Data and code from: Disentangling the drivers of decadal body size decline in an insect population

While climate warming is widely predicted to reduce body size of ectotherms, evidence for this trend is mixed. Body size depends not only on temperature but also on other factors, such as food quality and intraspecific competition. Because temperature trends or other long-term environmental factors may affect population size and food sources, attributing trends in average body size to temperature requires the separation of potentially confounding effects. We evaluated trends in the body size of the midge Tanytarsus gracilentus and potential drivers (water temperature, population size, and food quality) between 1977 and 2015 at Lake Mývatn, Iceland. Although temperatures increased at Mývatn over this period, there was only a slight (non-significant) decrease in midge adult body size, contrary to theoretical expectations. Using a state-space model including multiple predictors, body size was negatively associated with both water temperature and midge population abundance, and it was posit..., Midges (Diptera: Chironomidae) were captured using window traps (Jónsson et al., 1986) at two locations around Lake Mývatn, Iceland. They were innumerated to species and separated into two cohorts (early summer and late summer), matching the voltinism pattern of the focal species (Tanytarsus gracilentus) which overwinter as larvae (Gardarsson et al., 2004; Lindegaard & Jónasson, 1979; Einarsson et al. 2002, 2004). Archived midges were used to measure carbon stable isotopes (McCormick et al. 2022) and wing lengths were measured from arculus to tip on 15-20 individuals per generation (where abundances were adequate). Because identifications of female midges is difficult and often impossible, the dataset includes only males. Air temperature data come from the Icelandic Meterological Office (https://www.vedur.is/). All processing to the data are included in scripts., , # Data and code for: Disentangling the drivers of decadal body size decline in an insect population

[https://doi.org/10.5061/dryad.79cnp5j2m](https://doi.org/10.5061/dryad.79cnp5j2m)

This analysis had two major goals:

1. characterize linear trends at Lake Mývatn, Iceland, between 1977 and 2015 in:
1. midge (*Tanytarsus gracilentus)* wing lengths
2. midge population size
3. midge stable carbon isotopes (13C)
4. air and water temperatures
2. characterize the impact of population size, stable isotope signature, and water temperature on midge wing lengths.

To approach this problem, we employed state space models that account for temporal autocorrelation and measurement error. We assessed significance by using boostrapped likelihood ratio tests (2,000 simulations for each term).

## Description of the data and file structure

This file consists of three subfolders: **Data**, **Output**, and **Scripts**.

**Data** includes the raw data on which the analyses were used.

* f...