In ectothermic species, changes in environmental conditions can be reflected in mitochondria, which are known for their high level of sensitivity and plasticity. These can result in either a functional change within the mitochondrion itself, a variation in total density within a tissue, or even a combination of the two. Therefore, experimental data must be presented with an adequate measure of mitochondrial density to be interpretable and scientifically backed. One of the most effective ways to quantify mitochondrial density (MD) is through transmission electron microscopy (TEM). However, proxies of mitochondrial density are often used in lieu. Commonly, quantifications of mitochondrial protein content, enzyme activity, and mitochondrial DNA copy numbers are employed as estimates of MD. The most frequently used estimates include measurements of citrate synthase (CS), complex I, and complex IV activity, but results vary greatly between studies. Indeed, mitochondrial plasticity, especially in ectotherms, may partly explain these discrepancies. This ongoing project therefore embodies a comprehensive approach evaluating if the use of mitochondrial density markers can be generalized in ectothermic animals. We show that despite cold-acclimated Drosophila (D. melanogaster) having higher complexe IV activity, and a tendency for increased CS activity, these commonly employed mitochondrial density markers were poor predictors of differences in various measurements of MD (i.e. number of mitochondria, fractional area, and surface density) evaluated by TEM. Conversely, neither differences were observed between enzymatic activities nor MD measurements between differentially acclimated beetles (Leptinotarsa decemlineata). The analysis of further density markers is ongoing.