Varying levels of melanotic encapsulation of gordiid hairworm cysts (Nematomorpha) by aquatic insect larvae: seasonal and host effects.

The defence reactions of insects to parasitic invaders are both varied and complex. Melanisation of pathogens is often an important step in insect immunity and can play a key role in isolating parasites. Within 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 melanotic encapsulation by aquatic insect larvae to dormant Gordius sp. hairworm (Phylum Nematomorpha) cysts, a relatively obscure group of parasites. Some of these insect species act as intermediate transport hosts in the complex life cycle of hairworms. Based on these new observations, we calculated the melanisation response 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 melanisation response. Also, we explored the effect of host body size on the total number of cysts it carries. We found that the total number of cysts does not affect the melanisation response of the host. Season did have an impact on the melanisation response in Olinga sp., with lower levels observed in the spring. Additionally, larger caddisfly larvae harboured more cysts than smaller ones. Since little is known about the cryptic interactions between hairworms and their intermediate hosts, this new information adds some complexity to this poorly understood group of parasites.
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