I inclose an ichnenmoned spider's cocoon, which I found on the mountain of Montreal early in May last. When I opened the cocoon, the larvæ had a bluish colour and were quite active. It produced about forty specimens of the fly, which I send you, as I have no means at hand of determining the species. There is, however, a very nice investigation in regard to the economy and modus operandi of this little ichneumon. That is, how does it reach the spider's eggs ? I cannot detect an ovipositor, and the body of the creature itself is only about a sixteenth of an inch long.