The diversity, intercorrelation, and macroevolutionary consequences of insect life histories

The life history of an organism describes its pattern of growth, reproduction, and survival, and thus is closely linked to fitness. Between species, life history traits vary widely and one of the major goals of life history theory is to explain how natural selection gives rise to this variation. However, there has been a lack of comparative life history studies focussing on insects, which comprise over half of the currently described macroscopic species. I conduct a class-level analysis of insect life history traits, and explore the associations between life history strategies, ecology, metamorphosis, and macroevolutionary processes.

In Orthoptera (grasshoppers, crickets and kin), I identify mostly ‘fast-slow’ variation in life history traits, except that larger (and otherwise ‘slower’) species have smaller, not larger, clutches of eggs. I find that this type of variation is closer to that of reptiles than that of either mammals or birds and discuss potential reasons for this similarity.

Across the whole of the insects, I find that the primary axes of life history variation are related to a) amount of reproductive investment, and b) how this investment is divided into clutches. Further orthogonal axes contain variables related to development time, then adult lifespan. I show associations between life histories and diet (parasitoids and ecto-parasitoids are particularly fast-lived), metamorphosis (ametabolous species have longer lifespans), but not habitat (no difference between aquatic and terrestrial species).

Finally, I find links between life history traits and diversification. Species richness is higher in orders and families with fast development times, and higher in families with high fecundity. Diversification rates are higher in families with high fecundity and those with short egg stages.

Together, my work raises questions about the generality of widely invoked patterns of life history covariation and highlights the need to test these patterns in a taxonomically broad sense.