and H 2 O, into and out of the egg; in particular, water vapour conductance ( G H 2 O ) is an essential process for optimal development of the embryo. Previous work has shown that common cuckoos ( Cuculus canorus ) have a lower than expected eggshell G H 2 O compared with their hosts. Here, we sought to test whether this is a trait found in other independently evolved avian brood parasites, and therefore reflects a general adaptation to a parasitic lifestyle. We analysed G H 2 O for seven species of brood parasites from four unique lineages as well as for their hosts, and combined this with species from the literature. We found lower than expected G H 2 O among all our observed brood parasites both compared with hosts (except for brown-headed cowbirds ( Molothrus ater )) and compared with the expected rates given their phylogenetic positions. These findings suggest that a lowered G H 2 O may be a general adaptation for brood parasitism, perhaps helping the parasite nestling to develop greater aerobic fitness. This article is part of the theme issue ‘The coevolutionary biology of brood parasitism: from mechanism to pattern’.

Cracking the egg: the use of modern and fossil eggs for ecological, environmental and biological interpretation

A myriad of extant and extinct vertebrates produce eggs. Eggs and eggshells provide a useful substrate for reconstructing environment, ecology and biology over a range of time scales from deep time to the present. In this review, methods for analysing and understanding records of diet, climate, environment and biology preserved in eggshells are presented. Topics covered include eggshell structure, assessing diagenesis, stable isotope geochemistry and morphological investigations of eggshell characteristics. This review emphasizes the use of eggshells in the modern and fossil record, as they allow for interpretation of characteristics of a wide variety of amniotes across geological history, uniquely informing environmental and ecological investigations.