Does the fact cordyceps mostly affect insects have something to do with chitin being polysaccharide?
My initial guess was no but there might actually be some evidence for this. For example, host-switching between fungus and insects apparently happens within relatively small taxa of cordyceps, suggesting that host specialization is not too dramatic.
Moreover, there does seem to be some evidence that chitin is a relatively important metabolic input for at least one cordyceps species.
I don't know that we could say that chitin is the thing that allows the host switching or that it is the most important nutrient for cordyceps or anything like that, but it looks like it at least helps.
It's worth noting that ecological characteristics may play at least as large of a role. The phylogenetic study noted that host switching likely occurred from cicada nymphs (which hibernate underground) to truffles (which have their main mass underground), so it's possibly just an opportunistic jump in the soil environment.
It would be interesting to see if there is a &quot;chitin sensor&quot; mechanism in cordyceps that yields a signal to cordyceps that there is a host to invade. But I don't think that I saw anything that suggests that (though it's far from unprecedented in biology).