The ecological function of insect egg micropyles

Insect egg micropyles are openings through the chorion allowing sperm entry for fertilisation. Micropyles are diverse structures showing remarkable variation in number, spatial arrangement and physical structure across extant insect orders. Despite being almost ubiquitous across insects, they have received little attention. As key morphological features of an immobile life stage, it is plausible that part of the diversity exihibited by micropyles is adaptive supporting other egg structures during embryo development. So whilst egg fertilization is the primary function of micropyles, they could aid embryo development and be shaped by natural, as well as sexual selection. Here I first used ancestral reconstruction to investigate micropyle presence, number and variation in primitive insects. Then, I used phylogenetic comparative analyses to explore the ecological function of micropyle number. I hypothesised that micropyle number correlated with: (i) aeropyle presence facilitating oxygen exchange; (ii) aquatic oviposition supporting development in water; and is influenced by (iii) critical bioclimatic variables. Across 24 hexapod orders the most likely ancestral state was one or two micropyles, interspecific variation was high and intraspecific variation low. Mean micropyle number ranged from zero in Entognatha, Strepsiptera and Thysanoptera to 120 in Panstrongylus geniculatus, Hemiptera, and over 100 on average for Apidae, Hymenoptera. Micropyle number was strongly positively related to: i) egg size, with larger eggs having more micropyles; ii) the presence of aeropyles; iii) annual precipitation, with eggs developing in habitats with low annual precipitation exhibiting fewer micropyles; and iv) negatively related to micropyle width, insect eggs having fewer larger micropyles or numerous smaller ones. However, aquatic oviposition did not affect micropyle number. Overall these findings point to an adaptive ecological function of egg micropyles in addition to their primary fertilisation function. This is consistent with the hypothesis that micropyles aid embryo survival, and so this almost-ubiquitous trait across insects is shaped by sexual and natural selection pressures during this critical life stage.