A global trend of declining abundance has been recorded across many insect populations, prompting investigations into potential causes. In European honeybees (Apis mellifera), factors including varroa mite and pesticides are contributing to a syndrome now widely known as 'colony collapse disorder'. Due to the direct economic value of this species, there has been a substantial research effort aimed at identifying the causal factors of this decline. This effort has been relatively successful, not least due to the tractability of bees as a study subject, including a strong understanding of the entire lifecycle. In contrast, insects with cryptic or poorly understood lifecycles, pose a far greater challenge for identifying causal factors of decline. Aquatic insects have also experienced decline, with up to 60% of caddisfly species vulnerable in some systems. Much of their life cycle is poorly understood, as eggs and larvae live underwater, whilst adults episodically disperse via flight, sometimes nocturnally. In Marysville, Victoria, monitoring of Hydrobiosidae caddisfly egg masses over 6 years has recorded decline in some species and a shift in community composition. Without a corresponding quantitative understanding of other life stages in relation to egg masses, the causes of decline cannot be determined. We assessed the numerical relationship between three stages of the lifecycle to see the entirety of species decline and compositional shift. A secondary aim was to collect location and habitat preference data for larvae. Comparing ratios of eggs:larvae:adults identified the loss rates through each stage. Replicated across riffles, these analyses also identify the relationship between inputs and outputs at each stage, with disconnection suggesting either strong density-dependence or dispersal processes. Together, these results improve our understanding of population dynamics and identify targets for future research on declines in Hydrobiosidae caddisflies.