Hairworms (phylum Nematomorpha) are a relatively obscure group of parasites with a complex life cycle including multiple invertebrate hosts, an aquatic and terrestrial phase, and five recognised life stages. The final aquatic life stage is a dormant cyst found within the tissues of myriad intermediate macroinvertebrate hosts, mostly insect larvae, inhabiting streams and rivers. Since relatively little is known on the cryptic interactions that occur between hairworms and their freshwater hosts (e.g., the internal defence reaction or IDR of hosts), our goal was to uncover some of these interactions and explore their dynamics. Therefore, with samples collected from a subalpine stream in New Zealand during two consecutive seasons (i.e., winter and spring), we observed and categorised different levels of melanin encapsulation, a type of IDR, by aquatic insect larvae to dormant Gordius sp. hairworm cysts. Based on these new observations, we calculated an intensity of IDR for an abundant species of caddisfly larvae (Olinga sp.) using the proportion of non-melanised cysts per individual host. We tested the hypothesis that season and total number of cysts in an infected host impact its IDR intensity. Also, we explored the effect of host body size on the total number of cysts it harbours. We found that the total number of cysts does not affect the IDR intensity of the host. However, season did have an impact on the IDR intensity in Olinga sp., with lower levels observed in the spring. Additionally, larger caddisfly larvae harboured more cysts than smaller ones. Since little is known on the hidden interactions between hairworms and their intermediate freshwater hosts, this new information adds some complexity to this poorly understood group of parasites.